Find below a catalog comprising 70 diverse planetary types, including carefully crafted artistic depictions and detailed explanations of what is currently known or hypothesized about each type.
Contents:
. chthonian planet
. cloudless gas giant
. cold eyeball planet
. coreless planet
. crater planet
. desert planet
. disrupted planet
. dwarf planet
. Earth analog planet
. Earth-like planet
. ecumenopolis planet
. ellipsoid planet
. exoplanet
. eyeball planet
. fluorine planet
. forest planet
. gas giant
. giant planet
. habitable planet
. helium planet
. hot eyeball planet
. hot Jupiter
. hot Neptune
. hycean planet
. ice giant
. ice planet
. iron planet
. jungle planet
. lava planet
. mega-Earth
. mesoplanet
. methane planet
. mini-Jupiter
. mini-Neptune
. mountain planet
. ocean planet
. phosphorus planet
. planetesimal
. protoplanet
. puffy planet
. ringed planet
. rogue planet
. silicate clouds gas giant
. silicate planet
. sub-brown dwarf
. sub-Earth
. sub-Neptune
. subsurface ocean planet
. sulfur planet
. super-earth
. super-Jupiter
. super-Neptune
. super-puff planet
. superhabitable planet
. supermassive terrestrial planet
. swamp planet
. terrestrial planet
. toroidal planet
. ultra-cool dwarf
. ultra-hot Jupiter
. ultra-hot Neptune
. ultra-short period planet
. water clouds gas giant
⇊ Let’s explore each and every one! ⇊
Alkali metal clouds gas giant
Above 900 K (630 °C/1160 °F), carbon monoxide becomes the dominant carbon-carrying molecule in a gas giant’s atmosphere (rather than methane). Furthermore, the abundance of alkali metals, such as sodium substantially increases, and spectral lines of sodium and potassium are predicted to be prominent in a gas giant’s spectrum. These planets form cloud decks of silicates and iron deep in their atmospheres, but this is not predicted to affect their spectrum. The Bond albedo of a class IV planet around a Sun-like star is predicted to be very low, at 0.03 because of the strong absorption by alkali metals. Gas giants of classes IV and V are referred to as hot Jupiters.
55 Cancri b was listed as a class IV planet.
HD 209458 b at 1300 K (1000 °C) would be another such planet, with a geometric albedo of, within error limits, zero; and in 2001, NASA witnessed atmospheric sodium in its transit, though less than predicted. This planet hosts an upper cloud deck absorbing so much heat that below it is a relatively cool stratosphere. The composition of this dark cloud, in the models, is assumed to be titanium/vanadium oxide (sometimes abbreviated “TiVO”), by analogy with red dwarfs, but its true composition is yet unknown; it could well be as per Sudarsky.
HD 189733 b, with measured temperatures 920–1200 K (650–930 °C), also qualifies as class IV. However, in late 2007 it was measured as deep blue, with an albedo over 0.14 (possibly due to the brighter glow of its “hot spot”). No stratosphere has been conclusively proven for it as yet.
TrES-2b was measured with the lowest albedo and therefore listed as a class IV gas giant or alkali metal clouds gas giant.
Keywords: Alkali metal clouds, alkali, metals, gas, giant, class IV, Sudarsky, planet, Jupiter, brown, blue, system, substellar, planet, carbon, monoxide, methane, sodium, spectrum, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, 55 Cancri, HD 209458, HD 189733, TrES-2b
——————
Ammonia clouds gas giant
Gaseous giants with appearances dominated by ammonia clouds or class I gas giants are found in the outer regions of a planetary system. They exist at temperatures less than about 150 K (−120 °C; −190 °F). The predicted Bond albedo of a class I planet around a star like the Sun is 0.57, compared with a value of 0.343 for Jupiter and 0.342 for Saturn. The discrepancy can be partially accounted for by taking into account non-equilibrium condensates such as tholins or phosphorus, which are responsible for the coloured clouds in the Jovian atmosphere, and are not modelled in the calculations.
The temperatures for a class I planet requires either a cool star or a distant orbit. The former may mean the star(s) are too dim to be visible, where the latter may mean the orbits are so large that their effect is too subtle to be detected until several observations of those orbits’ complete “years” (cf. Kepler’s third law). The increased mass of superjovians would make them easier to observe, however a superjovian of comparable age to Jupiter would have more internal heating, which could push it to a higher class.
As of 2015, 47 Ursae Majoris c and d could be Class I planets. Upsilon Andromedae e and 55 Cancri d may also be Class I planets.
Keywords: ammonia clouds, ammonia, gas, giant, class I, Sudarsky, planet, Jupiter, brown, yellow, beige, bands, spot, system, planet, cold, temperature, tholins, spectrum, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, 47 Ursae Majoris, Upsilon Andromedae, 55 Cancri
——————
Ammonia planet
An ammonia planet is an assumed class of planet with its surface covered in lakes or oceans of ammonia and/or with ammonia clouds in the atmosphere.
Ammonia planets tend to have similar climates to Earth’s, except it uses ammonia as a “variable gas” instead of water vapor as it is on Earth. For example, there is ammonia rain or ammonia snow instead of water rain or water snow. Those planets tend to be very cold, at around −150°F (−115°C), or warmer depending on the thickness of the atmosphere. Their atmospheres tend to be composed mostly of nitrogen and oxygen with variable amounts of ammonia and trace amounts of carbon dioxide and other gases.
The life-bearing potential of ammonia planets is considered fair. These unique worlds may harbor intriguing life forms that have adapted to extreme cold and employ ammonia as their primary solvent, contrasting with Earth’s life that relies on water as its solvent. Some of the plants on ammonia planet may use photosynthesis using light from the parent star while others perform chemosynthesis. It is predicted that plants may use ammonia and carbon dioxide to produce methylamine, nitrogen, and oxygen.
It is also predicted that lifeforms similar to animals might inhale oxygen and exhale carbon dioxide for respiration, just like animals here on Earth. Animals also may eat foods rich in amines and drink liquid ammonia. The main biogeochemical cycle on ammonia planets is the ammonia cycle compared to the carbon cycle here on Earth.
However, life on some ammonia planets may not be carbon-based, but silicon-based as they have better survivability to extreme cold than carbon-based life.
Keywords: ammonia planet, ammonia world, ammonia, planet, world, exoplanet, atmosphere, lakes, rain, snow, gases, planetary, science, space, astronomy, astrobiology, chemistry
——————
Barren planet
A barren planet is a class of planet with a hard surface made mainly of bare rocks and virtually no vegetation.
The life-bearing status on barren planets is fair. Typically barren planets don’t contain a lot of surface liquids (such as water), but some of these have abundant liquids underground in aquifers or caves where life can thrive in. Most of those planets are speculated to have relatively thin atmospheres.
Mercury, Venus, and Mars are barren planets in our solar system. There are nearly a hundred speculated barren planets around other stars as of 2015, including two out of three pulsar planets in orbit around PSR B1257+12, a diamond planet around PSR J1719-1438, all three around Kepler-42, and one around HD 10180.
Keywords: barren planet, barren world, planet, world, exoplanet, desolate, inhospitable, sterile, desert, rocky, Kepler, HD, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most massive gas giant planets and the least massive stars, approximately 13 to 80 times that of Jupiter. However, they can fuse deuterium and the most massive ones (> 65 MJ) can fuse lithium.
Astronomers classify self-luminous objects by spectral class, a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M, L, T, and Y. As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age.
Despite their name, to the naked eye, brown dwarfs would appear in different colors depending on their temperature. The warmest ones are possibly orange or red, while cooler brown dwarfs would likely appear magenta or black to the human eye. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.
As brown dwarfs have relatively low surface temperatures, they are not very bright at visible wavelengths, emitting most of their light in the infrared. However, with the advent of more capable infrared detecting devices, thousands of brown dwarfs have been identified. The nearest known brown dwarfs are located in the Luhman 16 system, a binary of L- and T-type brown dwarfs about 6.5 light-years from the Sun. Luhman 16 is the third closest system to the Sun after Alpha Centauri and Barnard’s Star.
Keywords: brown dwarf, brown, magenta, star, stellar, planetary, system, substellar, planet, fusion, hydrogen, deuterium, lithium, Luhman 16, type M, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Carbon planet
A carbon planet is a theoretical type of planet that contains more carbon than oxygen. Carbon is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.
Carbon planets could form if protoplanetary discs are carbon-rich and oxygen-poor. They would develop differently from Earth, Mars, and Venus, which are composed mostly of silicon–oxygen compounds. Different planetary systems have different carbon-to-oxygen ratios, with the Solar System’s terrestrial planets closer to being “oxygen planets” with C/O molar ratio of 0.55. 12% of stars have C/O ratios above 0.65, making them candidates for the carbon-rich planetary systems.
The exoplanet 55 Cancri e, orbiting a host star with C/O molar ratio of 0.78, is a possible example of a carbon planet. The pulsar planets PSR B1257+12 A, B and C may be carbon planets that formed from the disruption of a carbon-producing star. Carbon planets might also be located near the Galactic Center or globular clusters orbiting the galaxy, where stars have a higher carbon-to-oxygen ratio than the Sun. When old stars die, they spew out large quantities of carbon. As time passes and more and more generations of stars end, the concentration of carbon, and carbon planets, will increase.
Carbon planets would probably have an iron-rich core like the known terrestrial planets. Surrounding that would be molten silicon carbide and titanium carbide. Above that, a layer of carbon in the form of graphite, possibly with a kilometers-thick substratum of diamond if there is sufficient pressure. During volcanic eruptions, it is possible that diamonds from the interior could come up to the surface, resulting in mountains of diamonds and silicon carbides. The surface would contain frozen or liquid hydrocarbons (e.g., tar and methane) and carbon monoxide. A weather cycle is theoretically possible on carbon planets with an atmosphere, provided that the average surface temperature is below 77 °C.
However, carbon planets will probably be devoid of water, which cannot form because any oxygen delivered by comets or asteroids will react with the carbon on the surface. The atmosphere on a relatively cool carbon planet would consist primarily of carbon dioxide or carbon monoxide with a significant amount of carbon smog.
Carbon planets are predicted to be of similar diameter to silicate and water planets of the same mass, potentially making them difficult to distinguish. The equivalents of geologic features on Earth may also be present, but with different compositions. For instance, the rivers might consist of oils. If the temperature is low enough (below 350 K), then gasses may be able to photochemically synthesize into long-chain hydrocarbons, which could rain down onto the surface.
The spectra of carbon planets would lack water, but show the presence of carbonaceous substances, such as carbon monoxide.
Keywords: carbon planet, carbon world, carbon, planet, world, exoplanet, rocky, core, atmosphere, diamonds, rain, 55 Cancri, PSR B1257+12, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Chlorine planet
Chlorine planet is a hypothetical class of planet with surface covered in lakes or oceans of muriatic acid with clouds made of muriatic acid in the atmosphere.
Viewed from space, the oceans on chlorine planet would appear deep green. Lands would appear brownish to blackish green and clouds appear white with green tinge. The clouds formed when muriatic acid vapor condenses.
Chlorine planets tend to be smoggy and have precipitation but no thunderstorm. Unlike Earth, chlorine planets use muriatic acid as a “variable gas” instead of water. For example, there are muriatic acid rain or muriatic acid snow instead of water rain or water snow. Chlorine planets tend to be cold, at around −110°F (−79°C), about the sublimation point of carbon dioxide. Their atmospheres tend to be composed mostly of nitrogen, oxygen, and chlorine with variable amounts of muriatic acid vapor and trace amounts of dichloride monoxide, carbon dioxide, mustard gas, and other gases.
The life-bearing status on chlorine planets is fair, even though chlorine is toxic to many forms of life on Earth, but some forms of higher life somewhere in our galaxy may require chlorine to thrive. It is predicted that during the photosynthesis, muriatic acid is combined with carbon dioxide to produce formaldehyde, chlorine, and oxygen.
It is also predicted that animal-like lifeforms might drink muriatic acid and eat foods rich in formaldehyde. Animals may also inhale oxygen and exhale carbon dioxide like they do on Earth. The main biogeochemical cycle on chlorine planets is the chlorine cycle compared to the carbon cycle here on Earth.
Keywords: chlorine planet, chlorine world, chlorine, planet, world, exoplanet, atmosphere, ocean, muriatic acid, green, brown, clouds, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Chthonian planet
Chthonian planets are a hypothetical class of celestial objects resulting from the stripping away of a gas giant’s hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.
Transit-timing variation measurements indicate, for example, that Kepler-52b, Kepler-52c and Kepler-57b have maximum masses between 30 and 100 times the mass of Earth (although the actual masses could be much lower); with radii about two Earth radii, they might have densities larger than that of an iron planet of the same size. These exoplanets are orbiting very close to their stars and could be the remnant cores of evaporated gas giants or brown dwarfs. If cores are massive enough they could remain compressed for billions of years despite losing the atmospheric mass.
As there is a lack of gaseous “hot-super-Earths” between 2.2 and 3.8 Earth-radii exposed to over 650 Earth incident flux, it is assumed that exoplanets below such radii exposed to such stellar fluxes could have had their envelopes stripped by photoevaporation.
HD 209458 b is an example of a gas giant that is in the process of having its atmosphere stripped away, though it will not become a chthonian planet for many billions of years, if ever. A similar case would be Gliese 436b, which has already lost 10% of its atmosphere.
CoRoT-7b is the first exoplanet found that might be chthonian. Other researchers dispute this, and conclude CoRoT-7b was always a rocky planet and not the eroded core of a gas or ice giant, due to the young age of the star system.
In 2020, a high-density planet more massive than Neptune was found very close to its host star, within the Neptunian Desert. This world, TOI 849 b, may very well be a chthonian planet.
Keywords: chthonian planet, chthonian, planet, world, exoplanet, strip, atmosphere, gas, remnant, rocky, metallic, core, terrestrial, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Kepler-52, Kepler-57, HD 209458, Gliese 436, CoRoT-7, TOI 849
——————
Cloudless gas giant
Gaseous giants with equilibrium temperatures between about 350 K (170 °F, 80 °C) and 800 K (980 °F, 530 °C) do not form global cloud cover, because they lack suitable chemicals in the atmosphere to form clouds. (They would not form sulfuric acid clouds like Venus due to excess hydrogen.) These planets would appear as featureless azure-blue globes because of Rayleigh scattering and absorption by methane in their atmospheres, appearing like Jovian-mass versions of Uranus and Neptune. Because of the lack of a reflective cloud layer, the Bond albedo is low, around 0.12 for a class-III planet around a Sun-like star. They exist in the inner regions of a planetary system, roughly corresponding to the location of Mercury.
Possible class-III planets are HD 37124 b, HD 18742 b, HD 178911 Bb, 55 Cancri c, Upsilon Andromedae c, Kepler-89e, CoRoT-9b, HD 205739 b and Pollux b. Above 700 K (800 °F, 430 °C), sulfides and chlorides might provide cirrus-like clouds.
Keywords: cloudless planet, cloudless, gas, giant, gas giant, planet, world, exoplanet, class-III, atmosphere, gas, remnant, rocky, metallic, core, terrestrial, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, HD 37124, HD 18742, 55 Cancri, Kepler-89e, Upsilon Andromedae, CoRoT-9b
——————
Cold eyeball planet
An eyeball planet is a hypothetical type of tidally locked planet, for which tidal locking induces spatial features (for example in the geography or composition of the planet) resembling an eyeball. They are terrestrial planets where liquids may be present, in which tidal locking will induce a spatially dependent temperature gradient (the planet will be hotter on the side facing the star and colder on the other side).
A “cold” eyeball planet, usually farther from the star, will have liquid on the side facing the host star while the rest of its surface is made of ice and rocks.
Because most planetary bodies have a natural tendency toward becoming tidally locked to their host body on a long enough timeline, it is thought that eyeball planets may be common and could host life, particularly in planetary systems orbiting red and brown dwarf stars which have lifespans much longer than other main sequence stars.
Kepler-1652b is potentially an eyeball planet. The TRAPPIST-1 system may contain several such planets.
Keywords: eyeball planet, eyeball world, eyeball, planet, world, exoplanet, frozen, ice, ocean, cold, tidal lock, terrestrial, temperature, side, face, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Kepler-1652, TRAPPIST-1
——————
Coreless planet
A coreless planet is a theoretical type of terrestrial planet that has no metallic core and is thus effectively a giant rocky mantle. It can be formed in cooler regions and far from the star.
There are probably two ways in which a coreless planet may form: in the first, the planet accretes from chondrite-like fully oxidized water-rich material, where all the metallic iron is bound into silicate mineral crystals. Such planets may form in cooler regions farther from the central star. In the second, the planet accretes from both water-rich and iron metal-rich material. However, the metal iron reacts with water to form iron oxide and release hydrogen before differentiation of a metal core has taken place. Provided the iron droplets are well mixed and small enough (<1 centimeter), the predicted end result is that the iron is oxidized and trapped in the mantle, unable to form a core.
Earth’s magnetic field results from its flowing liquid metallic core, according to the dynamo theory, but in super-Earths the mass can produce high pressures with large viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Magnesium oxide, which is rocky on Earth, can be liquid at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.
The predicted sizes of coreless and cored planets are similar within a few percent, which makes it difficult to interpret the interior composition of exoplanets based on measured planetary masses and radii.
Keywords: coreless planet, coreless, planet, world, exoplanet, core, terrestrial, metallic, rocky, mantle, iron, silicate, magnetic field, interior, layers, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, super-Earth
——————
Crater planet
A crater planet is a class of planet with a surface pockmarked with impact craters, just like the surface of the Moon.
Crater planets tend to have brief periods of geologic activities and usually have little or no atmospheres, otherwise it would have eroded away much of the craters. Also, crater planets are usually small, about half the size of Earth and one-tenth the mass. Small, low-mass planets are not as active as their more massive cousins. Their lack of geologic activities mean that there are little or no volcanic and tectonic activities. Small, low-mass planets also have little gravity, so they don’t hold on the gases well, allowing the stellar winds to strip away the atmospheres easily.
The life-bearing status of crater planets are poor, although present-day microbial life could likely exist in the rocks and underground on Mars. Life may thrive underground in caves or in aquifers. Thriving underground is advantageous because the ground above can shield life from radiation and meteor impacts.
There are an estimated 169 billion crater planets in our galaxy alone, making it the second most abundant surface class of rocky planet after barren planets.
There are two crater planets in our solar system — Mercury and Mars. Both planets have little to no atmospheres and are geologically dead. Mercury’s surface is more heavily cratered than Mars. As of February 2014, there are only five probable crater exoplanets out of over a thousand — Kepler-42d (Idunn), KIC 12557548 b (Iberia), Kepler-37b (Cobis), Kepler-62c (Esus), and Kepler-102c (Dian Cecht).
Keywords: crater planet, crater world, crater, planet, impact, craters, Mercury, Mars, terrestrial, silicate, rocky, exoplanet, meteor, Moon, meteorite, asteroid, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Desert planet
A desert planet, also known as a dry planet, an arid planet, or a dune planet, is a type of terrestrial planet with a surface consistency similar to Earth’s hot deserts. Mars is arguably the only present example of a desert planet.
A 2011 study suggested that not only are life-sustaining desert planets possible but that they might be more common than Earth-like planets. The study found that, when modeled, desert planets had a much larger habitable zone than ocean planets. The same study also speculated that Venus may have once been a habitable desert planet as recently as 1 billion years ago. It is also predicted that Earth will become a desert planet within a billion years due to the Sun’s increasing luminosity.
A study conducted in 2013 concluded that hot desert planets without runaway greenhouse effect can exist in 0.5 AU around Sun-like stars. In that study, it was concluded that a minimum humidity of 1% is needed to wash off carbon dioxide from the atmosphere, but too much water can act as a greenhouse gas itself. Higher atmospheric pressures increase the range in which the water can remain liquid.
The concept of desert planet has become a common setting in science fiction, appearing as early as the 1956 film Forbidden Planet and Frank Herbert’s 1965 novel Dune. The environment of the desert planet Arrakis (also known as Dune) in the Dune franchise drew inspiration from the Middle East, particularly the Arabian Peninsula and Persian Gulf, as well as Mexico. Dune in turn inspired the desert planets which prominently appear in the Star Wars franchise, including the planets Tatooine, Geonosis, and Jakku.
Keywords: desert planet, desert world, desert, dry, arid, dune, planet, world, exoplanet, terrestrial, Mars, habitable, hot, greenhouse, Arrakis, Arabia, Middle East, Tatooine, Geonosis, Jakku, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Disrupted planet
A disrupted planet is a planet or exoplanet or, perhaps on a somewhat smaller scale, a planetary-mass object, planetesimal, moon, exomoon or asteroid that has been disrupted or destroyed by a nearby or passing astronomical body or object such as a star.
Necroplanetology is the related study of such a process. Nonetheless, the result of such a disruption may be the production of excessive amounts of related gas, dust and debris, which may eventually surround the parent star in the form of a circumstellar disk or debris disk. As a consequence, the orbiting debris field may be an “uneven ring of dust”, causing erratic light fluctuations in the apparent luminosity of the parent star, as may have been responsible for the oddly flickering light curves associated with the starlight observed from certain variable stars, such as that from Tabby’s Star (KIC 8462852), RZ Piscium and WD 1145+017. Excessive amounts of infrared radiation may be detected from such stars, suggestive evidence in itself that dust and debris may be orbiting the stars.
Examples of planets, or their related remnants, considered to have been a disrupted planet, or part of such a planet, include: ‘Oumuamua and WD 1145+017 b, as well as asteroids, hot Jupiters and those that are hypothetical planets, like Fifth planet, Phaeton, Planet V and Their.
Examples of parent stars considered to have disrupted a planet include: EPIC 204278916, Tabby’s Star (KIC 8462852), PDS 110, RZ Piscium, WD 1145+017 and 47 Ursae Majoris.
Keywords: disrupted planet, disrupted, planet, world, exoplanet, atmosphere, destroyed, necroplanetology, debris, remnant, vaporized, dust, vanish, zap, sublimate, disintegrate, melt, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that they may be geologically active bodies, an expectation that was borne out in 2015 by the Dawn mission to Ceres and the New Horizons mission to Pluto.
Astronomers are in general agreement that at least the nine largest candidates are dwarf planets: Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. Of these nine plus the tenth-largest candidate Salacia, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Only one, Sedna, has neither been visited nor has any known moons, making an accurate estimate of mass difficult.
There has been some debate as to whether the Pluto–Charon system should be considered a double dwarf planet. The IAU currently says Charon is not considered a dwarf planet but rather a satellite of Pluto, though the idea that Charon might qualify as a dwarf planet may be considered at a later date.
Keywords: dwarf planet, small, planet, orbit, Kuiper belt, hydrostatic equilibrium, orbital dominance, asteroid, tholins, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, Orcus, Salacia
——————
Earth analog planet
An Earth analog, also called an Earth analogue, Earth twin, or second Earth, is a planet or moon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer to any terrestrial planet.
The possibility is of particular interest to astrobiologists and astronomers under reasoning that the more similar a planet is to Earth, the more likely it is to be capable of sustaining complex extraterrestrial life. As such, it has long been speculated and the subject expressed in science, philosophy, science fiction and popular culture. Advocates of space colonization and space and survival have long sought an Earth analog for settlement. In the far future, humans might artificially produce an Earth analog by terraforming.
The mediocrity principle suggests that planets like Earth should be common in the Universe, while the Rare Earth hypothesis suggests that they are extremely rare. The thousands of exoplanetary star systems discovered so far are profoundly different from the Solar System, supporting the Rare Earth Hypothesis.
Astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarf stars within the Milky Way Galaxy. The nearest such planet could be expected to be within 12 light-years of the Earth, statistically. In September 2020, astronomers identified 24 superhabitable planets (planets better than Earth) contenders, from among more than 4000 confirmed exoplanets, based on astrophysical parameters, as well as the natural history of known life forms on the Earth.
On 11 January 2023, NASA scientists reported the detection of LHS 475 b, an Earth-like exoplanet – and the first exoplanet discovered by the James Webb Space Telescope.
Keywords: Earth, planet, world, Earth analog, Earth-like, twin, analogue, second, terrestrial, habitable, zone, liquid water, ocean, tectonic, exoplanet, atmosphere, complex, extraterrestrial, life, rare, Kepler, seti, galaxy, super habitable, near, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, LHS 475
——————
Earth-like planet
The planets considered Earth-like are rocky on their surface and about 0.5 to 1.5 times the size of Earth. They also land in the “habitable zone” of their orbiting stars, meaning they fall in the sweet spot of orbital distance — not too close and not too far away — and therefore have the potential to be able to support liquid water on their surfaces. The ability to support water is huge, as it means the planet could have the potential to support life.
These planets are common in the universe. One out of every two sun-like stars you see in the night sky has a rocky, Earth-like planet in its habitable zone.
We now know these planets exist, but we have yet to see them. To know more about a planet’s Earth-like properties, such as whether it hosts water or oxygen, further specific detection methods are needed. Thanks to recent breakthroughs, the technology now exists to capture a direct image of an Earth-like planet outside our solar system.
Keywords: Earth, planet, world, Earth analog, Earth 2.0, Earth-like, twin, analogue, second, terrestrial, habitable, zone, liquid water, ocean, tectonic, exoplanet, atmosphere, complex, extraterrestrial, life, rare, Kepler, seti, galaxy, super habitable, near, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, LHS 475
——————
Ecumenopolis planet
Ecumenopolis is the hypothetical concept of a planetwide city.
The idea lies in that in the future, urban areas and megalopolises would eventually fuse, and there would be a single continuous worldwide city as a progression from the current urbanization, population growth, transport and human networks. This concept was already current in science fiction in 1942, with Trantor in Isaac Asimov’s Foundation series. When made public, Doxiadis’ idea of ecumenopolis seemed “close to science fiction”, but today is “surprisingly pertinent”, especially as a consequence of globalization and Europeanization.
In science fiction, the ecumenopolis has become a frequent topic and popularized in 1999 by the fictional city planet Coruscant in the Star Wars franchise. In Dune, the Harkonnens’ home world of Giedi Prime is a heavily polluted ecumenopolis infamous for its gladiator arenas.
Keywords: city planet, city world, city, urban, planet, world, exoplanet, metropolis, populated, metropolitan, colony, galactic, empire, megacity, megalopolis, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Trantor, Coruscant, Giedi Prime
——————
Ellipsoid planet
Ellipsoid Planet is a type of planet with an extraordinary oval shape, distinct from the typical spherical planetary bodies commonly observed. The first documented ellipsoid planet, named WASP-103b, was discovered by astronomers utilizing the European Space Agency’s CHEOPS space telescope. WASP-103b is an ultra-short-period “ultra-hot Jupiter” gas giant exoplanet that orbits perilously close to its F-type star in just 0.9 days, earning its moniker as a “star-hugger.” This close proximity leads to significant tidal forces between the planet and its host star, causing the distinctive rugby ball-like deformation. The study of WASP-103b’s peculiar shape and internal composition may offer insights into its extreme environment and the effects of intense tidal heating from its nearby star.
As of now, the scientific community remains uncertain whether other physical processes beyond tidal forces induced by a nearby star could give rise to the formation of ellipsoid planets like WASP-103b.
Keywords: ellipsoid planet, world, planet, squashed, oval, ellipsoidal, ovoid, oblate, spheroid, tidal, forces, interaction, lock, round, exoplanet, WASP-103b, ultra-hot, Jupiter, star, atmosphere, planetary, science, space, astronomy, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. As of 1 August 2023, there are 5,484 confirmed exoplanets in 4,047 planetary systems, with 875 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and also much more about exoplanets, including composition, environmental conditions and potential for life.
There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an “Earth-sized” planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.
The least massive exoplanet known is Draugr, which is about twice the mass of the Moon. The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium), it is too massive to be a planet and might be a brown dwarf instead. Known orbital times for exoplanets vary from less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.
Almost all of the planets detected so far are within the Milky Way. However, there is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist. The nearest exoplanets are located 4.2 light-years from Earth and orbit Proxima Centauri, the closest star to the Sun.
The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star’s habitable zone (or sometimes called “goldilocks zone”), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.
Rogue planets are those that do not orbit any star. Such objects are considered a separate category of planets, especially if they are gas giants, often counted as sub-brown dwarfs The rogue planets in the Milky Way possibly number in the billions or more.
Keywords: exoplanet, exomoon, planet, world, habitable, zone, star, solar system, kepler, spectroscopy, transit, orbit, protoplanetary disk, binary, doppler, rogue planet, terrestrial, planetary, system, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Eyeball planet
An eyeball planet is a hypothetical type of tidally locked planet, for which tidal locking induces spatial features (for example in the geography or composition of the planet) resembling an eyeball. They are terrestrial planets where liquids may be present, in which tidal locking will induce a spatially dependent temperature gradient (the planet will be hotter on the side facing the star and colder on the other side). This temperature gradient may therefore limit the places in which liquid may exist on the surface of the planet to ring-or disk-shaped areas.
Such planets are further divided into “hot” and “cold” eyeball planets, depending on which side of the planet the liquid is present. A “hot” eyeball planet is usually closer to its host star, and the centre of the “eye”, facing the star (day side), is made of rock while liquid is present on the opposite side (night side). A “cold” eyeball planet, usually farther from the star, will have liquid on the side facing the host star while the rest of its surface is made of ice and rocks.
Because most planetary bodies have a natural tendency toward becoming tidally locked to their host body on a long enough timeline, it is thought that eyeball planets may be common and could host life, particularly in planetary systems orbiting red and brown dwarf stars which have lifespans much longer than other main sequence stars.
Kepler-1652b is potentially an eyeball planet. The TRAPPIST-1 system may contain several such planets.
Keywords: eyeball planet, eyeball world, planet, exoplanet, tidal, tidally, lock, locked, side, face, day, night, habitable, temperature, liquid, water, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Fluorine planet
Fluorine worlds are hypothetical planetary environments where fluorine-rich conditions prevail, potentially leading to unique geological and climatic characteristics. This concept is based on the understanding that fluorine, though relatively rare in the universe, is enriched in Earth’s crust and has interesting chemical properties that may have implications for habitability.
Fluorine is the 24th most abundant element in the universe, making up about 4 × 10−5% of all elements. On Earth, it is more abundant, ranking as the 13th most abundant element by weight percent in the crust (0.054%), just ahead of carbon. Fluorine shares a high affinity with common elements such as silicon, aluminum, calcium, and magnesium, which are also abundant in Earth’s crust and the universe. Consequently, fluorine is found bound within stable substances, especially outside liquid conditions.
While fluorine’s chemical properties make it an alternative substitute for oxygen, the latter is far more abundant both in the universe and Earth’s crust. Oxygen plays a central role in the chemistry of life on Earth, while fluorine, despite being present in the crust, appears to be largely ignored by the biosphere as a building block for life. This is likely due to the rarity of metabolically generated fluorinated compounds, as well as the low availability of fluorides in solution compared to other elements like oxygen and chlorine.
Fluorine gas is pale yellow or yellow-green in color when observed in large quantities. The yellow color is a result of the absorption and emission of specific wavelengths of light by the fluorine molecules. In gaseous form, fluorine exists as a diatomic molecule, where two fluorine atoms are bonded together. In its liquid state, it will not have the yellow-green color that is observed in its gaseous form. Instead, fluorine liquid would be a clear, transparent substance with no visible color.
No fluorine planets have yet been observed or discovered. The concept of “fluorine planets” is purely hypothetical and theoretical, based on scientific discussions about the possibility of life in fluorine-rich environments and the potential uses of fluorine in alternative biochemistry.
Since no fluorine planets have been observed or studied, it is not possible to determine their appearance or color as viewed from space. In reality, the color of a planet depends on various factors, including its composition, atmosphere, and the nature of its surface.
Planets in our solar system, for example, exhibit a wide range of colors due to different surface features, atmospheres, and chemical compositions.
Keywords: fluorine planet, fluorine world, fluorine, planet, world, exoplanet, yellow, green, fluorocarbon, chemistry, life, habitable, biology, biogenic, organic, compound, water, species, rainforest, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Forest planet
Forest planet is a theoretical class of terrestrial planet with most or nearly all of the surface covered with forests. Forest planets are most commonly referred in science fiction, such as Dagobah in Star Wars. There are currently no theoretical studies on the specific concept of forest planets.
Forest planets would provide habitats very suitable for complex life over much of the surface, resulting in life more diverse than there are on Earth. Around Sun-like stars, forest planets appear mostly green from space. Such planets are estimated to be rare in the universe and may comprise less than 0.1% of all planets in the Milky Way Galaxy. At present, the prospects of finding evidence of forests on exoplanets are slim.
The average height of trees, shrubs, and other organisms around the planet depend on the strength of planet’s gravity. Gravity affects how organisms grow and develop in certain ways. Because gravity acts as breaking mechanism for tree growth, shorter trees tend to be grown in higher gravity environments, while taller trees tend to be grown in lower gravity enviroments. The same mechanism also applies to animals; in addition, they’re stronger on higher gravity planets than in counterparts on Earth, and vice versa.
Forest planets tend to have atmospheres thicker than Earth’s but with similar proportions of gases, like nitrogen, oxygen, carbon dioxide, and methane. The combination between thicker atmosphere and greater amounts of greenhouse gases would mean heating be distributed more evenly around the planet. As a result, climate would be similar worldwide, allowing forests and wildlife to thrive over much of the planet’s surface.
Chlorophyll is a pigment causing plants to appear green on Earth. However, plants on other worlds don’t have to be green, depending on the color of parent stars. Planets orbiting around F, G, K, and M-type stars would be suitable for vegetation, but stars brighter than F would emit too much light for plants to absorb energy and thus would not survive. Around late K and M-type stars, plants would need to absorb nearly all of visible light wavelengths, resulting in gray to black plants, and corresponding forest planets would appear dark from space with geometric albedo less than 0.1. Around early to mid F-type stars, vegetations would need to absorb less energy and reflect more light, thus making plants light in color, and corresponding forest planets would appear bright from space with geometric albedo greater than 0.7. Around solar-type stars (from late F to mid G-type star), plants would be green like they are on Earth, but on planets orbiting late G to early K-type stars, vegetations would be brown.
Keywords: forest planet, forest world, planet, world, exoplanet, forest, green, trees, life, habitable, biology, wood, shrub, plant, pine, root, water, species, rainforest, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. Uranus and Neptune are a distinct class of giant planets, being composed mainly of heavier volatile substances (which are referred to as “ices”).
Jupiter and Saturn consist mostly of hydrogen and helium. They are thought to consist of an outer layer of compressed molecular hydrogen surrounding a layer of liquid metallic hydrogen, with probably a molten rocky core inside. The outermost portion of their hydrogen atmosphere contains many layers of visible clouds that are mostly composed of water (despite earlier certainty that there was no water anywhere else in the Solar System) and ammonia. The layer of metallic hydrogen located in the mid-interior makes up the bulk of every gas giant and is referred to as “metallic” because the very large atmospheric pressure turns hydrogen into an electrical conductor. The gas giants’ cores are thought to consist of heavier elements at such high temperatures (20,000 K [19,700 °C; 35,500 °F]) and pressures that their properties are not yet completely understood.
The defining differences between a very low-mass brown dwarf (which can have a mass as low as roughly 13 times that of Jupiter) and a gas giant are debated. One school of thought is based on formation; the other, on the physics of the interior. Part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history.
The term gas giant is, arguably, something of a misnomer because, throughout most of the volume of all giant planets, the pressure is so high that matter is not in gaseous form. Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases. The term has nevertheless caught on, because planetary scientists typically use “rock”, “gas”, and “ice” as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase the matter may appear in. In the outer Solar System, hydrogen and helium are referred to as “gases”; water, methane, and ammonia as “ices”; and silicates and metals as “rocks”.
Gas giants can be divided into five distinct classes according to their modeled physical atmospheric properties, and hence their appearance: ammonia clouds (I), water clouds (II), cloudless (III), alkali-metal clouds (IV), and silicate clouds (V). Jupiter and Saturn are both class I. Hot Jupiters are class IV or V.
Keywords: gas giant, gas planet, gas world, gas, giant, planet, exoplanet, Jovian, atmosphere, Jupiter, Saturn, hydrogen, helium, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Giant planet
The giant planets or Jovian planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. There are four known giant planets in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar giant planets have been identified orbiting other stars.
Subtypes of giant planets comprise gas giants, ice giants, massive solid planets, and super-puffs.
Because of the limited techniques currently available to detect exoplanets, many of those found to date have been of a size associated, in the Solar System, with giant planets. Many of the exoplanets are much closer to their parent stars and hence much hotter than the giant planets in the Solar System, making it possible that some of those planets are a type not observed in the Solar System. Considering the relative abundances of the elements in the universe (approximately 98% hydrogen and helium) it would be surprising to find a predominantly rocky planet more massive than Jupiter.
Keywords: giant planet, gas giant, jovian, ice giant, super-Earth, planet, world, exoplanet, extrasolar, Jupiter, Saturn, Uranus, Neptune, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Habitable planet
Habitable exoplanets and exomoons are thought to require orbiting at the right distance from the host star for liquid surface water to be present, in addition to various geophysical and geodynamical aspects, atmospheric density, radiation type and intensity, and the best star’s plasma environment. As of September 2022, 5,084 exoplanets have been confirmed, of which about 70 have a potentially habitable profile in terms of being in their circumstellar habitable zone and having a suitable mass and radius. JWST or a future space telescope could pick up a strong indication of possible life if it finds signs of an atmosphere like our own (oxygen, carbon dioxide, methane). Future telescopes might even pick up signs of photosynthesis or gases/molecules suggesting the presence of animal life. Intelligent, technological life might create atmospheric pollution, as it does on our planet, also detectable from afar
Keywords: habitable planet, habitable, planet, zone, world, exoplanet, life, extraterrestrial, alien, extremophiles, bacteria, inhabitable, livable, populated, earthlike, orbit, liquid, water, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Helium planet
A helium planet is a planet with a helium-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might form in a variety of ways.
There are several hypotheses for how a helium planet might form. A helium planet might form via hydrogen evaporation from a gaseous planet orbiting close to a star. The star will drive off lighter gases more effectively through evaporation than heavier gasses, and over time deplete the hydrogen, leaving a greater proportion of helium behind.
A scenario for forming helium planets from regular giant planets involves an ice giant, in an orbit so close to its host star that the hydrogen effectively boils out of the atmosphere, evaporating from and escaping the gravitational hold of the planet. The planet’s atmosphere will experience a large energy input and because light gases are more readily evaporated than heavier gases, the proportion of helium will steadily increase in the remaining atmosphere. Such a process will take some time to stabilize and completely drive out all the hydrogen, perhaps on the order of 10 billion years, depending on the precise physical conditions and the nature of the planet and the star. Hot Neptunes are candidates for such a scenario.
A helium-rich planetary object may also form from a low-mass white dwarf, which gets depleted of hydrogen via mass transfer in a close binary system with a second, massive object like a neutron star. One scenario involves an AM CVn type of symbiotic binary star composed of two helium-core white dwarfs surrounded by a circumbinary helium accretion disk formed during the mass transfer from the less massive to the more massive white dwarf. After it loses most of its mass, the less massive white dwarf may approach planetary mass.
Helium planets are expected to be distinguishable from regular hydrogen-dominated planets by strong evidence of carbon monoxide and carbon dioxide in the atmosphere. Due to hydrogen depletion, the expected methane in the atmosphere cannot form because there is no hydrogen for the carbon to combine with, hence carbon combines with oxygen instead, forming CO and CO2. Due to the atmospheric composition, helium planets are expected to be white or grey in appearance. Such a signature can be found in Gliese 436 b, which has a predominance of carbon monoxide and is hypothesized to be a helium planet.
Keywords: helium planet, helium world, helium, planet, world, exoplanet, gas, atmosphere, ice giant, carbon monoxide, core, white dwarf, carbon dioxide, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Gliese 436
——————
Hot eyeball planet
An eyeball planet is a hypothetical type of tidally locked planet, for which tidal locking induces spatial features (for example in the geography or composition of the planet) resembling an eyeball. They are terrestrial planets where liquids may be present, in which tidal locking will induce a spatially dependent temperature gradient (the planet will be hotter on the side facing the star and colder on the other side). This temperature gradient may therefore limit the places in which liquid may exist on the surface of the planet to ring or disk-shaped areas.
A “hot” eyeball planet is usually close to its host star, and the center of the “eye”, facing the star (dayside), is made of rock while liquid is present on the opposite side (night side).
Keywords: eyeball planet, eyeball world, eyeball, planet, world, exoplanet, hot eyeball planet, rock, hot, tidal lock, terrestrial, temperature, side, face, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Kepler-1652, TRAPPIST-1
——————
Hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (P < 10 days). The close proximity to their stars and high surface-atmosphere temperatures resulted in their informal name “hot Jupiters”.
Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method because the oscillations they induce in their parent stars’ motion are relatively large and rapid compared to those of other known types of planets. One of the best-known hot Jupiters is 51 Pegasi b.
Though there is diversity among hot Jupiters, they do share some common properties.
Their defining characteristics are their large masses and short orbital periods, spanning 0.36–11.8 Jupiter masses and 1.3–111 Earth days. The mass cannot be greater than approximately 13.6 Jupiter masses because then the pressure and temperature inside the planet would be high enough to cause deuterium fusion, and the planet would be a brown dwarf.
Most have nearly circular orbits (low eccentricities). It is thought that their orbits are circularized by perturbations from nearby stars or tidal forces. Whether they remain in these circular orbits for long periods of time or collide with their host stars depends on the coupling of their orbital and physical evolution, which are related through the dissipation of energy and tidal deformation.
Many have unusually low densities. The lowest one measured thus far is that of TrES-4b at 0.222 g/cm3. The large radii of hot Jupiters are not yet fully understood but it is thought that the expanded envelopes can be attributed to high stellar irradiation, high atmospheric opacities, possible internal energy sources, and orbits close enough to their stars for the outer layers of the planets to exceed their Roche limit and be pulled further outward.
Usually, they are tidally locked, with one side always facing its host star.
They are likely to have extreme and exotic atmospheres due to their short periods, relatively long days, and tidal locking.
Atmospheric dynamics models predict strong vertical stratification with intense winds and super-rotating equatorial jets driven by radiative forcing and the transfer of heat and momentum. Recent models also predict a variety of storms (vortices) that can mix their atmospheres and transport hot and cold regions of gas.
The day-night temperature difference at the photosphere is predicted to be substantial, approximately 500 K for a model based on HD 209458b.
They appear to be more common around F- and G-type stars and less so around K-type stars. Hot Jupiters around red dwarfs are very rare. Generalizations about the distribution of these planets must take into account the various observational biases, but in general, their prevalence decreases exponentially as a function of the absolute stellar magnitude.
There are three schools of thought regarding the possible origin of hot Jupiters. One possibility is that they were formed in situ at the distances at which they are currently observed. Another possibility is that they were formed at a distance but later migrated inward. Such a shift in position might occur due to interactions with gas and dust during the solar nebula phase. It might also occur as a result of a close encounter with another large object destabilizing a Jupiter’s orbit.
Keywords: hot Jupiter, hot, Jupiter, gas giant, gas, giant, hydrogen, temperature, short period, orbit, tidal lock, planet, world, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, 51 Pegasi
——————
Hot Neptune
A hot Neptune or Hoptune is a type of giant planet with a mass similar to that of Uranus or Neptune orbiting close to its star, normally within less than 1 AU. The first hot Neptune to be discovered with certainty was Gliese 436 b in 2007, an exoplanet about 33 light years away. Recent observations have revealed a larger potential population of hot Neptunes in the Milky Way than was previously thought. Hot Neptunes may have formed either in situ or ex situ.
Because of their close proximity to their parent stars, hot Neptunes have a much greater rate and chance of transiting their star as seen from a farther outlying point, than planets of the same mass in larger orbits. This increases the chances of discovering them by transit-based observation methods.
Transiting hot Neptunes include Gliese 436 b and HAT-P-11b. The exoplanet Dulcinea (or HD 160691 c) discovered in 2004 might also be a hot Neptune. Another may be Kepler-56b, which has a mass somewhat larger than Neptune’s and orbits its star at 0.1 AU, closer than Mercury orbits the Sun.
If these planets formed ex situ, i.e., by migrating to their current locations while growing, they may contain large quantities of frozen volatiles and amorphous ices. Otherwise, if they formed in situ, their inventory of heavy elements should be made entirely of refractory materials. Yet, regardless of the mode of formation, hot Neptunes should contain large fractions (by mass) of gases, primarily hydrogen and helium, which also account for most of their volume.
Keywords: hot, Neptune, Hoptune, hot Neptune, planet, gas giant, gas, giant, hydrogen, temperature, short period, orbit, tidal lock, exoplanet, atmosphere, , exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Gliese 436, HAT-P-11
——————
Hycean planet
A hycean planet is a hypothetical type of planet, described as a hot, water-covered planet with a hydrogen atmosphere. The presence of extraterrestrial liquid water makes Hycean planets promising candidates for planetary habitability. According to researchers, density data imply that both rocky Super-Earths and Sub-Neptunes can fit this type, and it is thus expected that they will be common exoplanets. Currently, there are no confirmed hycean planets, but the Kepler mission detected many candidates.
Hycean planets could be considerably larger than what habitable planets were previously thought to be, with radii reaching 2.6 R⊕ (2.3 R⊕) and masses of 10 M⊕ (5 M⊕). Moreover, the habitable zone of such planets could be considerably larger than that of Earth-like planets. The planetary equilibrium temperature can reach 500 K (227 °C; 440 °F) at late M-dwarfs.
There could also exist tidally locked ‘Dark Hycean’ planets (habitable only on the side of permanent night) or ‘Cold Hycean’ planets (with negligible irradiation). Hycean worlds could be soon investigated for biosignatures by terrestrial telescopes and space telescopes like the James Webb Space Telescope.
The term “Hycean planet” was coined in 2021 by a team of exoplanet researchers at the University of Cambridge, as a portmanteau of “hydrogen” and “ocean”, used to describe planets that are thought to have large oceans and hydrogen-rich atmospheres. Hycean planets are thought to be common around red dwarf stars and are considered to be a promising place to search for life beyond Earth.
Although the presence of water may help them be habitable planets, their habitability may be limited by a possible runaway greenhouse effect. Hydrogen reacts differently to starlight’s wavelengths than heavier gases like nitrogen and oxygen. If the planet orbits the star at one Astronomical unit (AU), the temperature would be so high that the oceans would boil and water would become vapor. Current calculations locate the habitable zone where water would remain liquid at 1.6 AU, if the atmospheric pressure is similar to Earth’s, or at 3.85 AU if it is the more likely tenfold to twentyfold pressure. All current Hycean planet candidates are located within the area where oceans would boil and are thus unlikely to have actual oceans of liquid water.
Hycean planets have hydrogen-rich atmospheres. The atmospheres on Hycean planets are thought to be made up of hydrogen, helium, and water vapor.
They are thought to be covered in oceans. The oceans on Hycean planets are thought to be much deeper than the oceans on Earth.
They are thought to be common around red dwarf stars. Red dwarf stars are the most common type of star in the Milky Way galaxy.
They are considered to be a promising place to search for life beyond Earth. Hycean planets have the ingredients necessary for life, including liquid water, energy, and organic molecules.
The discovery of Hycean planets may represent a new frontier in the search for life beyond Earth. These planets are thought to be very different from Earth, but they could still be home to forms of life. Astronomers plan to use telescopes like the James Webb Space Telescope to search for Hycean planets and to learn more about their potential for human habitability.
One such candidate planet is K2-18b, which orbits a faint star with a period of about 33 days. It could have liquid water, contains a considerably high amount of hydrogen gas in its atmosphere, and is far enough from its star. It clearly resides in its star’s habitable zone. It is discovered to contain water in its atmosphere. Such candidate planets can be studied for biomarkers.
Keywords: hycean planet, hycean world, planet, world, exoplanet, hydrogen, ocean, atmosphere, habitable, zone, extraterrestrial, alien, life, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Ice giant
An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.
In astrophysics and planetary science the term “ices” refers to volatile chemical compounds with freezing points above about 100 K, such as water, ammonia, or methane, with freezing points of 273 K (0°C), 195 K (−78°C), and 91 K (−182°C), respectively. In the 1990s, it was determined that Uranus and Neptune are a distinct class of giant planets, separate from the other giant planets, Jupiter and Saturn, which are gas giants predominantly composed of hydrogen and helium.
As such, Neptune and Uranus are now referred to as ice giants. Lacking well-defined solid surfaces, they are primarily composed of gases and liquids. Their constituent compounds were solids when they were primarily incorporated into the planets during their formation, either directly in the form of ice or trapped in water ice. Today, very little of the water in Uranus and Neptune remains in the form of ice. Instead, water primarily exists as a supercritical fluid at the temperatures and pressures within them.
The ice giants are primarily composed of heavier than hydrogen and helium elements. Based on the abundance of elements in the universe, oxygen, carbon, nitrogen, and sulfur are most likely. Although the ice giants have hydrogen envelopes, these are much smaller. They account for less than 20% of their mass. Their hydrogen also never reaches the depths necessary for the pressure to create metallic hydrogen. These envelopes nevertheless limit observation of the ice giants’ interiors, and thereby the information on their composition and evolution.
Although Uranus and Neptune are referred to as ice giant planets, it is thought that there is a supercritical water-ammonia ocean beneath their clouds, which accounts for about two-thirds of their total mass.
Keywords: ice giant, ice, planet, world, exoplanet, atmosphere, Uranus, Neptune, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Ice planet
An ice planet or icy planet is a type of planet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere.
Under a geophysical definition of a planet, the small icy worlds of the Solar System qualify as icy planets. These include most of the planetary-mass moons, such as Ganymede, Titan, Enceladus, and Triton; and also the known dwarf planets, such as Ceres, Pluto, and Eris. In June 2020, NASA scientists reported that it is likely that exoplanets with oceans, including some with oceans that may lie beneath a layer of surface ice, may be common in the Milky Way galaxy, based on mathematical modeling studies.
An ice planet’s surface can be composed of water, methane, ammonia, carbon dioxide (known as “dry ice”), carbon monoxide, nitrogen, and other volatiles, depending on its surface temperature. Ice planets would have surface temperatures below 260 K (−13 °C) if composed primarily of water, below 180 K (−93 °C) if primarily composed of CO2 and ammonia, and below 80 K (−193 °C) if composed primarily of methane.
On the surface, ice planets are hostile to life forms like those living on Earth because they are extremely cold. Many ice worlds likely have subsurface oceans, warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water would provide habitable conditions for life, including fish, plankton, and microorganisms. Subsurface plants as we know them could not exist because there is no sunlight to use for photosynthesis. Microorganisms can produce nutrients using specific chemicals (chemosynthesis) that may provide food and energy for other organisms. Some planets, if conditions are right, may have significant atmospheres and surface liquids like Saturn’s moon Titan, which could be habitable for exotic forms of life.
There are several extrasolar ice planet candidates, including OGLE-2005-BLG-390Lb, OGLE-2013-BLG-0341LB b and MOA-2007-BLG-192Lb.
Keywords: ice planet, icy world, snowball, planet, world, exoplanet, ice, icy, cryo, frozen, temperature, water, ammonia, methane, cracks, white, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Ganymede, Titan, Enceladus, Triton, Europa, Ceres, Pluto, Eris
——————
Iron planet
An iron planet is a type of planet that consists primarily of an iron-rich core with little or no mantle. Mercury is the largest celestial body of this type in the Solar System (as the other terrestrial planets are silicate planets), but larger iron-rich exoplanets may exist.
Iron is the sixth most abundant element in the universe by mass after hydrogen, helium, oxygen, carbon, and neon.
Iron-rich planets may be the remnants of normal metal/silicate rocky planets whose rocky mantles were stripped away by giant impacts. Some are thought to consist of diamond fields. Current planet formation models predict iron-rich planets will form in close-in orbits or orbit massive stars where the protoplanetary disk presumably consists of iron-rich material.
Iron-rich planets are smaller and denser than other types of planets of comparable mass. Such planets would have no plate tectonics or strong magnetic field as they cool rapidly after formation. These planets are not like Earth. Since water and iron are unstable over geological timescales, wet iron planets in the Goldilocks zone may be covered by lakes of iron carbonyl and other exotic volatiles rather than water.
In science fiction, such a planet has been called a “Cannonball”.
An extrasolar planet candidate that may be composed mainly of iron is Kepler-974b.
Keywords: iron planet, iron world, planet, core, mantle, terrestrial, silicates iron-rich, metal, diamond field, exoplanet, cannonball, dense, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Kepler-974, Mercury, carbonyl
——————
Jungle planet
A jungle planet is a theoretical class of terrestrial planet with most or nearly all of the surface covered with jungles.
Jungle planets would provide habitats very suitable for complex life over much of the surface, resulting in life more diverse than there is on Earth. Around Sun-like stars, jungle planets appear mostly green from space. Such planets are rare in the universe, estimated to comprise less than 0.1% of all planets in the Milky Way Galaxy.
Jungle planets may tend to have atmospheres thicker than Earth’s but with similar proportions of gases, like nitrogen, oxygen, carbon dioxide, and methane. The combination between a thicker atmosphere and greater amounts of greenhouse gases would mean heating be distributed more evenly around the planet. As a result, the climate would be similar worldwide, allowing life to thrive over much of the planet’s surface. Because of the mentioned atmospheric tendencies, three quarters of all planets with life are believed to be jungle planets.
Keywords: jungle planet, jungle world, planet, jungle, forest, swamp, plants, vegetation, life, biodiversity, exoplanet, green, purple, brown, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Lava planet
A lava planet is a type of terrestrial planet with a surface mostly or entirely covered by molten lava. Situations, where such planets could exist, include a young terrestrial planet just after its formation, a planet that has recently suffered a large collision event, or a planet orbiting very close to its star, causing intense irradiation and tidal forces.
Long-lasting lava planets would probably orbit extremely close to their parent star. In planets with eccentric orbits, the gravity from the nearby star would distort the planet periodically, with the resulting friction producing internal heat. This tidal heating could melt rocks into magma, which would then erupt through volcanoes. This would be similar to the Solar System moon Io, orbiting close to its parent Jupiter. Io is the most geologically active world in the Solar System, with hundreds of volcanic centers and extensive lava flows. Lava worlds orbiting extremely closely to the parent star may possibly have even more volcanic activity than Io, leading some astronomers to use the term super-Io. These “super-Io” exoplanets may resemble Io with extensive sulfur concentrated on their surfaces that are associated with continuous active volcanism.
However, tidal heating is not the only factor shaping a lava planet. In addition to tidal heating from orbiting close to their parent star, the intense stellar irradiation could melt the surface crust directly into lava. The entire star-facing surface of a tidally locked planet could be left covered in a lava ocean while the nightside may have lava lakes or even lava rain caused by the condensation of vaporized rock from the dayside. The mass of the planet would also be a factor. The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets than Earth expected to exhibit plate tectonics and thus more intense volcanic activity. Also, a Mega Earth may retain so much internal heat from its formation that a solid crust cannot form.
Protoplanets tend to have intense volcanic activity resulting from large amounts of internal heating just after formation, even relatively small planets that orbit far from their parent stars. Lava planets can also result from giant impacts; Earth was briefly a lava planet after being impacted by a Mars-sized body that formed the Moon.
Lava planets have low geometric albedos of around 0.1 and molten lava on the surface can cool and harden to form quenched glass.
There are no known lava worlds in the Solar System and the existence of extrasolar lava planets remains unknown. Several known exoplanets are likely lava worlds, given their small enough masses, sizes, and orbits. Likely lava exoplanets include COROT-7b, Kepler-10b, and Kepler-78b.
Keywords: lava planet, lava world, lava, magma, planet, exoplanet, terrestrial, volcó, melt, rock, tidal, tidally locked, heat, hot, Io, super-Io, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, mega-Earth, proto, Earth, protoplanet, impact, Theia, Moon, COROT-7b, Kepler-10b, Kepler-78b
——————
Mega-Earth
A mega-Earth is a proposed neologism for a massive terrestrial exoplanet that is at least ten times the mass of Earth. Mega-Earths would be substantially more massive than super-Earths (terrestrial and ocean planets with masses around 5–10 MEarth). The term “mega-Earth” was coined in 2014, when Kepler-10c was revealed to be a Neptune-mass planet with a density considerably greater than that of Earth, though it has since been determined to be a typical volatile-rich planet weighing just under half that mass.
K2-56b, also designated BD+20594b, is a much more likely mega-Earth, with about 16 Earth’s mass and 2.2 Earth’s radius. At the time of its discovery in 2016, it had the highest chance of being rocky for a planet its size, with a posterior probability that it is dense enough to be terrestrial at about 0.43. For comparison, at the time the corresponding probability for Kepler-10c was calculated as 0.1, and as 0.002 for Kepler-131b.
Kepler-145b is one of the most massive planets classified as mega-Earths, with a mass of 37.1 Earth’s mass and a radius of 2.65 Earth’s radius, so large that it could belong to a sub-category of mega-Earths known as supermassive terrestrial planets (SMTP). It likely has an Earth-like composition of rock and iron without any volatiles. A similar mega-Earth, K2-66b, has a mass of about 21.3 ME and a radius of about 2.49 REarth, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere.
Kepler-277b and Kepler-277c are a pair of planets orbiting the same star, both thought to be mega-Earths with masses of about 87.4 ME and 64.2 ME, and radii of about 2.92 RE and 3.36 RE, respectively.
Keywords: mega-Earth, mega, Earth, planet, world, terrestrial, exoplanet, mass, massive, supermassive, solid, iron, core, silicate, atmosphere, super-Earth, rocky, density, radius, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Kepler-10c, Neptune, K2-56b, BD+20594b, Kepler-131b, Kepler-145b, K2-66b, Kepler-277, Kepler-277b, Kepler-277c
——————
Mesoplanet
Mesoplanets are planetary-mass objects with sizes smaller than Mercury but larger than Ceres. The term was coined by Isaac Asimov. Assuming size is defined in relation to the equatorial radius, mesoplanets should be approximately 500 km to 2,500 km in radius.
Asimov noted that the Solar System has many planetary bodies and stated that lines dividing “major planets” from minor planets were necessarily arbitrary. Asimov then pointed out that there was a large gap in size between Mercury, the smallest planetary body that was considered to be undoubtedly a major planet, and Ceres, the largest planetary body that was considered to be undoubtedly a minor planet. Only one planetary body known at the time, Pluto, fell within the gap. Rather than arbitrarily decide whether Pluto belonged with the major planets or the minor planets, Asimov suggested that any planetary body that fell within the size gap between Mercury and Ceres be called a mesoplanet, because mesos means “middle” in Greek.
Today, the known objects that would be included by this definition are Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, probably Sedna, and perhaps Orcus. These eight, together with Ceres, are the objects astronomers generally agree are dwarf planets; other smaller bodies have been proposed, but astronomers disagree about their dwarf planethood.
Keywords: mesoplanet, mesoworld, meso, planet, world, small, body, dwarf, dwarf planet, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Ceres, Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus
——————
Methane planet
A methane planet is an assumed class of planet with its surface covered in lakes or oceans of methane with methane clouds in the atmosphere like it is on Titan, the largest moon of Saturn.
Viewed from space, a methane planet would appear blue to aqua green because methane absorbs red light and reflects blue and green light. Some methane clouds appear white because they contain phosphorus while others are orange because they contain tholins.
Methane planets tend to have similar climates to Earth’s, except it uses methane as a “variable gas” instead of water vapor as it is on Earth. For example, there is methane rain or methane snow instead of water rain or water snow. Those planets tend to be frigid, at around −290°F (−179°C). Their atmospheres tend to be composed mostly of nitrogen and oxygen with variable amounts of methane and trace amounts of nitric oxide and other gases.
Methane planets tend to be hazy like Titan, because stellar radiation break methane molecules apart and form different hydrocarbons, such as ethane (C2H6), diacetylene (C4H2), methylacetylene (C3H4), acetylene (C2H2) and propane (C3H8). This breakdown can also produce non-hydrocarbons such as carbon dioxide (CO2), carbon monoxide (CO), hydrogen cyanide (HCN), cyanogen ((CN)2), and cyanoacetylene (C3HN).
The life-bearing status on methane planets is fair. Life on methane planets should be able to adapt to extreme cold and use methane as a solvent, whereas life on Earth uses water as a solvent. Under a hazy atmosphere, plant life would rely on chemosynthesis as there is not enough light for photosynthesis because methane planets probably orbit far from their parent stars. It is predicted that plant-like life may use methane (CH4) and nitric oxide (NO) to produce methanol (CH3OH), nitrogen (N2), and oxygen (O2):
It is also predicted that animal-like life would take in oxygen and release nitric oxide instead of carbon dioxide. Animals would also eat foods rich in methanol and drink liquid methane. The main biogeochemical cycle on a methane planet is the methane cycle compared to the carbon cycle here on Earth.
However, life on some methane planets may not be carbon-based but silicon-based as they have better survivability to extreme cold than carbon-based life.
Keywords: methane planet, methane world, planet, world, exoplanet, lakes, ocean, clouds, climate, atmosphere, rain, snow, gas, Titan, life, solvent, water, hydrocarbons, ethane, diacetylene, carbon, dioxide, monoxide, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Mini-Jupiter
A mini-Jupiter planet is a small planet with a hydrogen/helium envelope. This is a type of celestial body that falls within a specific range of radius and mass, intermediate between that of Earth and Neptune. This intriguing class of exoplanets presents a distinct set of characteristics that distinguish them from rocky planets like Earth and gas giants like Jupiter. The classification of these planets has sparked ongoing debates within the scientific community, as they challenge conventional definitions of planetary compositions. As technology and observational methods have advanced, astronomers are gaining greater insights into the nature and potential diversity of these mini-Jupiter planets, shedding light on their formation, atmospheres, and significance in our understanding of planetary systems beyond our own.
Keywords: mini, Jupiter, mini-Jupiter, mini Jupiter, planet, world, Earth, Neptune, radio, mass, mini-Neptune, rocky, gas, giant, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, classification, type, sphere
——————
Mini-Neptune
A mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock, or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles).
A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having, as a result, a total radius between 1.7 and 3.9 Earth radii. The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than 1.7 RE and planets greater than 3.9 RE, namely ice giants and gas giants.
Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune. Without a thick atmosphere, it would be classified as an ocean planet instead. An estimated dividing line between a rocky planet and a gaseous planet is around 1.6–2.0 Earth radii. Planets with larger radii and measured masses are mostly low-density and require an extended atmosphere to simultaneously explain their masses and radii, and observations show that planets larger than approximately 1.6 Earth radius (and more massive than approximately 6 Earth masses) contain significant amounts of volatiles or H–He gas, likely acquired during formation. Such planets appear to have a diversity of compositions that is not well-explained by a single mass–radius relation as that found for denser, rocky planets.
The lower limit for mass can vary widely for different planets depending on their compositions; the dividing mass can vary from as low as one to as high as 20 ME. Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out. A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.
Neptune-like planets are considerably rarer than sub-Neptunes, despite being only slightly bigger. This “radius cliff” separates sub-Neptunes (radius < 3 Earth radii) from Neptunes (radius > 3 Earth radii). This is thought to arise because, during formation when gas is accreting, the atmospheres of planets of that size reach the pressures required to force the hydrogen into the magma ocean, stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas.
The smallest known extrasolar planet that might be a gas dwarf is Kepler-138d, which is less massive than Earth but has a 60% larger volume and therefore has a density of 2.1 g/cm3) that indicates either a substantial water content or possibly a thick gas envelope. However, more recent evidence suggests that it may be more dense than previously thought, and could be an ocean planet instead.
Keywords: mini-Neptune, mini, Neptune, gas dwarf, transitional, planet, world, planet, world, exoplanet, atmosphere, mass, radius, Earth, sub-Neptune, Neptune-like, ocean, hydrogen, helium, dense, gas, ice, giant, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Kepler-138d
——————
Mountain planet
A mountain planet is a theoretical class of planets with mountain ranges covering most or all of the surface.
The life-bearing status on mountain planets is fair. Some habitable mountain planets would feature water in river valleys, as well as in lakes and oceans.
There are no mountain planets in our solar system, but there are five speculated mountain exoplanets as of February 2014, including a pulsar planet and four planets detected by Kepler.
Keywords: mountain planet, mountain world, mountain, planet, world, exoplanet, elevation, hill, volcano, mount, mons, glacier, orogeny, river, valley, highland, plateau, mountain range, peak, mountainous, slope, sea level, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Ocean planet
An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalasso en, such as lava (the case of Io), ammonia (in a eutectic mixture with water, as is likely the case of Titan’s inner ocean) or hydrocarbons (like on Titan’s surface, which could be the most abundant kind of exo-sea). The study of extraterrestrial oceans is referred to as planetary oceanography.
Earth is the only astronomical object known to presently have bodies of liquid water on its surface, although several exoplanets have been found with the right conditions to support liquid water. There are also considerable amounts of subsurface water found on Earth, mostly in the form of aquifers. For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy. The characteristics of ocean worlds provide clues to their history and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to originate and host life.
In June 2020, NASA scientists reported that it is likely that exoplanets with oceans are common in the Milky Way galaxy, based on mathematical modeling studies.
Ocean worlds are of extreme interest to astrobiologists for their potential to develop life and sustain biological activity over geological timescales. Major moons and dwarf planets in the Solar System thought to harbor subsurface oceans are of substantial interest because they can realistically be reached and studied by space probes, in contrast to exoplanets, which are tens if not hundreds or thousands of light-years away, far beyond the reach of current human technology. The best-established water worlds in the Solar System, other than the Earth, are Callisto, Enceladus, Europa, Ganymede, and Titan. Europa and Enceladus are considered among the most compelling targets for exploration due to their comparatively thin outer crusts and observations of cryovolcanism.
A host of other bodies in the Solar System are considered candidates to host subsurface oceans based upon a single type of observation or by theoretical modeling, including Ariel, Titania, Umbriel, Ceres, Dione, Eris, Mimas, Miranda, Oberon, Pluto, and Triton.
Outside the Solar System, exoplanets that have been described as candidate ocean worlds include GJ 1214 b, Kepler-22b, Kepler-62e, Kepler-62f, and the planets of Kepler-11 and TRAPPIST-1.
More recently, the exoplanets TOI-1452 b, Kepler-138c, and Kepler-138d have been found to have densities consistent with large fractions of their mass being composed of water. Additionally, models of the massive rocky planet LHS 1140 b suggest its surface may be covered in a deep ocean.
Although 70.8% of all Earth’s surface is covered in water, water accounts for only 0.05% of Earth’s mass. An extraterrestrial ocean could be so deep and dense that even at high temperatures the pressure would turn the water into ice. The immense pressures in the lower regions of such oceans could lead to the formation of a mantle of exotic forms of ice such as ice V. This ice would not necessarily be as cold as conventional ice. If the planet is close enough to its star that the water reaches its boiling point, the water will become supercritical and lack a well-defined surface. Even on cooler water-dominated planets, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing a very strong greenhouse effect. Such planets would have to be small enough not to be able to retain a thick envelope of hydrogen and helium or be close enough to their primary star to be stripped of these light elements. Otherwise, they would form a warmer version of an ice giant instead, like Uranus and Neptune.
Keywords: ocean world, ocean planet, panthalassic planet, maritime world, water world, aquaplanet, ocean, water, planet, world, exoplanet, hydrosphere, sea, exosea, exo-sea, surface, atmosphere, planetary, oceanography, Earth, Callisto, Enceladus, Europa, Ganymede, Titan, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Ariel, Titania, Umbriel, Ceres, Dione, Eris, Mimas, Miranda, Oberon, Pluto, Triton, GJ 1214 b, Kepler-22b, Kepler-62e, Kepler-62f, Kepler-11, TRAPPIST-1, TOI-1452 b, Kepler-138c, and Kepler-138d, LHS 1140 b
——————
Phosphorus planet
A phosphorus planet is a hypothetical class of planet with a surface covered in lakes or oceans of phosphoric acid with clouds made of phosphoric acid in the atmosphere. Kepler-55c is speculated to be a phosphorus planet.
Viewed from space, a phosphorus planet would appear white or reddish, just like phosphorus itself. Phosphorus planets’ main climates are precipitation and “white fog.” Unlike Earth, phosphorus planets would use phosphoric acid as a “variable gas” instead of water vapor as it is on Earth. For example, there is phosphoric acid rain or phosphoric acid snow instead of water rain or water snow. Those planets would tend to be hot, at around 340°F (171°C). Their atmospheres would tend to be composed mostly of phosphorus trioxide (P4O6) with variable amounts of phosphoric acid vapor (H3PO4) and trace amounts of phosphorus trichloride (PCl3), phosphine (PH3), carbon dioxide (CO2), and other gases.
The life-bearing status on phosphorus planets is fair, even though phosphorus is toxic to many forms of life on Earth. Some forms of life may require phosphorus in the form of compounds to thrive. It is predicted that during a photosynthesis-like process, phosphoric acid (H3PO4) and phosphine (PH3) may combine with carbon dioxide (CO2) to produce triphenylphosphate (OP(OC2H5)3), phosphorus trioxide (P4O6), and oxygen (O2).
It is also predicted that animal-like life may drink phosphoric acid and eat foods rich in organophosphates. Like terrestrial animals, they would inhale oxygen and exhale carbon dioxide on phosphorus planets. The main biogeochemical cycle on phosphorus planets would be the phosphorus cycle compared to the carbon cycle here on Earth.
Keywords: phosphorus planet, phosphorus world, phosphorus, planet, world, exoplanet, atmosphere, phosphoric, acid, vapor, phosphine, carbon, dioxide, gas, liquid, climate, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System about 4.6 billion years ago, they aid the study of its formation.
A widely accepted theory of planet formation, the so-called planetesimal hypothesis, the Chamberlin–Moulton planetesimal hypothesis, and that of Viktor Safronov, states that planets form from cosmic dust grains that collide and stick to form ever-larger bodies. Once a body reaches around a kilometer in size, its constituent grains can attract each other directly through mutual gravity, enormously aiding further growth into moon-sized protoplanets. Smaller bodies must instead rely on Brownian motion or turbulence to cause the collisions leading to sticking. The mechanics of collisions and mechanisms of sticking are intricate. Alternatively, planetesimals may form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk—or via the concentration and gravitational collapse of swarms of larger particles in streaming instabilities. Many planetesimals eventually break apart during violent collisions, as 4 Vesta and 90 Antiope may have, but a few of the largest ones may survive such encounters and grow into protoplanets and, later, planets.
It has been inferred that about 3.8 billion years ago, after a period known as the Late Heavy Bombardment, most of the planetesimals within the Solar System had either been ejected from the Solar System entirely, into distant eccentric orbits such as the Oort cloud or had collided with larger objects due to the regular gravitational nudges from the giant planets (particularly Jupiter and Neptune). A few planetesimals may have been captured as moons, such as Phobos and Deimos (the moons of Mars) and many of the small high-inclination moons of the giant planets.
Planetesimals that have survived to the current day are valuable to science because they contain information about the formation of the Solar System. Although their exteriors are subjected to intense solar radiation that can alter their chemistry, their interiors contain pristine material essentially untouched since the planetesimal was formed. This makes each planetesimal a ‘time capsule’, and their composition might reveal the conditions in the Solar Nebula from which our planetary system was formed. The most primitive planetesimals visited by spacecraft are the contact binary Arrokoth.
Keywords: planetesimal, protoplanet, planet, world, protoplanetary disk, debris disk, Chamberlin–Moultonexo, Viktor Safronov, dust, grains, blob, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Vesta, Antiope, Arrokoth, stain, splotch, blob, blot, spot, amorphous, shape, smudge, homunculus, goop, glob, gunk, spatter, rock
——————
Protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other’s orbits and collide, gradually coalescing into the dominant planets.
It is thought that the collisions of planetesimals created a few hundred larger planetary embryos. Over the course of hundreds of millions of years, they collided with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first “generation” of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually, only a handful of embryos were left, which collided to complete the assembly of the planets properly.
Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones, their heavier elements sank to the center, whereas lighter elements rose to the surface. Such a process is known as planetary differentiation. The composition of some meteorites shows that differentiation took place in some asteroids.
In the case of the Solar System, it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar to Ceres and Pluto with masses of about 10^22 to 10^23 kg and were a few thousand kilometers in diameter.
According to the giant impact hypothesis the Moon formed from a colossal impact of a hypothetical protoplanet called Theia with Earth, early in the Solar System’s history.
In the inner Solar System, the three protoplanets to survive more-or-less intact are the asteroids Ceres, Pallas, and Vesta. Psyche is likely the survivor of a violent hit-and-run with another object that stripped off the outer, rocky layers of a protoplanet. The asteroid Metis may also have a similar origin history to that of Psyche. The asteroid Lutetia also has characteristics that resemble a protoplanet. Kuiper-belt dwarf planets have also been referred to as protoplanets. Because iron meteorites have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in the asteroid belt that since have been disrupted and that are the source of these meteorites.
In February 2013 astronomers made the first direct observation of a candidate protoplanet forming in a disk of gas and dust around a distant star, HD 100546. Subsequent observations suggest that several protoplanets may be present in the gas disk.
Another protoplanet, AB Aur b, may be in the earliest observed stage of formation for a gas giant. It is located in the gas disk of the star AB Aurigae. AB Aur b is among the largest exoplanets identified, and has a distant orbit, three times as far as Neptune is from the Earth’s sun. Observations of AB Aur b may challenge conventional thinking about how planets are formed. It was viewed by the Subaru Telescope and the Hubble Space Telescope.
Rings, gaps, spirals, dust concentrations, and shadows in protoplanetary disks could be caused by protoplanets. These structures are not completely understood and are therefore not seen as proof of the presence of a protoplanet.
One new emerging way to study the effect of protoplanets on the disk is molecular line observations of protoplanetary disks in the form of gas velocity maps. HD 97048 b is the first protoplanet detected by disk kinematics in the form of a kink in the gas velocity map. Other disks like the ones around IM Lupi or HD 163296 show similar kinks in their gas velocity map. Another candidate exoplanet, called HD 169142 b, was first directly imaged in 2014. HD 169142 b additionally shows multiple lines of evidence to be a protoplanet.
Keywords: protoplanet, planetesimal, planet, world, asteroid, Ceres, Pallas, Vesta, Psyche, Pluto, Metis, Lutetia, melting, embryo, Solar System, asteroid belt, differentiated, metal, iron, meteorite, interior, core, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Moon, impact, Theia, Kuiper, belt, dwarf planet, HD 100546, AB Aur b, AB Aurigae, HD 97048 b, IM Lupi, HD 163296, HD 169142 b, HD 169142 b
——————
Puffy planet
Gas giants with a large radius and very low density are sometimes called “puffy planets” or “hot Saturns”, due to their density being similar to Saturn’s. Puffy planets orbit close to their stars so that the intense heat from the star combined with internal heating within the planet will help inflate the atmosphere. Six large-radius low-density planets have been detected by the transit method. In order of discovery, they are HAT-P-1b, CoRoT-1b, TrES-4, WASP-12b, WASP-17b, and Kepler-7b. Some hot Jupiters detected by the radial-velocity method may be puffy planets. Most of these planets are around or below Jupiter’s mass as more massive planets have stronger gravity keeping them at roughly Jupiter’s size. Indeed, hot Jupiters with masses below Jupiter, and temperatures above 1800 Kelvin, are so inflated and puffed out that they are all on unstable evolutionary paths which eventually lead to Roche-Lobe overflow and the evaporation and loss of the planet’s atmosphere.
Even when taking surface heating from the star into account, many transiting hot Jupiters have a larger radius than expected. This could be caused by the interaction between atmospheric winds and the planet’s magnetosphere creating an electric current through the planet that heats it up, causing it to expand. The hotter the planet, the greater the atmospheric ionization, and thus the greater the magnitude of the interaction and the larger the electric current, leading to more heating and expansion of the planet. This theory matches the observation that planetary temperature is correlated with inflated planetary radii.
Keywords: puffy planet, puff planet, puff, puffy, hot Saturn, planet, world, Jupiter, Saturn, gas, giant, gas giant, radius, density, inflated, puffed, Roche-Lobe, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Ringed planet
A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets and is a common component of satellite systems around giant planets like Saturn. A ring system around a planet is also known as a planetary ring system.
The most prominent and most famous planetary rings in the Solar System are those around Saturn, but the other three giant planets (Jupiter, Uranus, and Neptune) also have ring systems. There are also dust rings around the Sun at the distances of Mercury, Venus, and Earth, in mean motion resonance with these planets. Recent evidence suggests that ring systems may also be found around other types of astronomical objects, including minor planets, moons, brown dwarfs, and other stars.
There are three ways that thicker planetary rings have been proposed to have formed: from the material of the protoplanetary disk that was within the Roche limit of the planet and thus could not coalesce to form moons, from the debris of a moon that was disrupted by a large impact, or from the debris of a moon that was disrupted by tidal stresses when it passed within the planet’s Roche limit. Most rings were thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it now appears that Saturn’s rings might be quite old, dating to the early days of the Solar System.
Fainter planetary rings can form as a result of meteoroid impacts with moons orbiting around the planet or, in the case of Saturn’s E-ring, the ejecta of cryovolcanic material.
The composition of ring particles varies; they may be silicate or icy dust. Larger rocks and boulders may also be present, and in 2007 tidal effects from eight ‘moonlets’ only a few hundred meters across were detected within Saturn’s rings. The maximum size of a ring particle is determined by the specific strength of the material it is made of, its density, and the tidal force at its altitude. The tidal force is proportional to the average density inside the radius of the ring, or to the mass of the planet divided by the radius of the ring cubed. It is also inversely proportional to the square of the orbital period of the ring.
Sometimes rings will have “shepherd” moons, small moons that orbit near the inner or outer edges of rings or within gaps in the rings. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon’s orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself.
It is also predicted that Phobos, a moon of Mars, will break up and form a planetary ring in about 50 million years. Its low orbit, with an orbital period that is shorter than a Martian day, is decaying due to tidal deceleration.
Because all giant planets of the Solar System have rings, the existence of exoplanets with rings is plausible. Although particles of ice, the material that is predominant in the rings of Saturn, can only exist around planets beyond the frost line, within this line rings consisting of rocky material can be stable in the long term. Such ring systems can be detected for planets observed by the transit method by additional reduction of the light of the central star if their opacity is sufficient. As of 2020, one candidate extrasolar ring system has been found by this method, around HIP 41378 f.
Fomalhaut b was found to be large and unclearly defined when detected in 2008. This was hypothesized to either be due to a cloud of dust attracted from the dust disc of the star, or a possible ring system, though in 2020 Fomalhaut b itself was determined to very likely be an expanding debris cloud from a collision of asteroids rather than a planet. Similarly, Proxima Centauri c has been observed to be far brighter than expected for its low mass of 7 Earth masses, which may be attributed to a ring system of about 5 RJ.
A sequence of occultations of the star 1SWASP J140747.93-394542.6 observed in 2007 over 56 days was interpreted as a transit of a ring system of a (not directly observed) substellar companion dubbed “J1407b”. This ring system has an attributed radius of about 90 million km (about 200 times that of Saturn’s rings). In press releases, the term “super Saturn” was used. However, the age of this stellar system is only about 16 million years, which suggests that this structure, if real, is more likely a circumplanetary disk rather than a stable ring system in an evolved planetary system. The ring was observed to have a 0.0267 AU-wide gap at a radial distance of 0.4 AU. Simulations suggest that this gap is more likely the result of an embedded moon than the resonance effects of an external moon.
Keywords: ring planet, ringed planet, ring world, ring, belt, disc, dust, rings, planet, world, Saturn, Jupiter, Uranus, Neptune, chunks, ice, asteroids, Roche limit, debris, silicate, icy, exoplanet, atmosphere, planetary, ring system, tidal, shepherd, moon, sscience, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Phobos, Mars, HIP 41378 f, Proxima Centauri c, super Saturn
——————
Rogue planet
A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound, or wandering planet) is an interstellar object of planetary mass that is not gravitationally bound to any star or brown dwarf. Rogue planets originate from planetary systems in which they are formed and later ejected. They can also form on their own, outside a planetary system. The Milky Way alone may have billions to trillions of rogue planets, a range the upcoming Nancy Grace Roman Space Telescope will likely be able to narrow down.
Some planetary-mass objects may have formed in a similar way to stars, and the International Astronomical Union has proposed that such objects be called sub-brown dwarfs. A possible example is Cha 110913−773444, which may have been ejected and become a rogue planet, or formed on its own to become a sub-brown dwarf.
Astronomers have used the Herschel Space Observatory and the Very Large Telescope to observe a very young free-floating planetary-mass object, OTS 44, and demonstrate that the processes characterizing the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. Herschel’s far-infrared observations have shown that OTS 44 is surrounded by a disk of at least 10 Earth masses and thus could eventually form a mini planetary system. Spectroscopic observations of OTS 44 with the SINFONI spectrograph at the Very Large Telescope have revealed that the disk is actively accreting matter, similar to the disks of young stars. In December 2013, a candidate exomoon of a rogue planet (MOA-2011-BLG-262) was announced.
In October 2020, OGLE-2016-BLG-1928, an Earth-mass rogue planet, was discovered.
Keywords: rogue planet, free-floating planet, FFP, interstellar, nomad, orphan, starless, unbound, wandering, interstellar, planet, object, planetary, system, mass, gravitational, exoplanet, ejected, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Cha 110913−773444, OTS 44, MOA-2011-BLG-262, OGLE-2016-BLG-1928
——————
Silicate clouds gas giant
For the very hottest gas giants, with temperatures above 1400 K (2100 °F, 1100 °C) or cooler planets with lower gravity than Jupiter, the silicate and iron cloud decks are predicted to lie high up in the atmosphere. These planets are class V and their predicted Bond albedo around a Sun-like star is 0.55, due to reflection by the cloud decks. At such temperatures, a gas giant may glow red from thermal radiation but the reflected light generally overwhelms thermal radiation. For stars of visual apparent magnitude under 4.50, such planets are theoretically visible to our instruments. Examples of such planets might include 51 Pegasi b and Upsilon Andromedae b. HAT-P-11b and those other extrasolar gas giants found by the Kepler telescope might be possible class V planets, such as Kepler-7b, HAT-P-7b, or Kepler-13b.
Keywords: silicate clouds planet, silicate, clouds, gas, giant, gas giant, planet, world, exoplanet, class V, iron, atmosphere, gas, remnant, rocky, metallic, core, terrestrial, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, 51 Pegasi, HAT-P-11b, Upsilon Andromedae, Kepler-7b, HAT-P-7b, Kepler-13b
——————
Silicate planet
Silicate Planets are the most common type of terrestrial planet, especially in our solar system. Earth, Mars, and Venus are all examples of silicate planets. Silicate planets are composed of a rocky silicon-based mantle with a frequent presence of a metallic core, usually composed of iron, allowing silicate planets to experience tectonic activity and have a magnetic field. Silicate planets have a density of between roughly 3 to 6 g/cm3, with the densities of planets generally getting lower further out in the star system.
The most basic type of silicate planet is a Rocky Planet, which is compositionally similar to the Moon.
Keywords: silicate planet, silicate world, silicate, planet, world, rocky, rock, silicon, mantle, metal, core, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Sub-brown dwarf
A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the limiting mass for thermonuclear fusion of deuterium (about 13 MJ). Some researchers call them rogue planets whereas others call them planetary-mass brown dwarfs. They are sometimes categorized as Y spectral class brown dwarfs.
Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion) but there is no consensus amongst astronomers on whether the formation process should be taken into account when classifying an object as a planet. Free-floating sub-brown dwarfs can be observationally indistinguishable from rogue planets, which originally formed around a star and were ejected from orbit. Similarly, a sub-brown dwarf formed free-floating in a star cluster may be captured into orbit around a star, making distinguishing sub-brown dwarfs and large planets also difficult. A definition for the term “sub-brown dwarf” was put forward by the IAU Working Group on Extra-Solar Planets (WGESP), which defined it as a free-floating body found in young star clusters below the lower mass cut-off of brown dwarfs.
The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 Jupiter mass (MJ). This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas.
There is no consensus on whether companions of stars and companions of brown dwarfs should be considered sub-brown dwarfs or planets.
Keywords: sub-brown, dwarf, sub, brown dwarf, planet, class Y, rogue planet, exoplanet, WD 0806-661 B, DT Virginis c, FW Tauri b, HD 106906 b, ROXs 42Bb, 2MASS J04414489+2301513, 2M1207b, WISE 0855–0714, S Ori 52, UGPS 0722-05, Cha 110913-773444, CFBDSIR 2149−0403, OTS 44, giant planet, hot Jupiter, red dwarf, substellar, object, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere
——————
Sub-Earth planet
A sub-Earth is a planet “substantially less massive” than Earth and Venus. In the Solar System, this category includes Mercury and Mars. Sub-Earth exoplanets are among the most difficult type to detect because their small sizes and masses produce the weakest signal. Despite the difficulty, one of the first exoplanets found was a sub-Earth around a millisecond pulsar PSR B1257+12. The smallest known is WD 1145+017 b with a size of 0.15 Earth radii, or somewhat smaller than Pluto. However, WD 1145+017 b is not massive enough to qualify as a sub-Earth classical planet and is instead defined as a minor, or dwarf, planet. It is orbiting within a thick cloud of dust and gas as chunks of itself continually break off to then spiral in towards the star, and within around 5,000 years it will have more-or-less disintegrated.
The Kepler space telescope opened up a new realm of sub-Earth discoveries. On January 10, 2012, Kepler discovered the first three sub-Earths around an ordinary star, Kepler-42. As of June 2014, Kepler has 45 confirmed planets that are smaller than Earth, with 17 of them being smaller than 0.8 Rⴲ. In addition, there are over 310 planet candidates with an estimated radius of <1 Rⴲ, with 135 of them being smaller than 0.8 Rⴲ.
There is suspected to be a sub-Earth orbiting Proxima Centauri, the closest star to the Sun. The mass of Proxima d is believed to be between that of Mars and Venus.
Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away their atmospheres. Due to their small sizes, and unless there are significant tidal forces when orbiting close to the parent star, sub-Earths also have short periods of geologic activity.
Keywords: sub-Earth planet, sub-Earth world, sub-Earth, planet, world, mass, Mercury, Mars, exoplanet, atmosphere, planetary, science, space, astronomy, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, PSR B1257+12, WD 1145+017 b, Kepler-42, Proxima Centauri, Proxima d
——————
Sub-Neptune
The term sub-Neptune can refer to a planet with a smaller radius than Neptune even though it may have a larger mass or to a planet with a smaller mass than Neptune even though it may have a larger radius like a super-puff and both meanings can even be used in the same publication.
Neptune-like planets are considerably rarer than sub-Neptune-sized planets, despite being only slightly bigger. This “radius cliff” separates sub-Neptunes (<3 RE) from Neptunes (>3 RE). This radius cliff is thought to arise because during formation when gas is accreting, the atmospheres of planets that size reach the pressures required to force the hydrogen into the magma ocean stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas.
Keywords: sub-Neptune, sub Neptune, Neptune, planet, world, radius, mass, radius cliff, exoplanet, gas, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, super-Earth, mini-Neptune, mega-Earth
——————
Subsurface ocean planet
Subsurface ocean planets and subsurface ocean worlds are celestial bodies that exhibit evidence of containing vast bodies of liquid water beneath their icy or rocky surfaces.
The existence of these hidden oceans has ignited significant interest and speculation in the realm of planetary science due to their potential implications for habitability and the search for extraterrestrial life. The concept is grounded in the fundamental role of water as a prerequisite for life, as evidenced by Earth’s own biodiversity that thrives in aquatic environments. While Earth remains the sole known planet with liquid water on its surface, emerging research suggests that sub-surface ocean worlds could hold the key to unlocking novel insights into the possibility of life beyond our home planet.
Known subsurface ocean worlds within our Solar System include Europa, Enceladus, Titan, Triton, Dione, Ganymede, Callisto, and potentially Pluto.
Keywords: subsurface ocean planet, subsurface ocean world, subsurface, ocean, planet, world, water, sea, exoplanet, alien, habitability, extraterrestrial, life, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, Europa, Enceladus, Titan, Triton, Dione, Ganymede, Callisto, Pluto
——————
Sulfur planet
A sulfur planet is a theoretical class of planet with a surface covered in lakes or oceans of sulfuric acid with sulfuric acid clouds in the atmosphere. Kepler-52c, Kepler-47b, Tau Ceti e, and KIC 5522786 b are speculated sulfur planets as of 2014. Venus has sulfuric acid clouds in the atmosphere, but Venus is not considered a sulfur planet because there is not quite enough sulfur on its surface. Instead, much of the surface is covered in solidified lava.
Viewed from space, a sulfur planet would appear yellow, just like the element sulfur itself. However, the color of clouds can be slightly different depending on their chemistry of condensation nuclei.
Sulfur planets may tend to have similar climates to Earth’s, except they use sulfuric acid as a “variable gas” instead of water vapor as it is on Earth. For example, there may be sulfuric acid rain or sulfuric acid snow instead of water rain or water snow. Those planets are predicted to be warmer than Earth, at around 150°F (66°C). Their atmospheres probably tend to be composed mostly of nitrogen and oxygen with variable amounts of sulfuric acid and trace amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide, and other gases.
The life-bearing status on sulfur planets is good. Life on sulfur planets is similar to Earth except they use sulfuric acid as a solvent, whereas life on Earth uses water as a solvent. Plant-like life on a sulfur planet may use photosynthesis using light from the parent star. It is predicted that plant-like life may use sulfuric acid and carbon dioxide to produce thioacetic acid, sulfur dioxide, and oxygen.
It is also predicted that animal-like life in this environment should inhale oxygen and exhale carbon dioxide like animals do on Earth. Animals may be able to drink sulfuric acid instead of water, and they may be able to eat sulfur-rich foods. The main biogeochemical cycle on sulfur planets is the sulfur cycle compared to the carbon cycle here on Earth.
Keywords: sulfur planet, sulfur world, sulfur, planet, world, exoplanet, atmosphere, sulfuric, acid, carbon, dioxide, hydrogen, sulfide, gas, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Super-Earth
A Super-Earth is a type of exoplanet with a mass higher than Earth’s, but substantially below those of the Solar System’s ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth’s, respectively. The term “super-Earth” refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term “gas dwarfs” may be more accurate for those at the higher end of the mass scale, although “mini-Neptunes” is a more common term.
In general, super-Earths are defined by their masses, and the term does not imply temperatures, compositions, orbital properties, habitability, or environments. While sources generally agree on an upper bound of 10 Earth masses (~69% of the mass of Uranus, which is the Solar System’s giant planet with the least mass), the lower bound varies from 1 or 1.9 to 5, with various other definitions appearing in the popular media. The term “super-Earth” is also used by astronomers to refer to planets bigger than Earth-like planets (from 0.8 to 1.2 Earth-radius), but smaller than mini-Neptunes (from 2 to 4 Earth-radii). This definition was made by the Kepler space telescope personnel. Some authors further suggest that the term Super-Earth might be limited to rocky planets without a significant atmosphere, or planets that have not just atmospheres but also solid surfaces or oceans with a sharp boundary between liquid and atmosphere, which the four giant planets in the Solar System do not have. Planets above 10 Earth masses are termed massive solid planets, mega-Earths, or gas giant planets, depending on whether they are mostly rock and ice or mostly gas.
Due to the larger mass of super-Earths, their physical characteristics may differ from Earth’s; theoretical models for super-Earths provide four possible main compositions according to their density: low-density super-Earths are inferred to be composed mainly of hydrogen and helium (mini-Neptunes); super-Earths of intermediate density are inferred to either have water as a major constituent (ocean planets), or have a denser core enshrouded with an extended gaseous envelope (gas dwarf or sub-Neptune). A super-Earth of high density is believed to be rocky and/or metallic, like Earth and the other terrestrial planets of the Solar System. A super-Earth’s interior could be undifferentiated, partially differentiated, or completely differentiated into layers of different compositions. A study on Gliese 876 d by a team around Diana Valencia revealed that it would be possible to infer from a radius measured by the transit method of detecting planets and the mass of the relevant planet what the structural composition is. For Gliese 876 d, calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet.
The limit between rocky planets and planets with a thick gaseous envelope is calculated with theoretical models. Calculating the effect of the active XUV saturation phase of G-type stars over the loss of the primitive nebula-captured hydrogen envelopes in extrasolar planets, it’s obtained that planets with a core mass of more than 1.5 Earth-mass (1.15 Earth-radius max.), most likely cannot get rid of their nebula captured hydrogen envelopes during their whole lifetime. Other calculations point out that the limit between envelope-free rocky super-Earths and sub-Neptunes is around 1.75 Earth-radii, as 2 Earth-radii would be the upper limit to be rocky (a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 2.0 GPa or 20,000 bar). Whether or not the primitive nebula-captured H/He envelope of a super-Earth is entirely lost after formation also depends on the orbital distance. For example, formation and evolution calculations of the Kepler-11 planetary system show that the two innermost planets Kepler-11b and c, whose calculated mass is ≈2 M and between ≈5 and 6 M respectively (which are within measurement errors), are extremely vulnerable to envelope loss. In particular, the complete removal of the primordial H/He envelope by energetic stellar photons appears almost inevitable in the case of Kepler-11b, regardless of its formation hypothesis.
If a super-Earth is detectable by both the radial velocity and the transit methods, then both its mass and its radius can be determined; thus its average bulk density can be calculated. The actual empirical observations are giving similar results as theoretical models, as it’s found that planets larger than approximately 1.6 Earth radius (more massive than approximately 6 Earth masses) contain significant fractions of volatiles or H/He gas (such planets appear to have a diversity of compositions that is not well-explained by a single mass-radius relation as that found in rocky planets). After measuring 65 super-Earths smaller than 4 Earth-radii, the empirical data points out that Gas Dwarves would be the most usual composition: there is a trend where planets with radii up to 1.5 Earth-radii increase in density with increasing radius, but above 1.5 radii the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles by volume overlying a rocky core. Another discovery about exoplanets’ composition is that about the gap or rarity observed for planets between 1.5 and 2.0 Earth-radii, which is explained by a bimodal formation of planets (rocky Super-Earths below 1.75 and sub-Neptunes with thick gas envelopes being above such radii).
Additional studies, conducted with lasers at the Lawrence Livermore National Laboratory and at the OMEGA laboratory at the University of Rochester show that the magnesium-silicate internal regions of the planet would undergo phase changes under the immense pressures and temperatures of a super-Earth planet and that the different phases of this liquid magnesium silicate would separate into layers.
Keywords: super-Earth, super, Earth, planet, world, terrestrial, mass, Uranus, Neptune, exoplanet, atmosphere, rocky, ocean world, mini-Neptune, gas dwarf, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, LHS 1140b, Gliese 876, Kepler-11
——————
Super-Jupiter
A super-Jupiter is a gas giant exoplanet that is more massive than the planet Jupiter. For example, companions at the planet–brown dwarf borderline have been called super-Jupiters, such as around the star Kappa Andromedae.
By 2011 there were 180 known super-Jupiters, some hot, some cold. Even though they are more massive than Jupiter, they remain about the same size as Jupiter up to 80 Jupiter masses. This means that their surface gravity and density go up proportionally to their mass. The increased mass compresses the planet due to gravity, thus keeping it from being larger. In comparison, planets somewhat lighter than Jupiter can be larger, so-called “puffy planets” (gas giants with a large diameter but low density).
CoRoT-3b, with a mass around 22 Jupiter masses, is predicted to have an average density of 26.4 g/cm3, greater than osmium (22.6 g/cm3), the densest natural element under standard conditions. Extreme compression of matter inside it causes the high density because it is likely composed mainly of hydrogen. The surface gravity is also high, over 50 times that of Earth.
In 2012, the super-Jupiter Kappa Andromedae b was imaged around the star Kappa Andromedae, orbiting it about 1.8 times the distance at which Neptune orbits the Sun.
Keywords: super-Jupiter, super Jupiter, super, Jupiter, planet, world, hot, cold, mass, massive, density, gravity, exoplanet, ice, gas, giant, super-Earth, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Kappa Andromedae, CoRoT-3b, Kepler-1704b
——————
Super-Neptune
A super-Neptune is a planet that is more massive than the planet Neptune. These planets are generally described as being around 5–7 times as large as Earth with estimated masses of 20–80 ME; beyond this, they are generally referred to as gas giants. A planet falling within this mass range may also be referred to as a sub-Saturn.
There have been relatively few discoveries of planets of this kind. The mass gap between Neptune-like and Jupiter-like planets is thought to exist because of “runaway accretion” occurring for protoplanets of more than 20 ME—once this mass threshold is crossed, they accumulate much additional mass (due to gravity increasing with mass and the presence of material in an accretion disk) and grow into planets the size of Jupiter or even larger.
Known examples include Kepler-101b, HAT-P-11b, and K2-33b.
Keywords: super-Neptune, super Neptune, super, Neptune, planet, world, sub-Saturn, gas, giant, ice, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Kepler-101b, HAT-P-11b, K2-33b
——————
Super-puff planet
A super-puff is a type of exoplanet with a mass only a few times larger than Earth’s but with a radius larger than that of Neptune, giving it a very low mean density. They are cooler and less massive than the inflated low-density hot-Jupiters.
The most extreme examples known are the three planets around Kepler-51 which are all Jupiter-sized but with densities below 0.1 g/cm3. These planets were discovered in 2012 but their low densities were not discovered until 2014. Another example is Kepler-87c.
One hypothesis is that a super-puff has continuous outflows of dust to the top of its atmosphere (for example, Gliese 3470 b)- so the apparent surface is really dust at the top of the atmosphere. Another possibility is that some of the super-puff planets are smaller planets with large ring systems, like HIP 41378 f.
Keywords: super-puff, super puff, super, puff, puffy, planet, world, mass, radius, density, low density, hot, Jupiter, dust, cloud, outflow, gas, giant, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Kepler-51, Kepler-87c, Gliese 3470 b, ring, system, HIP 41378 f
——————
Superhabitable planet
A superhabitable planet is a theoretical type of exoplanet or exomoon that could provide more suitable conditions for the emergence and evolution of life compared to Earth. The concept was introduced by René Heller and John Armstrong in 2014 as a response to the limitations of the habitable zone concept. The authors argue that a planet’s position within its host star’s habitable zone is insufficient to determine its potential habitability. They propose that factors such as a denser atmosphere, larger size, and different star types could contribute to creating environments more conducive to life.
Heller and Armstrong emphasize that the habitability of rocky planets extends beyond the stellar habitable zone. They suggest that tidal heating could make terrestrial or icy worlds habitable beyond this zone, as seen in the case of Jupiter’s moon Europa. The authors advocate for a biocentric approach to identifying habitable planets, using characteristics like stellar type, mass, and planetary system location.
According to their proposal, superhabitable planets would likely be larger, warmer, and older than Earth, orbiting stars like K-type main-sequence stars. These stars emit less ultraviolet radiation and are more stable on the main sequence, allowing for longer periods of potential life emergence and evolution.
Heller and Armstrong define certain characteristics for a planet to be considered superhabitable, including a larger size (around 2 Earth masses with 1.3 Earth radii), a denser atmosphere, and a higher concentration of oxygen and greenhouse gases, resulting in an average temperature of about 25°C (77°F). They suggest that such conditions could lead to more diverse flora and fauna, particularly in shallow ocean waters.
Star type plays a crucial role in superhabitability, with K-type and low-luminosity G-type stars being favorable due to their longer lifetimes and lower ultraviolet radiation emissions. The authors highlight the potential benefits of superhabitable planets having more complex terrains, larger liquid water surfaces, and shallower oceans, fostering diverse aquatic species.
Superhabitable planets may display differences in appearance compared to Earth, potentially due to their denser atmospheres and increased air density. Plant life might have distinct colors and cover more of the planet’s surface. A warmer, more stable climate with homogenous land distribution could promote extensive life growth.
Keywords: superhabitable, planet, world, habitable, habitability, exoplanet, Earth, Earth analog, Earth-like, life, alien, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Kepler-69c, Kepler-1126b
——————
Supermassive terrestrial planet
Supermassive terrestrial planets (SMTP), a subset within the category of mega-Earth exoplanets, are proposed neologisms describing massive terrestrial planets that possess a minimum mass of ten times that of Earth. These planets constitute a distinct group within the broader mega-Earth classification, which encompasses terrestrial exoplanets with masses ranging from approximately 5 to 10 Earth masses.
Notable examples of SMTPs include K2-56b, characterized by a mass of around 16 Earth masses and a radius of 2.2 times Earth’s, and Kepler-145b, one of the most massive planets within the mega-Earth classification, boasting a mass of 37.1 Earth masses and a radius of 2.65 times Earth’s. These supermassive terrestrial planets often exhibit Earth-like compositions with significant rock and iron components, distinguishing them from volatile-rich counterparts in the mega-Earth spectrum.
Keywords: supermassive terrestrial planet, SMTP, supermassive, terrestrial, category, mega-Earth, planet, world, radius, Earth-like, density, rock, iron, silicate, rocky, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, K2-56b, Kepler-145b
——————
Swamp planet
A swamp planet, also referred to as a swamp world, is a speculative planetary concept characterized by its predominant or complete coverage of bogs and swamps. While no confirmed instances of such planets have been observed as of current knowledge, the notion of swamp planets is rooted in astrobiological and geological considerations. These hypothetical worlds are imagined to possess extensive areas of standing water, shallow marshes, and waterlogged terrain, which could potentially lead to unique environmental conditions and the evolution of distinct ecosystems.
These hypothetical worlds have become a common setting in various science fiction franchises, offering a distinctive backdrop for storytelling and exploration of alien ecosystems, often portraying them as intriguing and mysterious realms teeming with life adapted to the wetland environment.
Keywords: swamp planet, swamp world, swamp, planet, world, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, bog, flood, wetland, marsh, everglades, inundate, deluge, swampland, wetlands, lake, pond, creek, lagoon
——————
Terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth’s Moon, Io, and sometimes Europa – may also be considered terrestrial planets; and so may be the rocky protoplanet-asteroids Pallas and Vesta. The terms “terrestrial planet” and “telluric planet” are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.
Terrestrial planets have a solid planetary surface, making them substantially different from the larger gaseous planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.
Most of the planets discovered outside the Solar System are giant planets because they are more easily detectable. But since 2005, hundreds of potentially terrestrial extrasolar planets have also been found, with several being confirmed as terrestrial. Most of these are super-Earths, i.e. planets with masses between Earth’s and Neptune’s; super-Earths may be gas planets or terrestrial, depending on their mass and other parameters.
It is likely that most known super-Earths are in fact gas planets similar to Neptune, as examination of the relationship between mass and radius of exoplanets (and thus density trends) shows a transition point at about two Earth masses. This suggests that this is the point at which significant gas envelopes accumulate. In particular, Earth and Venus may already be close to the largest possible size at which a planet can usually remain rocky. Exceptions to this are very close to their stars (and thus would have had their volatile atmospheres boiled away).
In 2005, the first planets orbiting a main-sequence star and which show signs of being terrestrial planets were found: Gliese 876 d and OGLE-2005-BLG-390Lb. From 2007 to 2010, three potential terrestrial planets were found orbiting within the Gliese 581 planetary system. Two of them, Gliese 581c and Gliese 581d, as well as a disputed planet, Gliese 581g, are massive super-Earths orbiting in or close to the habitable zone of the star, so they could potentially be habitable, with Earth-like temperatures.
Another possibly terrestrial planet, HD 85512 b, was discovered in 2011; it has at least 3.6 times the mass of Earth. The radius and composition of all these planets are unknown.
The first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler Mission, specifically designed to discover Earth-size planets around other stars using the transit method. In the same year, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including six that are “Earth-size” or “super-Earth-size” and in the habitable zone of their star. Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range.
Keywords: terrestrial planet, telluric, rocky, silicate, rocky, planet, world, planet, world, solid, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, Mercury, Venus, Earth, Mars, Earth’s Moon, Io, Europa, Pallas, Vesta, Gliese 876 d, OGLE-2005-BLG-390Lb, Gliese 581, HD 85512 b, Kepler-10b
——————
Toroidal planet
A toroidal planet is a hypothetical type of telluric exoplanet with a toroidal or doughnut shape. While no firm theoretical understanding as to how toroidal planets could form naturally is necessarily known, the shape itself is potentially quasi-stable and is analogous to the physical parameters of a speculatively constructible megastructure in self-suspension, such as a Dyson Ring, ringworld, Stanford torus, or Bishop Ring.
At sufficiently large enough scales, rigid matter such as the typical silicate-ferrous composition of rocky planets behaves fluidly and satisfies the condition for evaluating the mechanics of toroidal self-gravitating fluid bodies in context. A rotating mass in the form of a torus allows an effective balance between the gravitational attraction and the force due to centrifugal acceleration when the angular momentum is adequately large. Ring-shaped masses without a relatively massive central nucleus in equilibrium have been analyzed in the past by Henri Poincaré (1885), Frank W. Dyson (1892), and Sophie Kowalewsky (1885), wherein a condition is allowable for a toroidal rotating mass to be stable with respect to a displacement leading to another toroid. Dyson (1893) investigated other types of distortions and found that the rotating toroidal mass is secularly stable against “fluted” and “twisted” displacements but can become unstable against beaded displacements in which the torus is thicker in some meridians but thinner in some others. In the simple model of parallel sections, beaded instability commences when the aspect ratio of major to minor radius exceeds 3.
Wong (1974) found that toroidal fluid bodies are stable against axisymmetric perturbations for which the corresponding Maclaurin sequence is unstable, yet in the case of non-axisymmetric perturbation at any point on the sequence is unstable. Prior to this, Chandrasekhar (1965, 1967), and Bardeen (1971), had shown that a Maclaurin spheroid with an eccentricity ≥ 0.98523 is unstable against displacements leading to toroidal shapes and that this Newtonian instability is excited by the effects of general relativity. Eriguchi and Sugimoto (1981) improved on this result, and Ansorg, Kleinwachter & Meinel (2003) achieved near-machine accuracy, which allowed them to study bifurcation sequences in detail and correct erroneous results.
Since the existence of toroidal planets is strictly hypothetical, no empirical basis for protoplanetary formation has been established. One homolog is a synestia, a loosely connected doughnut-shaped mass of vaporized rock, proposed by Simon J. Lock and Sarah T. Stewart-Mukhopadhyay to have been responsible for the isotopic similarity in composition, particularly the difference in volatiles, of the Earth-Moon system that occurred during the early-stage process of formation, according to the leading giant-impact hypothesis. The computer modeling incorporated a smoothed particle hydrodynamics code for a series of overlapping constant-density spheroids to obtain the result of a transitional region with a corotating inner region connected to a disk-like outer region.
To date, no distinctly torus-shaped planet has ever been observed. Given how improbable their occurrence, it is extremely unlikely any will ever be observationally confirmed to exist even within our cosmological horizon.
Keywords: toroidal planet, toroidal world, toroidal, doughnut planet, doughnut, planet, doughnut-shaped, shape, world, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, Dyson Ring, ringworld, Stanford, torus, Bishop, ring, synestia
——————
Ultra-cool dwarf
An ultra-cool dwarf is a stellar or sub-stellar object of spectral class M that has an effective temperature lower than 2,700 K (2,430 °C; 4,400 °F). This category of dwarf stars was introduced in 1997 by J. Davy Kirkpatrick, Todd J. Henry, and Michael J. Irwin. It originally included very low mass M-dwarf stars with spectral types of M7 but was later expanded to encompass stars ranging from the coldest known to brown dwarfs as cool as spectral type T6.5. Altogether, ultra-cool dwarves represent about 15% of the astronomical objects in the stellar neighborhood of the Sun. One of the best known examples is TRAPPIST-1.
Models of the formation of planets suggest that due to their low masses and the small size of their proto-planetary disks, these stars could host a relatively abundant population of terrestrial Earth-like planets ranging from Mercury-sized to Earth-sized bodies, rather than a population of super-Earths and Jupiter-massed planets. The discovery of the TRAPPIST-1 planetary system, consisting of seven Earth-like planets, would appear to validate this accretion model.
Due to their slow hydrogen fusion, when compared to other types of low-mass stars the life spans of ultra-cool dwarves are estimated to be at least several hundred billion years, with the smallest among them living for about 12 trillion years. As the age of the universe is only 13.8 billion years, all ultra-cool dwarf stars are therefore relatively young. Models predict that at the ends of their lives the smallest of these stars will become blue dwarfs rather than expanding into red giants.
After the detection of bursts of radio emission from the M9 ultracool dwarf LP 944-20 in 2001, a number of astrophysicists began observation campaigns at the Arecibo Observatory and the Very Large Array to search for additional objects emitting radio waves. To date hundreds of ultra-cool dwarves have been observed with these radio telescopes and of these stars, more than a dozen radio-emitting ultra-cool dwarves have been identified. These surveys indicate that approximately 5-10% of ultracool dwarves emit radio waves. These observation campaigns identified the noteworthy 2MASS J10475385+2124234, which has a temperature of 800-900 K making it the coolest known radio-emitting brown dwarf. 2MASS J10475385+2124234 is a T6.5 brown dwarf that retains a magnetic field with a strength greater than 1.7 kG, making it some 3000 times more intense than Earth’s magnetic field.
Keywords: ultra-cool dwarf, ultra-cool, dwarf, ultra, cool, cold, planet, world, planet, world, M9, ultracool, LP 944-20, exoplanet, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, stellar, sub-stellar, star, red dwarf, brown dwarf, class M, temperature, dwarf star, hydrogen, fusion, low-mass, radio waves, 2MASS J10475385+2124234
——————
Ultra-hot Jupiter
Ultra-hot Jupiters are hot Jupiters with a dayside temperature greater than 2,200 K. In such dayside atmospheres, most molecules dissociate into their constituent atoms and circulate to the nightside where they recombine into molecules again.
One example is TOI-1431b, announced by the University of Southern Queensland in April 2021, which has an orbital period of just two and a half days. Its dayside temperature is 2,700 K (2,427 °C), making it hotter than 40% of stars in our galaxy. The nightside temperature is 2,600 K (2,300 °C).
Keywords: ultra-hot Jupiter, ultra, hot, Jupiter, dayside, temperature, nightside, tidally locked, gas, giant, planet, world, exoplanet, atmosphere, planetary, science, space, astronomy, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Ultra-hot Neptune
Ultra-hot Neptunes are hot Neptunes with very extreme high-temperature ranges.
LTT 9779 b is the first ultra-hot Neptune discovered with an orbital period of 19 hours and an atmospheric temperature of over 1700 degrees Celsius. Being so close to its star and with a mass around twice that of Neptune, its atmosphere should have evaporated into space so its existence requires an unusual explanation. A candidate planet around Vega slightly more massive than Neptune was detected in 2021. It orbits Vega, an A-class star, every 2.43 days, and with a temperature of about 2500 degrees Celsius would be the second-hottest planet on record if confirmed.
Keywords: ultra-hot Neptune, ultra, hot, Neptune, Jupiter, dayside, temperature, nightside, tidal, lock, gas, giant, planet, world, exoplanet, atmosphere, planetary, science, space, astronomy, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere
——————
Ultra-short period planet
An ultra-short period (USP) planet is a type of exoplanet with orbital period less than one day. At this short distance, tidal interactions lead to relatively rapid orbital and spin evolution. Therefore when there is a USP planet around a mature main-sequence star it is most likely that the planet has a circular orbit and is tidally locked. There are not many USP planets with sizes exceeding 2 Earth radii. About one out of 200 Sun-like stars (G dwarfs) has an ultra-short-period planet. There is a strong dependence of the occurrence rate on the mass of the host star. The occurrence rate falls from (1.1 ± 0.4)% for M dwarfs to (0.15 ± 0.05)% for F dwarfs. Mostly the USP planets seem consistent with an Earth-like composition of 70% rock and 30% iron, but K2-229b has a higher density suggesting a more massive iron core. WASP-47e and 55 Cnc e have a lower density and are compatible with pure rock, or a rocky-iron body surrounded by a layer of water (or other volatiles).
Studies of TOI-561b found that it is an USP planet with the lowest density (3.8 ± 0.5 g cm−3) as of April 2022. The low density of this planet is explained with a massive water layer, no H/He envelope, as well as a predicted water steam atmosphere. The steam atmosphere could be detected with JWST in the future. More complex models might be needed to fully explain the unusual properties of TOI-561b.
A difference between hot Jupiters and terrestrial USP planets is the proximity of planetary companions. Hot Jupiters are rarely found with other planets within a factor of 2–3 in orbital period or distance. In contrast, terrestrial USP planets almost always have longer-period planetary companions. The period ratio between adjacent planets tends to be larger if one of them is a USP planet suggesting the USP planet has undergone tidal orbital decay which may still be ongoing. USP planets also tend to have higher mutual inclinations with adjacent planets than for pairs of planets in wider orbits, suggesting that USP planets have experienced inclination excitation in addition to orbital decay.
There are several known giant planets with a period shorter than one day. Their occurrence must be lower by at least an order of magnitude than that of terrestrial USP planets.
It had been proposed that USP planets were the rocky cores of evaporated hot Jupiters, however the metallicity of the host stars of USP planets is lower than that of hot Jupiters’ stars so it seems more likely that USP planets are the cores of evaporated gas dwarfs.
A study by the TESS-Keck Survey using 17 USP planets found that USP planets predominantly have Earth-like compositions with an iron core mass of about 32% and have masses below runaway accretion. USPs are also almost always found in multiple-planet systems around stars with solar metallicity.
Keywords: ultra-short period planet, USP planet, ultra-short-period, ultra, short, period, planet, USP, world, terrestrial, rock, iron, core, WASP-47e, 55 Cnc e, exoplanet, orbit, day, spin, revolution, tidally locked, tidal, lock, atmosphere, planetary, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, class, type, sphere, K2-229b, TOI-561b, hot Jupiter
——————
Water clouds gas giant
Gaseous giants in class II are too warm to form ammonia clouds; instead, their clouds are made up of water vapor. These characteristics are expected for planets with temperatures below around 250 K (−23 °C; −10 °F). Water clouds are more reflective than ammonia clouds, and the predicted Bond albedo of a class II planet around a Sun-like star is 0.81. Even though the clouds on such a planet would be similar to those of Earth, the atmosphere would still consist mainly of hydrogen and hydrogen-rich molecules such as methane.
Examples of possible class II planets: HD 45364 b and HD 45364 c, HD 28185 b, Gliese 876 b, Upsilon Andromedae d, 55 Cancri f, 47 Ursae Majoris b, PH2b, Kepler-90 h, HD 10180 g.
Keywords: water clouds, clouds, water, vapor, water vapor, gas, giant, class II, Sudarsky, planet, white, blue, pale, bands, spot, system, warm, temperature, spectrum, science, space, astronomy, astrobiology, astrochemistry, cosmology, celestial body, astronomical, object, classification, type, sphere, HD 45364, HD 28185, Gliese 876, Upsilon Andromedae, 55 Cancri, 7 Ursae Majoris, PH2b, Kepler-90, HD 10180
text sources: en.wikipedia.org / exoplanets.nasa.gov / original
* CC BY SA: This webpage’s planet-type images are all free to use anywhere in the sizes and resolutions provided above. Please credit the author as: “planet concept by Pablo Carlos Budassi / @thecelestialzoo”
Download fully licensed Planet Types HD bundle > here <
* Share this page with a friend using the following link:
* Check out other graphics from our team at this website’s ROOTPAGE