PLANET TYPES

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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”.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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):

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.

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.

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.

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.

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.

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

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.

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.

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 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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

* Check out other graphics from our team at this website’s ROOTPAGE

Alkali metal clouds gas giant

Ammonia clouds gas giant

Ammonia planet

Barren planet

Brown dwarf

Carbon planet

Chlorine planet

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