All terrestrial planets in theSolar System have the same basic structure, such as a central metalliccore (mostlyiron) with a surrounding silicatemantle.
The large rocky asteroid4 Vesta has a similar structure; possibly so does the smaller one21 Lutetia.[4] Another rocky asteroid2 Pallas is about the same size as Vesta, but is significantly less dense; it appears to have never differentiated a core and a mantle. The Earth'sMoon and Jupiter's moonIo have similar structures to terrestrial planets, but Earth's Moon has a much smaller iron core. Another Jovian moonEuropa has a similar density but has a significant ice layer on the surface: for this reason, it is sometimes considered anicy planet instead.
Relative masses of the terrestrial planets of the Solar System, and the Moon (shown here as Luna)The inner planets (sizes to scale). From left to right: Earth, Mars, Venus and Mercury.
The Solar System has four terrestrial planets under the dynamical definition:Mercury,Venus,Earth andMars. The Earth's Moon as well as Jupiter's moons Io and Europa would also count geophysically, as well as perhaps the large protoplanet-asteroidsPallas andVesta (though those are borderline cases). Among these bodies, only the Earth has an active surfacehydrosphere. Europa is believed to have an active hydrosphere under its ice layer.
During the formation of the Solar System, there were many terrestrialplanetesimals andproto-planets, but most merged with or were ejected by the four terrestrial planets, leaving only Pallas and Vesta to survive more or less intact. These two were likely bothdwarf planets in the past, but have been battered out of equilibrium shapes by impacts. Some other protoplanets began to accrete and differentiate but suffered catastrophic collisions that left only a metallic or rocky core, like16 Psyche[4] or8 Flora respectively.[6] ManyS-type[6] andM-type asteroids may be such fragments.[7]
The other round bodies from theasteroid belt outward are geophysicallyicy planets. They are similar to terrestrial planets in that they have a solid surface, but are composed of ice and rock rather than of rock and metal. These include the dwarf planets, such asCeres,Pluto andEris, which are found today only in the regions beyond theformation snow line where water ice was stable under direct sunlight in the early Solar System. It also includes the other round moons, which are ice-rock (e.g.Ganymede,Callisto,Titan, andTriton) or even almost pure (at least 99%) ice (Tethys andIapetus). Some of these bodies are known to have subsurface hydrospheres (Ganymede, Callisto,Enceladus, and Titan), like Europa, and it is also possible for some others (e.g. Ceres,Mimas,Dione,Miranda,Ariel, Triton, and Pluto).[8][9] Titan even has surface bodies of liquid, albeit liquidmethane rather than water. Jupiter's Ganymede, though icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets.
The nameTerran world has been suggested to define all solid worlds (bodies assuming a rounded shape), without regard to their composition. It would thus include both terrestrial and icy planets.[10]
The uncompressed density of a terrestrial planet is the average density its materials would have at zeropressure. A greater uncompressed density indicates a greater metal content. Uncompressed density differs from the true average density (also often called "bulk" density) because compression within planet cores increases their density; the average density depends on planet size, temperature distribution, and material stiffness as well as composition.
Calculations to estimate uncompressed density inherently require a model of the planet's structure. Where there have been landers or multiple orbiting spacecraft, these models are constrained by seismological data and also moment of inertia data derived from the spacecraft's orbits. Where such data is not available, uncertainties are inevitably higher.[11]
The uncompressed densities of the rounded terrestrial bodies directly orbiting the Sun trend towards lower values as the distance from theSun increases, consistent with the temperature gradient that would have existed within the primordial solar nebula. The Galilean satellites show a similar trend going outwards from Jupiter; however, no such trend is observable for the icy satellites of Saturn or Uranus.[12] The icy worlds typically have densities less than 2 g·cm−3. Eris is significantly denser (2.43±0.05 g·cm−3), and may be mostly rocky with some surface ice, like Europa.[2] It is unknown whether extrasolar terrestrial planets in general will follow such a trend.
Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable.[14][15][16] But since 2005, hundreds of potentially terrestrial extrasolar planets have also been found, with several being confirmed as terrestrial. Most of these aresuper-Earths, i.e. planets with masses between Earth's and Neptune's; super-Earths may begas planets or terrestrial, depending on their mass and other parameters.
During the early 1990s, the first extrasolar planets were discovered orbiting thepulsarPSR B1257+12, with masses of 0.02, 4.3, and 3.9 times that of Earth, bypulsar timing.
When51 Pegasi b, the first planet found around a star still undergoingfusion, was discovered, many astronomers assumed it to be a gigantic terrestrial,[citation needed] because it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. It was later found to be a gas giant.
In 2005, the first planets orbiting a main-sequence star and which showed signs of being terrestrial planets were found:Gliese 876 d andOGLE-2005-BLG-390Lb. Gliese 876 d orbits the red dwarfGliese 876, 15light years from Earth, and has a mass seven to nine times that of Earth and an orbital period of just two Earth days. OGLE-2005-BLG-390Lb has about 5.5 times the mass of Earth and orbits a star about 21,000 light-years away in the constellation Scorpius.From 2007 to 2010, three (possibly four) potential terrestrial planets were found orbiting within theGliese 581 planetary system. The smallest,Gliese 581e, is only about 1.9 Earth masses,[17] but orbits very close to the star.[18] Two others,Gliese 581c and the disputedGliese 581d, are more-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.[19]The radius and composition of all these planets are unknown.
In 2016, statistical modeling of the relationship between a planet's mass and radius using a broken power law appeared to suggest that the transition point between rocky, terrestrial worlds and mini-Neptunes without a defined surface was in fact very close to Earth and Venus's, suggesting that rocky worlds much larger than our own are in fact quite rare.[10] This resulted in some advocating for the retirement of the term "super-earth" as being scientifically misleading.[23] Since 2016 the catalog of known exoplanets has increased significantly, and there have been several published refinements of the mass-radius model. As of 2024, the expected transition point between rocky and intermediate-mass planets sits at roughly 4.4 earth masses, and roughly 1.6 earth radii.[24]
In 2013, astronomers reported, based onKepler space mission data, that there could be as many as 40 billion Earth- and super-Earth-sizedplanets orbiting in thehabitable zones ofSun-like stars andred dwarfs within theMilky Way.[28][29][30] Eleven billion of these estimated planets may be orbiting Sun-like stars.[31] The nearest such planet may be 12 light-years away, according to the scientists.[28][29] However, this does not give estimates for the number of extrasolar terrestrial planets, because there are planets as small as Earth that have been shown to be gas planets (seeKepler-138d).[32]
Estimates show that about 80% of potentially habitable worlds are covered by land, and about 20% are ocean planets. Planets with ratios more like those of Earth, which was 30% land and 70% ocean, only make up 1% of these worlds.[33]
A theoretical class of planets, composed of a metal core surrounded by primarily carbon-based minerals. They may be considered a type of terrestrial planet if the metal content dominates. The Solar System contains no carbon planets but does havecarbonaceous asteroids, such as Ceres andHygiea. It is unknown if Ceres has a rocky or metallic core.[35]
A theoretical type of solid planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other solid planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass, and is sometimes called an iron planet,[36] though its surface is made of silicates and is iron-poor. Iron planets are thought to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron.
A type of solid planet with an icy surface of volatiles. In the Solar System, mostplanetary-mass moons (such as Titan, Triton, and Enceladus) and many dwarf planets (such as Pluto and Eris) have such a composition. Europa is sometimes considered an icy planet due to its surface ice, but its higher density indicates that its interior is mostly rocky. Such planets can have internal saltwater oceans andcryovolcanoes erupting liquid water (i.e. an internal hydrosphere, like Europa or Enceladus); they can have an atmosphere and hydrosphere made from methane or nitrogen (like Titan). A metallic core is possible, as exists on Ganymede.[2]
A theoretical type of solid planet that consists of silicate rock but has no metallic core, i.e. the opposite of an iron planet. Although the Solar System contains no coreless planets,chondrite asteroids and meteorites are common in the Solar System. Ceres and Pallas have mineral compositions similar to carbonaceous chondrites, though Pallas is significantly less hydrated.[37] Coreless planets are thought to form farther from the star where volatile oxidizing material is more common.
^Russell, David (2017). Geophysical Classification of Planets, Dwarf Planets, and Moons (Report).arXiv:1308.0616.
^abAsphaug, E.; Reufer, A. (2014). "Mercury and other iron-rich planetary bodies as relics of inefficient accretion".Nature Geoscience.7 (8):564–568.Bibcode:2014NatGe...7..564A.doi:10.1038/NGEO2189.
^abGaffey, Michael (1984). "Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites".Icarus.60 (1):83–114.Bibcode:1984Icar...60...83G.doi:10.1016/0019-1035(84)90140-4.
^Hardersen, Paul S.; Gaffey, Michael J. & Abell, Paul A. (2005). "Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroid".Icarus.175 (1): 141.Bibcode:2005Icar..175..141H.doi:10.1016/j.icarus.2004.10.017.
^Namely: KOI 326.01 [Rp=0.85], KOI 701.03 [Rp=1.73], KOI 268.01 [Rp=1.75], KOI 1026.01 [Rp=1.77], KOI 854.01 [Rp=1.91], KOI 70.03 [Rp=1.96] – Table 6). A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.Grant, Andrew (8 March 2011)."Exclusive: "Most Earth-Like" Exoplanet Gets Major Demotion—It Isn't Habitable".[blogs.discovermagazine.com/80beats 80beats].Discover Magazine. Archived fromthe original on 9 March 2011. Retrieved9 March 2011.
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