Terrestrial_planet - Wikiparency

# Terrestrial planet  **Microsim — three.js (Wikitube framework):** *Rock inside, gas and ice outside (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=composition&embed=1" data-title="Rock inside, gas and ice outside (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=composition`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: press l to show the labels and name the four rocky planets inside the frost line, Mercury, Venus, Earth and Mars, against the gas and ice giants beyond it; drag to a view from above to see how tightly the rocky group is packed, all within about 1.5 AU of the Sun; then press o to hide the orbits and compare the two groups by colour alone.* A **terrestrial planet**, also called a **rocky planet** or **telluric planet**, is a planet made mainly of silicate rock and metal, with a solid surface.[^lakdawalla2020] The name comes from the Latin words for Earth, *Terra* and *Tellus*. In the Solar System the terrestrial planets recognised by the International Astronomical Union are the four closest to the [[Sun]]: [[Mercury_(planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]].[^jhuapl2020] Under a geophysical definition of a planet, which counts any body massive enough to be rounded by its own gravity, the [[Moon]], [[Io_(moon)|Io]] and sometimes [[Europa_(moon)|Europa]] also qualify, and occasionally the large rocky asteroids [[4_Vesta|Vesta]] and Pallas.[^lakdawalla2020][^russell2017] Terrestrial planets differ sharply from the [[Gas_giant|gas giants]] and [[Ice_giant|ice giants]], which are made largely of [[Hydrogen|hydrogen]], [[Helium|helium]] and water in various states and have no solid surface at accessible depths. The explorer at the top of this page opens on the composition view, which shows the division directly: the rocky planets close to the Sun, inside the [[Frost_line_(astrophysics)|frost line]], and the giants outside it, with [[Jupiter]] and [[Saturn]] holding over 90% of the mass orbiting the Sun. This article covers the structure of rocky planets, the Solar System's examples and the trend in their densities, rocky planets around other stars, and proposed types of solid planets. ## Structure The terrestrial planets of the Solar System share a basic structure: a central core of metal, mostly [[Iron|iron]], surrounded by a mantle of silicate rock and topped by a thinner [[Crust_(geology)|crust]]. The layering is a result of differentiation: when a young planet is hot enough to melt, dense iron sinks to the centre and lighter silicates float above it. The large asteroid Vesta has the same structure, and the smaller asteroid 21 Lutetia may too.[^asphaug2014] Pallas, similar in size to Vesta, is much less dense and appears never to have separated a core from a mantle.[^marsset2020] The Moon and Io resemble the terrestrial planets inside, although the Moon's iron core is small; Europa has a similar density but a thick outer shell of ice, so it is sometimes classed as an icy world instead.[^lakdawalla2020] The size of the core varies widely. Mercury's metallic core holds an unusually large fraction of the planet, and the planet has been described as an iron planet, though its surface rocks are poor in iron.[^hauck2019] One explanation is that Mercury, and other iron-rich bodies, are the survivors of inefficient accretion in collisions that stripped away much of their rocky mantles.[^asphaug2014] Surfaces record what has acted on them. Terrestrial planets can carry impact craters, mountains, volcanoes and canyons, and their appearance depends on whether a liquid has eroded them and whether tectonic activity has renewed them. The atmospheres of the terrestrial planets are secondary: they were released from the interior by volcanic outgassing or delivered by impacts of comets and asteroids, after the planets formed. The giant planets, by contrast, keep primary atmospheres captured directly from the gas of the solar nebula.[^schombert2004] Small rocky planets could not hold such primary gas, because their weak gravity and warm surroundings let light hydrogen and helium escape. ## Terrestrial planets within the Solar System Four planets are terrestrial under the dynamical definition. Geophysically the Moon, Io and Europa count as well, with Vesta and Pallas as borderline cases.[^lakdawalla2020] Of all of these, only Earth has liquid water at its surface; Europa is thought to hide an ocean beneath its ice.[^hendrix2019] The present group is what remains of a much larger population. During the formation of the Solar System there were many rocky planetesimals and protoplanets, but most merged into the four terrestrial planets or were ejected, leaving Vesta and Pallas more or less intact; both may once have been rounded, before impacts battered them out of shape.[^marsset2020] Other protoplanets began to differentiate and were then broken apart, leaving metallic cores such as the asteroid 16 Psyche or rocky fragments such as 8 Flora; many S-type and M-type asteroids may be such pieces.[^asphaug2014][^gaffey1984][^hardersen2005] Rounded bodies farther from the Sun are mostly icy. [[Ceres_(dwarf_planet)|Ceres]], [[Pluto]] and [[Eris_(dwarf_planet)|Eris]], and most large moons of the giant planets, such as [[Ganymede_(moon)|Ganymede]], [[Callisto_(moon)|Callisto]], [[Titan_(moon)|Titan]] and [[Triton_(moon)|Triton]], have solid surfaces but are made of ice and rock rather than rock and metal. Several of them, including Ganymede, Callisto, [[Enceladus]] and Titan, are known or thought to have subsurface oceans, and a young ocean has recently been inferred inside Saturn's small moon Mimas.[^hendrix2019][^lainey2024] ### Density trends A planet's mean density mixes two effects: what it is made of and how hard its own weight squeezes it. The uncompressed density, the density its materials would have at zero pressure, isolates composition; a higher value means more metal. Estimating it requires a model of the interior, constrained where possible by seismic data and by the planet's moment of inertia measured from spacecraft orbits.[^caltech-notes] Among the rounded rocky bodies orbiting the Sun, uncompressed density falls with distance, as expected from a nebula that was hotter close in, where only metal-rich solids could condense; the Galilean moons show a similar outward trend from Jupiter.[^lewis2004] | Body | Mean density (g/cm³) | Uncompressed density (g/cm³) | Distance from Sun (AU) | |---|---|---|---| | Mercury | 5.43 | 5.3 | 0.39 | | Venus | 5.24 | 4.4 | 0.72 | | Earth | 5.51 | 4.4 | 1.00 | | Moon | 3.34 | 3.3 | 1.00 | | Mars | 3.93 | 3.8 | 1.52 | | Io | 3.53 | — | 5.20 | | Europa | 3.01 | — | 5.20 | Mean densities are from the NASA fact sheets; uncompressed values from Lewis and, for the Moon, Szurgot; distances are semi-major axes.[^nasa-fs][^nasa-jovian][^lewis2004][^szurgot2017][^jpl-t1] Earth is denser than Mercury on average, 5.51 against 5.43 g/cm³, but only because its greater mass compresses its interior more; with compression removed, Mercury, at 5.3 against Earth's 4.4, is the more metal-rich (derived from the table).[^lewis2004] Icy worlds typically have densities below 2 g/cm³; Eris, at about 2.43 g/cm³, may be mostly rock with a surface of ice.[^lakdawalla2020] ## Extrasolar terrestrial planets Most of the first exoplanets found were giants, because large planets are easier to detect by the Doppler and transit methods.[^haswell2010][^perryman2011] The very first confirmed exoplanets, however, were small: in 1992 pulsar timing revealed planets around the pulsar PSR B1257+12, later found to have masses of about 0.02, 4.3 and 3.9 times Earth's.[^wolszczan1992][^konacki2003] When 51 Pegasi b, the first planet found around a Sun-like star, turned up in 1995 only 0.052 AU from its star, it was a surprise, since no giant planet was expected so close to a star, but its minimum mass of about half Jupiter's marked it as a gas giant rather than an enormous rocky world.[^mayor1995] In 2005 the first candidates for rocky planets around main-sequence stars appeared. Gliese 876 d, orbiting a red dwarf about 15 light-years away, has a mass of about 7.5 Earth masses and a period of under two days; OGLE-2005-BLG-390Lb, found by gravitational microlensing, has about 5.5 Earth masses and orbits a star about 21,000 light-years away.[^rivera2005][^beaulieu2006] Gliese 581 e, announced in 2009, has a minimum mass of about 1.9 Earth masses, and HD 85512 b, announced in 2011, at least 3.6.[^mayor2009][^pepe2011] For all of these only the mass is known, not the radius, so their compositions were uncertain. Kepler-10b, reported in 2011, was the Kepler mission's first rocky planet, with both its mass and its radius measured.[^batalha2011] The same year Kepler released 1,235 planet candidates, some of them Earth-sized.[^borucki2011] Where "rocky" ends is now measured statistically. A 2016 analysis of the mass–radius relation found a break near Earth and Venus, above which planets mostly carry thick gas envelopes, suggesting that rocky planets much larger than Earth are rare.[^chen2017] With far more planets now catalogued, a 2024 revision puts the transition at roughly 4.4 Earth masses and 1.6 Earth radii.[^muller2024] Some planets of about Earth's mass have turned out to be gassy rather than rocky.[^cfa2014] In 2020 microlensing revealed a candidate rogue planet of roughly Earth's mass, OGLE-2016-BLG-1928L, drifting through the galaxy without a star.[^mroz2020] ### List of terrestrial exoplanets Planets with a measured density of at least 5 g/cm³ and a mass below Neptune's are very likely rocky. Examples found by the Kepler mission include Kepler-10b and Kepler-78b, the latter an Earth-sized planet circling its star in 8.5 hours with a density similar to Earth's.[^batalha2011][^howard2013] A density above 5 g/cm³ at such masses implies a large fraction of iron and rock, since even a modest envelope of hydrogen or a thick layer of water would pull the average density well down. ### Frequency In 2013 astronomers analysing Kepler data estimated that about 22% of Sun-like stars host an Earth-sized planet in their habitable zone.[^petigura2013] Scaled to the [[Milky_Way|Milky Way]], that implies tens of billions of such planets, perhaps 40 billion around Sun-like stars and red dwarfs together and 11 billion around Sun-like stars alone, and the nearest might be only about 12 light-years away.[^overbye2013][^khan2013] These are counts of Earth-sized planets in the [[Habitable_zone|habitable zone]], not of terrestrial planets, since planets of Earth's size can be gaseous.[^cfa2014] ## Types Several classes of solid planet have been proposed, differing in what they are made of.[^naeye2007] A **silicate planet**, like Venus, Earth or Mars, has a rocky silicate mantle around an iron core; all terrestrial planets of the Solar System are of this kind. An **iron planet** is a theoretical type made almost entirely of iron, and so denser and smaller than other solid planets of the same mass; such planets would form close to a star in an iron-rich disc. Mercury, whose metallic core makes up an unusually large share of its mass, is the Solar System's nearest example.[^hauck2019] A **coreless planet** is the opposite, silicate rock with no metallic core, expected to form farther from the star, where oxidising volatiles are common and iron stays bound in minerals; Ceres and Pallas have compositions like carbonaceous chondrites, though Pallas is much less hydrated.[^marsset2020] A **carbon planet** would have a metal core surrounded mainly by carbon-based minerals. The Solar System has none, although it contains carbonaceous asteroids such as Ceres.[^castillorogez2017] An **icy planet** has a surface of frozen volatiles; most planetary-mass moons, such as Titan, Triton and Enceladus, and many dwarf planets, such as Pluto and Eris, are icy, and some have subsurface oceans or cryovolcanoes. Ganymede, though icy, has a metallic core like the rocky bodies.[^lakdawalla2020][^hendrix2019] ## See also - [[Planetary_system]] - [[Habitable_zone]] - [[Formation_and_evolution_of_the_Solar_System]] - [[Gas_giant]] · [[Ice_giant]] - [[IAU_definition_of_planet]] - Earth analog · Chthonian planet · Planetary habitability ## References [^lakdawalla2020]: Lakdawalla, E. (21 April 2020). "What is a planet?". The Planetary Society. https://www.planetary.org/worlds/what-is-a-planet [^jhuapl2020]: Johns Hopkins University Applied Physics Laboratory (17 July 2020). "Types of planets". Interstellar Probe. https://interstellarprobe.jhuapl.edu/Resources/News-and-Gallery/#Gallery [^russell2017]: Russell, D. (2017). "Geophysical classification of planets, dwarf planets, and moons". arXiv:1308.0616. https://arxiv.org/abs/1308.0616 [^asphaug2014]: Asphaug, E.; Reufer, A. (2014). "Mercury and other iron-rich planetary bodies as relics of inefficient accretion". *Nature Geoscience* 7: 564–568. https://doi.org/10.1038/ngeo2189 [^marsset2020]: Marsset, M.; Brož, M.; Vernazza, P.; et al. (2020). "The violent collisional history of aqueously evolved (2) Pallas". *Nature Astronomy* 4: 569–576. https://doi.org/10.1038/s41550-019-1007-5 [^hauck2019]: Hauck, S. A.; Johnson, C. L. (2019). "Mercury: inside the iron planet". *Elements* 15: 21–26. https://doi.org/10.2138/gselements.15.1.21 [^schombert2004]: Schombert, J. (2004). "Lecture 14: Terrestrial planet atmospheres (primary atmospheres)". University of Oregon. http://abyss.uoregon.edu/~js/ast121/lectures/lec14.html [^hendrix2019]: Hendrix, A. R.; Hurford, T. A.; Barge, L. M.; et al. (2019). "The NASA Roadmap to Ocean Worlds". *Astrobiology* 19: 1–27. https://doi.org/10.1089/ast.2018.1955 [^gaffey1984]: Gaffey, M. J. (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: 83–114. https://doi.org/10.1016/0019-1035(84)90140-4 [^hardersen2005]: Hardersen, P. S.; Gaffey, M. J.; Abell, P. A. (2005). "Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids". *Icarus* 175: 141–158. https://doi.org/10.1016/j.icarus.2004.10.017 [^lainey2024]: Lainey, V.; Rambaux, N.; Tobie, G.; et al. (2024). "A recently formed ocean inside Saturn's moon Mimas". *Nature* 626: 280–282. https://doi.org/10.1038/s41586-023-06975-9 [^caltech-notes]: California Institute of Technology. "Mass–radius relationships in planetary formation", Ge 131 course notes. http://web.gps.caltech.edu/~mbrown/classes/ge131/notes/djs08.pdf [^lewis2004]: Lewis, J. S. (2004). *Physics and Chemistry of the Solar System* (2nd ed.). Academic Press, p. 265. ISBN 978-0-12-446744-6. [^nasa-fs]: NASA NSSDCA. "Planetary Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/ (fetched 2026-09-18). [^nasa-jovian]: NASA NSSDCA. "Jovian Satellite Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/joviansatfact.html (fetched 2026-09-18). [^szurgot2017]: Szurgot, M. (2017). "Uncompressed density of the Moon, lunar mantle and core". *Workshop on Modern Analytical Methods Applied to Earth and Planetary Sciences*, abstract 6007. https://www.hou.usra.edu/meetings/methods2017/pdf/6007.pdf [^jpl-t1]: JPL Solar System Dynamics. "Approximate positions of the planets", Table 1. https://ssd.jpl.nasa.gov/planets/approx_pos.html [^haswell2010]: Haswell, C. A. (2010). *Transiting Exoplanets*. Cambridge University Press. ISBN 978-0-521-13938-0. [^perryman2011]: Perryman, M. (2011). *The Exoplanet Handbook*. Cambridge University Press. ISBN 978-0-521-76559-6. [^wolszczan1992]: Wolszczan, A.; Frail, D. A. (1992). "A planetary system around the millisecond pulsar PSR1257+12". *Nature* 355: 145–147. https://doi.org/10.1038/355145a0 [^konacki2003]: Konacki, M.; Wolszczan, A. (2003). "Masses and orbital inclinations of planets in the PSR B1257+12 system". *The Astrophysical Journal* 591: L147–L150. https://doi.org/10.1086/377093 [^mayor1995]: Mayor, M.; Queloz, D. (1995). 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NASA/JPL presentation. http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183286.pdf ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Terrestrial_planet) : [Wikitube](https://en.wikitube.io/wiki/Terrestrial_planet) · pinned revision [1370790178](https://en.wikipedia.org/w/index.php?oldid=1370790178) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-038 · explorer state `?view=composition`.*