Editing Terrestrial planet (section)

==Terrestrial planets within the Solar System==
[[File:Masses of terrestrial planets.png|thumb|right|Relative masses of the terrestrial planets of the Solar System, and the Moon (shown here as Luna)]]
[[File:Telluric planets size comparison.jpg|thumb|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 (planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]]. The Earth's Moon as well as Jupiter's moons Io and Europa would also count geophysically, as well as perhaps the large protoplanet-asteroids [[2 Pallas|Pallas]] and [[4 Vesta|Vesta]] (though those are borderline cases). Among these bodies, only the Earth has an active surface [[hydrosphere]]. Europa is believed to have an active hydrosphere under its ice layer.

During the formation of the Solar System, there were many terrestrial [[planetesimal]]s and [[proto-planet]]s, 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 both [[dwarf planet]]s 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, like [[16 Psyche]]<ref name=Asphaug-Reufer-2014/> or [[8 Flora]] respectively.<ref name=Gaffey1984>{{cite journal
  |last=Gaffey |first=Michael
  |title=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
  |journal=Icarus
  |volume=60
  |issue=1 |pages=83–114 |date=1984
  |doi=10.1016/0019-1035(84)90140-4
  |bibcode=1984Icar...60...83G}}</ref> Many [[S-type asteroid|S-type]]<ref name=Gaffey1984/> and [[M-type asteroid|M-type]] asteroids may be such fragments.<ref name=Hardersen-Gaffey-Abell-2005>
{{cite journal
 |first1=Paul S.     |last1=Hardersen
 |first2=Michael J.  |last2=Gaffey
 |first3=Paul A.     |last3=Abell
 |name-list-style=amp
 |year=2005
 |title=Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroid
 |journal=Icarus
 |volume=175 |issue=1 |page=141
 |bibcode=2005Icar..175..141H  |doi=10.1016/j.icarus.2004.10.017
}}
</ref>

The other round bodies from the [[asteroid belt]] outward are geophysically ''icy 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 as [[Ceres (dwarf planet)|Ceres]], [[Pluto]] and [[Eris (dwarf planet)|Eris]], which are found today only in the regions beyond the [[Frost line (astrophysics)|formation 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 (moon)|Ganymede]], [[Callisto (moon)|Callisto]], [[Titan (moon)|Titan]], and [[Triton (moon)|Triton]]) or even almost pure (at least 99%) ice ([[Tethys (moon)|Tethys]] and [[Iapetus (moon)|Iapetus]]). 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 (moon)|Dione]], [[Miranda (moon)|Miranda]], [[Ariel (moon)|Ariel]], Triton, and Pluto).<ref name='OW Roadmap 2019'>{{cite journal | last1 = Hendrix | first1 = Amanda R. | last2 = Hurford | first2 = Terry A. | last3 = Barge | first3 = Laura M. | last4 = Bland | first4 = Michael T. | last5 = Bowman | first5 = Jeff S. | last6 = Brinckerhoff | first6 = William | last7 = Buratti | first7 = Bonnie J. | last8 = Cable | first8 = Morgan L. | last9 = Castillo-Rogez | first9 = Julie | last10 = Collins | first10 = Geoffrey C. | display-authors = etal | year = 2019| title = The NASA Roadmap to Ocean Worlds | journal = Astrobiology | volume =  19| issue = 1 | pages =  1–27| doi = 10.1089/ast.2018.1955 | pmid = 30346215 | pmc = 6338575 | bibcode = 2019AsBio..19....1H | doi-access = free }}</ref><ref>{{Cite journal |last1=Lainey |first1=V. |last2=Rambaux |first2=N. |last3=Tobie |first3=G. |last4=Cooper |first4=N. |last5=Zhang |first5=Q. |last6=Noyelles |first6=B. |last7=Baillié |first7=K. |date=February 7, 2024 |title=A recently formed ocean inside Saturn's moon Mimas |url=https://www.nature.com/articles/s41586-023-06975-9 |journal=Nature |language=en |volume=626 |issue=7998 |pages=280–282 |doi=10.1038/s41586-023-06975-9 |pmid=38326592 |bibcode=2024Natur.626..280L |s2cid=267546453 |issn=1476-4687}}</ref> Titan even has surface bodies of liquid, albeit liquid [[methane]] rather than water. Jupiter's Ganymede, though icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets.

The name ''Terran'' 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.<ref name=ChenKipping/>

===Density trends===
The uncompressed density of a terrestrial planet is the average density its materials would have at zero [[pressure]]. 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.<ref>{{cite web|url=http://web.gps.caltech.edu/~mbrown/classes/ge131/notes/djs08.pdf|title=Course materials on "mass-radius relationships" in planetary formation.|website=caltech.edu|access-date=2 May 2018|url-status=live|archive-url=https://web.archive.org/web/20171222105139/http://web.gps.caltech.edu/~mbrown/classes/ge131/notes/djs08.pdf|archive-date=22 December 2017}}</ref>

The uncompressed densities of the rounded terrestrial bodies directly orbiting the Sun trend towards lower values as the distance from the [[Sun]] 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.<ref>{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=265 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}}</ref> The icy worlds typically have densities less than 2 g·cm<sup>−3</sup>. Eris is significantly denser ({{val|2.43|0.05|u=g·cm<sup>−3</sup>}}), and may be mostly rocky with some surface ice, like Europa.<ref name=planetarysociety/> It is unknown whether extrasolar terrestrial planets in general will follow such a trend.

The data in the tables below are mostly taken from a [[list of gravitationally rounded objects of the Solar System]] and [[planetary-mass moon]]. All distances from the Sun are averages.

{{col-begin}}
{{col-break}}
{| class="wikitable" style="text-align: center;"
|+ Densities of the terrestrial geophysical planets<br />(including borderline Pallas and Vesta)
|-
! scope="col" rowspan="2" | Object
! scope="col" colspan="2" | Density (g·cm<sup>−3</sup>)
! scope="col" rowspan="2" | Distance from Sun ([[Astronomical unit|AU]])
|-
! scope="col" | Mean
! scope="col" | Uncompressed
|-
| style="text-align: left;" | [[Mercury (planet)|Mercury]]
| 5.4
| 5.3
| 0.39
|-
| style="text-align: left;" | [[Venus]]
| 5.2
| 4.4
| 0.72
|-
| style="text-align: left;" | [[Earth]]
| 5.5
| 4.4
| rowspan=2| 1.0
|-
| style="text-align: left;" | [[Moon]]
| 3.3
| 3.3<ref>{{cite conference |title=Uncompressed density of the Moon, lunar mantle and core |last=Szurgot |first=Marian |conference=Workshop on Modern Analytical Methods Applied to Earth, Budapest, Hungary |year=2017 |url=https://www.hou.usra.edu/meetings/methods2017/pdf/6007.pdf}}</ref>
|-
| style="text-align: left;" | [[Mars]]
| 3.9
| 3.8
| 1.52
|-
| style="text-align: left;" | [[4 Vesta|Vesta]]
| 3.5
| 3.5
| 2.36
|-
| style="text-align: left;" | [[2 Pallas|Pallas]]
| 2.9
| 2.9
| 2.77
|-
| style="text-align: left;" | [[Io (moon)|Io]]
| 3.5
| 3.5
| rowspan=2| 5.20
|-
| style="text-align: left;" | [[Europa (moon)|Europa]]
| 3.0
| 3.0
|}
{{col-break}}
{| class="wikitable" style="text-align: center;"
|+ Densities of some icy geophysical planets<br />(other [[Kuiper Belt Objects|KBO]] densities are poorly known)
|-
! scope="col" | Object
! scope="col" | Density (g·cm<sup>−3</sup>)
! scope="col" | Distance from Sun (AU)
|-
| style="text-align: left;" | [[Ceres (dwarf planet)|Ceres]]
| 2.2
| 2.77
|-
| style="text-align: left;" | [[10 Hygiea|Hygiea]]
| 2.1
| 3.14
|-
| style="text-align: left;" | [[Ganymede (moon)|Ganymede]]
| 1.9
| rowspan=2|5.20
|-
| style="text-align: left;" | [[Callisto (moon)|Callisto]]
| 1.8
|-
| style="text-align: left;" | [[Mimas (moon)|Mimas]]
| 1.2
| rowspan=7|9.54
|-
| style="text-align: left;" | [[Enceladus]]
| 1.6
|-
| style="text-align: left;" | [[Tethys (moon)|Tethys]]
| 1.0
|-
| style="text-align: left;" | [[Dione (moon)|Dione]]
| 1.5
|-
| style="text-align: left;" | [[Rhea (moon)|Rhea]]
| 1.2
|-
| style="text-align: left;" | [[Titan (moon)|Titan]]
| 1.9
|-
| style="text-align: left;" | [[Iapetus (moon)|Iapetus]]
| 1.1
|-
| style="text-align: left;" | [[Miranda (moon)|Miranda]]
| 1.2
| rowspan=5|19.2
|-
| style="text-align: left;" | [[Ariel (moon)|Ariel]]
| 1.6
|-
| style="text-align: left;" | [[Umbriel]]
| 1.5
|-
| style="text-align: left;" | [[Titania (moon)|Titania]]
| 1.7
|-
| style="text-align: left;" | [[Oberon (moon)|Oberon]]
| 1.6
|-
| style="text-align: left;" | [[Triton (moon)|Triton]]
| 2.1
| 30.1
|-
| style="text-align: left;" | [[Pluto]]
| 1.9
| rowspan=2|39.5
|-
| style="text-align: left;" | [[Charon (moon)|Charon]]
| 1.7
|-
| style="text-align: left;" | [[50000 Quaoar|Quaoar]]
| 1.7
| 43.7
|-
| style="text-align: left;" | [[Eris (dwarf planet)|Eris]]
| 2.4
| 67.9
|}
{{col-end}}