Terrestrial planet

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Solar System's terrestrial planets[edit]

Relative masses of the terrestrial planets of the Solar System, including the Moon (designated here as "Luna")



Dwarf planets, such as Ceres and Pluto, and large small Solar System bodies are similar to terrestrial planets in the fact that they do have a solid surface, but are, on average, composed of more icy materials (Ceres and Pluto have densities 2.17 and 1.87 g cm−3, respectively, and Haumea's density is similar to Pallas's 2.8 g cm−3).

Density trends[edit]

The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. A greater uncompressed density indicates greater metal content. Uncompressed density differs from the true average density because compression within planet cores increases their density; the average density depends on planet size as well as composition.

Densities of the terrestrial planets
Object Density (g cm−3) Semi-major axis (AU)
Mean Uncompressed
Mercury 5.4 5.3 0.39
Venus 5.2 4.4 0.72
Earth 5.5 4.4 1.0
Mars 3.9 3.8 1.5

The density of terrestrial planets trends towards lower values as the distance from the Sun increases. The rocky minor planet Vesta orbiting outside of Mars is less dense than Mars still, at 3.4 g cm−3.

It is unknown whether extrasolar terrestrial planets in general will also follow this trend.

Extrasolar terrestrial planets[edit]

Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable.[2][3][4] But since 2005, hundreds of potentially terrestrial extrasolar planets have been found, with several being confirmed as terrestrial. Most of these are super-Earths, i.e. planets with masses between Earth's and Neptune's; super-Earths may be gas planets or terrestrial, depending on their mass and other parameters.



In 2005, the first planets around main-sequence stars that may be terrestrial were found: Gliese 876 d, has a mass 7 to 9 times that of Earth and an orbital period of just two Earth days. It orbits the red dwarf Gliese 876, 15 light years from Earth. OGLE-2005-BLG-390Lb, about 5.5 times the mass of Earth, 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 the red dwarf Gliese 581. The smallest, Gliese 581 e, is only about 1.9 Earth mass,[5] but orbits very close to the star. An ideal terrestrial planet would be 2 Earth masses with a 25-day orbital period around a red dwarf.[6] Two others, Gliese 581 c and Gliese 581 d, as well as a disputed planet, Gliese 581 g, 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.[7] The radius and composition of all these planets are unknown.


The first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler Mission, specifically designed to discover Earth-like planets around other stars using the transit method.[8]

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" (i.e. they have a radius less than 2 Earth radii)[9] and in the habitable zone.[10] Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range (see image).

List of terrestrial exoplanets[edit]





In 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth- and super-Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.[11][12][13] 11 billion of these estimated planets may be orbiting Sun-like stars.[14] The nearest such planet may be 12 light-years away, according to the scientists.[11][12] 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 (see KOI-314c).[15]


Artist's impression of a carbon planet

Several possible classifications for terrestrial planets have been proposed:[16]

Silicate planet
Iron planet
Coreless planet
Carbon planet (also called "diamond planet")

See also[edit]


  2. ^ Carole Haswell, Transiting Exoplanets
  3. ^ Michael Perryman, The Exoplanet Handbook
  4. ^ Sara Seager, Exoplanets
  5. ^ "Lightest exoplanet yet discovered". ESO (ESO 15/09 – Science Release). 21 April 2009. Retrieved 15 July 2009. 
  8. ^ http://www.bbc.com/news/science-environment-17454005
  10. ^ Borucki, William J.; Koch, David G; Basri, Gibor; Batalha, Natalie; Brown, Timothy M.; et al. (1 February 2011). "Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data" (PDF). arXiv. Retrieved 16 February 2011. 
  13. ^ Staff (January 7, 2013). "17 Billion Earth-Size Alien Planets Inhabit Milky Way". Space.com. Retrieved January 8, 2013. 
  15. ^ http://www.cfa.harvard.edu/news/2014-01
  16. ^ Naeye, Bob (24 September 2007). "Scientists Model a Cornucopia of Earth-sized Planets". NASA, Goddard Space Flight Center. Retrieved 23 October 2013. 

External links[edit]