Terrestrial planet

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The terrestrial planets of the Solar System: Mercury, Venus, Earth and Mars, sized to scale

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, i.e. Mercury, Venus, Earth, and Mars. Among astronomers who use the geophysical definition of a planet, the Moon, Io and Europa may also be considered terrestrial planets.[1][2][3] 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. These planets are located between the Sun and the asteroid belt.

Terrestrial planets have a solid planetary surface, making them substantially different from the larger gaseous planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.

Structure

All terrestrial planets in the Solar System have the same basic structure, such as a central metallic core (mostly iron) with a surrounding silicate mantle. The Earth's Moon is similar, but has a much smaller iron core; other natural satellites, such as Io, Europa, and Titan, also have internal structures similar to that of terrestrial planets.

Terrestrial planets can have surface structures such as canyons, craters, mountains, volcanoes, and others, depending on the presence of an erosive liquid and / or tectonic activity.

Terrestrial planets have secondary atmospheres, generated by volcanic out-gassing or from comet impact debris. This contrasts with the outer, giant planets, whose atmospheres are primary; primary atmospheres were captured directly from the original solar nebula.[4]

Solar System's terrestrial planets

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: Mercury, Venus, Earth and Mars. Only one terrestrial planet, Earth, has an active hydrosphere.

During the formation of the Solar System, there were many terrestrial planetesimals and proto-planets, but most merged with or were ejected by the four terrestrial planets, leaving only a few such as 4 Vesta to survive.

Dwarf planets, such as Ceres, Pluto and Eris, 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. Some small Solar System bodies such as Vesta are quite rocky, or in the case of 16 Psyche even metallic like Mercury, while others such as 2 Pallas are icier.

Most planetary-mass moons are ice-rock or even primarily ice. The three exceptions are Earth's moon, which has a composition much like the Earth's mantle, Jupiter's Io, which is silicate and volcanic, and Jupiter's Europa, which is believed to have an active hydrosphere.

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 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.

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 1.52

The uncompressed density of terrestrial planets trends towards lower values as the distance from the Sun increases. For example, the rocky minor planet Vesta orbiting outside of Mars at 2.36 AU is less dense than Mars, at 3.5 g·cm−3, and icier Pallas, orbiting at 2.77 AU, is less dense still at 2.9 g·cm−3.

Earth's Moon has a density of 3.3 g·cm−3 and Jupiter's satellites Io and Europa are 3.5 and 3.0 g·cm−3; other large satellites are icier and typically have densities less than 2 g·cm−3.[5][6] The dwarf planets Ceres, Pluto and Eris have densities of 2.2, 1.9 and 2.5 g·cm−3, respectively. (At one point Ceres was sometimes distinguished as a 'terrestrial dwarf', vs Pluto as an 'ice dwarf', but the distinction is no longer tenable. It now appears that Ceres formed in the outer Solar System and is itself quite icy.)

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 orbits. Where such data is not available, uncertainties are inevitably higher.[7] It is unknown whether extrasolar terrestrial planets in general will show to follow this trend.

Extrasolar terrestrial planets

See also: Super-Earth, Mega-Earth, and List of nearest terrestrial exoplanet candidates

Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable.[8][9][10] But since 2005, hundreds of potentially terrestrial extrasolar planets have also 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.

During the early 1990s, the first extrasolar planets were discovered orbiting the pulsar PSR B1257+12, with masses of 0.02, 4.3, and 3.9 times that of Earth's, by pulsar timing.

When 51 Pegasi b, the first planet found around a star still undergoing fusion, 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 show signs of being terrestrial planets, were found: Gliese 876 d and OGLE-2005-BLG-390Lb. Gliese 876 d orbits the red dwarf Gliese 876, 15 light 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, 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 the Gliese 581 planetary system. The smallest, Gliese 581e, is only about 1.9 Earth masses,[11] but orbits very close to the star.[12] Two others, Gliese 581c and Gliese 581d, as well as a disputed planet, Gliese 581g, 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.[13] The radius and composition of all these planets are unknown.

Sizes of Kepler planet candidates based on 2,740 candidates orbiting 2,036 stars as of 4 November 2013 (NASA).

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.[14]

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)[15] and in the habitable zone of their star.[16] 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).

In September 2020, astronomers using microlensing techniques reported the detection, for the first time, of an earth-mass rogue planet (named OGLE-2016-BLG-1928) unbounded by any star, and free floating in the Milky Way galaxy.[17][18][19]

List of terrestrial exoplanets

See also: List of nearest terrestrial exoplanet candidates

The following exoplanets have a density of at least 5 g/cm3 and a mass below Neptune's and are thus very likely terrestrial:

Kepler-10b, Kepler-20b, Kepler-36b, Kepler-48d, Kepler 68c, Kepler-78b, Kepler-89b, Kepler-93b, Kepler-97b, Kepler-99b, Kepler-100b, Kepler-101c, Kepler-102b, Kepler-102d, Kepler-113b, Kepler-131b, Kepler-131c, Kepler-138c, Kepler-406b, Kepler-406c, Kepler-409b.

Frequency

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.[20][21][22] 11 billion of these estimated planets may be orbiting Sun-like stars.[23] The nearest such planet may be 12 light-years away, according to the scientists.[20][21] 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 Kepler-138d).[24]

Types

Further information: List of planet types
Artist's impression of a carbon planet

Several possible classifications for terrestrial planets have been proposed:[25]

Silicate planet
The standard type of terrestrial planet seen in the Solar System, made primarily of silicon-based rocky mantle with a metallic (iron) core.
Carbon planet (also called "diamond planet")
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 have carbonaceous asteroids.
Iron planet
A theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass. 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.
Coreless planet
A theoretical type of terrestrial 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. Coreless planets are thought to form farther from the star where volatile oxidizing material is more common.

See also

References

  1. ^ Types of Planets, The Johns Hopkins University Applied Physics Laboratory LLC, 2020-07-17
  2. ^ Emily Lakdawalla et al., What Is A Planet? The Planetary Society, 21 April 2020
  3. ^ David Russell (2017) Geophysical Classification of Planets, Dwarf Planets, and Moons, v.4
  4. ^ Schombert, James (2004). "Lecture 14 Terrestrial planet atmospheres (primary atmospheres)". Department of Physics. Astronomy 121 Lecture Notes. University of Oregon. Archived from the original on 13 July 2011. Retrieved 22 December 2009. {{cite web}}: External link in |author= (help)
  5. ^ NASA: Moons of Jupiter
  6. ^ Space: The Earth's Moon density
  7. ^ "Course materials on "mass-radius relationships" in planetary formation" (PDF). caltech.edu. Archived (PDF) from the original on 22 December 2017. Retrieved 2 May 2018.
  8. ^ Carole Haswell, Transiting Exoplanets Archived 7 November 2015 at the Wayback Machine
  9. ^ Michael Perryman, The Exoplanet Handbook Archived 7 November 2015 at the Wayback Machine
  10. ^ Sara Seager, Exoplanets Archived 7 November 2015 at the Wayback Machine
  11. ^ "Lightest exoplanet yet discovered". ESO (ESO 15/09 – Science Release). 21 April 2009. Archived from the original on 5 July 2009. Retrieved 15 July 2009.
  12. ^ M. Mayor; X. Bonfils; T. Forveille; X. Delfosse; S. Udry; J.-L. Bertaux; H. Beust; F. Bouchy; C. Lovis; F. Pepe; C. Perrier; D. Queloz; N. C. Santos (2009). "The HARPS search for southern extra-solar planets,XVIII. An Earth-mass planet in the GJ 581 planetary system". Astronomy & Astrophysics. 507 (1): 487–494. arXiv:0906.2780. Bibcode:2009A&A...507..487M. doi:10.1051/0004-6361/200912172. S2CID 2983930.
  13. ^ Kaufman, Rachel (30 August 2011). "New Planet May Be Among Most Earthlike – Weather Permitting, Alien world could host liquid water if it has 50 percent cloud cover, study says". National Geographic News. Archived from the original on 23 September 2011. Retrieved 5 September 2011.
  14. ^ Rincon, Paul (22 March 2012). "Thousand-year wait for Titan rain". Archived from the original on 25 December 2017 – via www.bbc.com.
  15. ^ 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 from the original on 9 March 2011. Retrieved 9 March 2011.
  16. ^ Borucki, William J; et al. (2011). "Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data". The Astrophysical Journal. 736 (1): 19. arXiv:1102.0541. Bibcode:2011ApJ...736...19B. doi:10.1088/0004-637X/736/1/19. S2CID 15233153.
  17. ^ Gough, Evan (1 October 2020). "A Rogue Earth-Mass Planet Has Been Discovered Freely Floating in the Milky Way Without a Star". Universe Today. Retrieved 2 October 2020.
  18. ^ Mroz, Przemek; et al. (29 September 2020). "A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event". The Astrophysical Journal. 903 (1): L11. arXiv:2009.12377v1. Bibcode:2020ApJ...903L..11M. doi:10.3847/2041-8213/abbfad. S2CID 221971000.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  19. ^ Redd, Nola Taylor (19 October 2020). "Rogue Rocky Planet Found Adrift in the Milky Way - The diminutive world and others like it could help astronomers probe the mysteries of planet formation". Scientific American. Retrieved 19 October 2020.
  20. ^ a b Overbye, Dennis (4 November 2013). "Far-Off Planets Like the Earth Dot the Galaxy". New York Times. Archived from the original on 5 November 2013. Retrieved 5 November 2013.
  21. ^ a b Petigura, Eric A.; Howard, Andrew W.; Marcy, Geoffrey W. (31 October 2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences of the United States of America. 110 (48): 19273–19278. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033. Archived from the original on 9 November 2013. Retrieved 5 November 2013.
  22. ^ Staff (7 January 2013). "17 Billion Earth-Size Alien Planets Inhabit Milky Way". Space.com. Archived from the original on 6 October 2014. Retrieved 8 January 2013.
  23. ^ Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Archived from the original on 6 November 2013. Retrieved 5 November 2013.
  24. ^ "Newfound Planet is Earth-mass But Gassy". harvard.edu. 3 January 2014. Archived from the original on 28 October 2017. Retrieved 2 May 2018.
  25. ^ Naeye, Bob (24 September 2007). "Scientists Model a Cornucopia of Earth-sized Planets". NASA, Goddard Space Flight Center. Archived from the original on 24 January 2012. Retrieved 23 October 2013.

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