Terrestrial planet

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The terrestrial planets of the Solar System: Mercury, Venus, Earth and Mars, sized to scale Terrestrial planet sizes2.jpg
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 are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars. 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.

Contents

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. [1]

Solar System's terrestrial planets

Relative masses of the terrestrial planets of the Solar System, and the Moon (shown here as Luna) Masses of terrestrial planets.png
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. Telluric planets size comparison.jpg
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, is known to have an active hydrosphere.

During the formation of the Solar System, there were probably many more terrestrial planetesimals, but most merged with or were ejected by the four terrestrial planets.

Dwarf planets, such as Ceres, Pluto and Eris, and 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, Pluto and Eris have densities of 2.17, 1.87 and 2.52 g·cm−3, respectively, and Haumea's, Makemake's and Gonggong's density is 2.02, 1.98 and 1.75 respectively g·cm−3). The Earth's Moon has a density of 3.4 g·cm−3 and Jupiter's satellites, Io, 3.528 and Europa, 3.013 g·cm−3; other satellites typically have densities less than 2 g·cm−3. [2] [3]

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
ObjectDensity (g·cm−3)Semi-major axis (AU)
MeanUncompressed
Mercury 5.45.30.39
Venus 5.24.40.72
Earth 5.54.41.0
Mars 3.93.81.52

The uncompressed 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.

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. [4] It is unknown whether extrasolar terrestrial planets in general will show to follow this trend.

Extrasolar terrestrial planets

Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable. [5] [6] [7] 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, [8] but orbits very close to the star. An ideal[ vague ] terrestrial planet would be two Earth masses,[ why? ] with a 25-day orbital period[ why? ] around a red dwarf[ why? ]. [9] 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. [10] 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). Size of Kepler Planet Candidates.jpg
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. [11]

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) [12] and in the habitable zone of their star. [13] 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 unbounded by any star, and free floating in the Milky Way galaxy. [14] [15]

List of terrestrial exoplanets

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.

The Neptune-mass planet Kepler-10c also has a density >5 g/cm3 and is thus very likely terrestrial.

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. [16] [17] [18] 11 billion of these estimated planets may be orbiting Sun-like stars. [19] The nearest such planet may be 12 light-years away, according to the scientists. [16] [17] 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). [20]

Types

Artist's impression of a carbon planet Carbon Planet.JPG
Artist's impression of a carbon planet

Several possible classifications for terrestrial planets have been proposed: [21]

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

Related Research Articles

Circumstellar habitable zone Zone around a star where surface liquid water may exist on a planet

In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the CHZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

Gliese 581d extrasolar planet

Gliese 581d is an extrasolar planet orbiting within the Gliese 581 system, approximately 20.4 light-years away in the Libra constellation. It is the third planet claimed in the system and the fifth in order from the star.

Super-Earth Type of planet

A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.

Stéphane Udry Swiss astronomer

Stéphane Udry is an astronomer at the Geneva Observatory in Switzerland, whose current work is primarily the search for extra-solar planets. He and his team, in 2007, discovered a possibly terrestrial planet in the habitable zone of the Gliese 581 planetary system, approximately 20 light years away in the constellation Libra. He also led the observational team that discovered HD 85512 b, another most promisingly habitable exoplanet.

Exoplanetology, or exoplanetary science, is an integrated field of astronomical science dedicated to the search for and study of exoplanets. It employs an interdisciplinary approach which includes astrobiology, astrophysics, astronomy, astrochemistry, astrogeology, geochemistry, and planetary science.

Gliese 581e extrasolar planet

Gliese 581e or Gl 581e is an extrasolar planet orbiting within the Gliese 581 system, located approximately 20.4 light-years away from Earth in the Libra constellation. It is the third planet discovered in the system and the first in order from the star.

Discoveries of exoplanets Detecting planets located outside the Solar System

An exoplanet is a planet located outside the Solar System. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such until 2016. No planet discovery has yet come from that evidence. However, the first scientific detection of an exoplanet began in 1988. Afterwards, the first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. As of 1 October 2020, there are 4,354 confirmed exoplanets in 3,218 systems, with 712 systems having more than one planet. This is a list of the most notable discoveries.

Kepler-22b Exoplanet orbiting around Kepler-22

Kepler-22b, also known by its Kepler object of interest designation KOI-087.01, is an extrasolar planet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about 638 light-years (196 pc) from Earth in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in December 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star, where liquid water could exist on the planet's surface. Kepler-22 is too dim to be seen with the naked eye.

Kepler-47 is a binary star system with three exoplanets in orbit around the pair of stars located about 1055 parsecs away from Earth. The first two planets announced are designated Kepler-47b, and Kepler-47c. Kepler-47 is the first circumbinary multi-planet system discovered by the Kepler mission. The outermost of the planets is a gas giant orbiting within the habitable zone of the stars. Because most stars are binary, the discovery that multi-planet systems can form in such a system has impacted previous theories of planetary formation.

Kepler-69c extrasolar planet

Kepler-69c is a confirmed super-Earth extrasolar planet, likely rocky, orbiting the Sun-like star Kepler-69, the outermost of two such planets discovered by NASA's Kepler spacecraft. It is located about 2,430 light-years from Earth.

Kepler-62 is a star somewhat cooler and smaller than the Sun in the constellation Lyra, 1200 light years from Earth. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission used to detect planets that may be transiting their stars. On April 18, 2013 it was announced that the star has five planets, two of which, Kepler-62e and Kepler-62f are within the star's habitable zone. The outermost, Kepler-62f, is likely a rocky planet.

Kepler-69 G-type star

Kepler-69 is a G-type main-sequence star similar to the Sun in the constellation Cygnus, located about 2,430 ly (750 pc) from Earth. On April 18, 2013 it was announced that the star has two planets. Although initial estimates indicated that the terrestrial planet Kepler-69c might be within the star's habitable zone, further analysis showed that the planet very likely is interior to the habitable zone and is far more analogous to Venus than to Earth and thus completely inhospitable.

Kepler-61b extrasolar planet

Kepler-61b is a super-Earth exoplanet orbiting within parts of the habitable zone of the K-type main-sequence star Kepler-61. It is located about 1,100 light-years from Earth in the constellation of Cygnus. It was discovered in 2013 using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured, by NASA's Kepler spacecraft.

Kepler-186f Extrasolar planet

Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is an exoplanet orbiting the red dwarf Kepler-186, about 582 light-years (178.5 parsecs, or nearly 5.298×1015 km) from the Earth. It is the first planet with a radius similar to Earth's to be discovered in the habitable zone of another star. NASA's Kepler space telescope detected it using the transit method, along with four additional planets orbiting much closer to the star (all modestly larger than Earth). Analysis of three years of data was required to find its signal. The results were presented initially at a conference on 19 March 2014 and some details were reported in the media at the time. The public announcement was on 17 April 2014, followed by publication in Science.

Kepler-444 is a triple star system, estimated to be 11.2 billion years old, approximately 119 light-years (36 pc) away from Earth in the constellation Lyra. On 27 January 2015, the Kepler spacecraft is reported to have confirmed the detection of five sub-Earth-sized rocky exoplanets orbiting the main star. The star is a K type star. According to NASA, no life as we know it could exist on these hot exoplanets, due to their close orbital distances to the host star.

Kepler-1652b is a super-Earth exoplanet, most likely rocky, orbiting within the habitable zone of the red dwarf Kepler-1652 about 822 light-years away in the Cygnus constellation. Discovered by NASA's Kepler spacecraft, Kepler-1652b was first announced as a candidate in 2013, but wasn't validated until four years later in 2017. It is a potential Mega-Earth with 160% Earth's radius and a density of around 9.9 g/cm³, suggesting a high mass and terrestrial composition. The planet orbits well within the habitable zone of its system, the region where liquid water can exist on a planet's surface. The Planetary Habitability Laboratory (PHL) has given Kepler-1652b a very high Earth Similarity Index (ESI) value of 0.85.

Kepler-277b is the second most massive and third-largest rocky planet ever discovered, with a mass close to that of Saturn. Discovered in 2014 by the Kepler Space Telescope, Kepler-277b is a sub-Neptune sized exoplanet with a very high mass and density for an object of its radius, suggesting a composition made mainly of rock and iron. Along with its sister planet, Kepler-277c, the planet's mass was determined using transit-timing variations (TTVs).

References

  1. 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.
  2. NASA: Moons of Jupiter
  3. Space: The Earth's Moon density
  4. "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.
  5. Carole Haswell, Transiting Exoplanets Archived 7 November 2015 at the Wayback Machine
  6. Michael Perryman, The Exoplanet Handbook Archived 7 November 2015 at the Wayback Machine
  7. Sara Seager, Exoplanets Archived 7 November 2015 at the Wayback Machine
  8. "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.
  9. 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.
  10. 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.
  11. Rincon, Paul (22 March 2012). "Thousand-year wait for Titan rain". Archived from the original on 25 December 2017 via www.bbc.com.
  12. 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". 80beats . Discover Magazine. Archived from the original on 9 March 2011. Retrieved 9 March 2011.External link in |work= (help)
  13. 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.
  14. 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.
  15. Mroz, Przemek; et al. (29 September 2020). "A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event" (PDF). arxiv . arXiv: 2009.12377v1 . Retrieved 2 October 2020.
  16. 1 2 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.
  17. 1 2 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.
  18. 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.
  19. 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.
  20. "Newfound Planet is Earth-mass But Gassy". harvard.edu. 3 January 2014. Archived from the original on 28 October 2017. Retrieved 2 May 2018.
  21. 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.