A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...

rocks In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...

or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun:

Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

, Venus, Earth and Mars. Among astronomers who use the

geophysical definition of a planet The International Union of Geological Sciences (IUGS) is the internationally recognized body charged with fostering agreement on nomenclature and classification across geoscientific disciplines. However, they have yet to create a formal definition ...

, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes

Europa Europa may refer to: Places * Europe * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Cliff ...

– may also be considered terrestrial planets; and so may be the rocky protoplanet-asteroids Pallas and Vesta.Emily Lakdawalla et al.
What Is A Planet?
The Planetary Society, 21 April 2020 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''. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists. 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 A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

. The large rocky asteroid

4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...

has a similar structure; possibly so does the smaller one

21 Lutetia ) , mp_category=Main belt , mpc_name=(21) Lutetia , orbit_ref = , epoch=May 31, 2020 ( JD 2459000.5) , semimajor=2.435 AU , perihelion=2.037 AU , aphelion=2.833 AU , eccentricity=0.16339 , period=3.80 yr (1388.1 d) , inclination=3.064� ...

. Another rocky asteroid

2 Pallas Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hyd ...

is about the same size as Vesta, but is significantly less dense; it appears to have never differentiated a core and a mantle. The Earth's Moon and Jupiter's moon Io have similar structures to the terrestrial planets, but Earth's Moon has a much smaller iron core. Another Jovian moon

has a similar density but has a significant ice layer on the surface: for this reason, it is sometimes considered an

icy planet An ice planet or icy planet is a type of planet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere. Under a geophysical definition of planet, the small icy worlds of the Solar System ...

instead. Terrestrial planets can have surface structures such as

canyon A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tenden ...

craters Crater may refer to: Landforms * Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet * Explosion crater, a hole formed in the ground produced by an explosion near or below the surf ...

, mountains, volcanoes, and others, depending on the presence at any time of an erosive liquid or tectonic activity or both. Terrestrial planets have

secondary atmosphere A secondary atmosphere is an atmosphere of a planet that did not form by accretion disc, accretion during the formation of the planet's star. A secondary atmosphere instead forms from internal volcano, volcanic activity, or by accumulation of materi ...

s, generated by volcanic out-gassing or from comet impact debris. This contrasts with the

outer {{Short pages monitorMost of the planets discovered outside the Solar System are giant planets, because they are more easily detectable. 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 planet A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...

s or terrestrial, depending on their mass and other parameters. It is likely that most known super-Earths are in fact gas planets similar to Neptune, as examination of the relationship between mass and radius of exoplanets (and thus density trends) shows a transition point at about two Earth masses. This suggests that this is the point at which significant gas envelopes accumulate. In particular, Earth and Venus may already be close to the largest possible size at which a planet can usually remain rocky. Exceptions to this are very close to their stars (and thus would have had their volatile atmospheres boiled away). During the early 1990s, the first extrasolar planets were discovered orbiting the

pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...

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 51 Pegasi b, officially named Dimidium , and formerly unofficially dubbed Bellerophon , is an extrasolar planet approximately away in the constellation of Pegasus. It was the first exoplanet to be discovered orbiting a main-sequence star, the S ...

, the first planet found around a star still undergoing fusion, was discovered, many astronomers assumed it to be a gigantic terrestrial, 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 Gliese 876 d is an exoplanet approximately 15 light-years away in the constellation of Aquarius. The planet was the third planet discovered orbiting the red dwarf Gliese 876. It was the lowest-mass extrasolar planet apart from the pulsar planets o ...

and OGLE-2005-BLG-390Lb. Gliese 876 d orbits the red dwarf Gliese 876, 15

light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s 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 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 (fourth if the disputed plane ...

, is only about 1.9 Earth masses, but orbits very close to the star. Two others,

Gliese 581c Gliese 581c (Gl 581c or GJ 581c) is a planet orbiting within the Gliese 581 system. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.5 times that of the Earth, it is classified as a s ...

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 HD 85512 b is an exoplanet orbiting HD 85512, a K-type main-sequence star approximately 37 light-years from Earth in the constellation of Vela. Due to its mass of at least 3.6 times the mass of Earth, HD 85512 b is classified as a rocky Earth ...

, was discovered in 2011; it has at least 3.6 times the mass of Earth. The radius and composition of all these planets are unknown. Size of Kepler Planet Candidates

The first confirmed terrestrial

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

Kepler-10b Kepler-10b is the first confirmed terrestrial planet to have been discovered outside the Solar System by the Kepler Space Telescope. Discovered after several months of data collection during the course of the NASA-directed Kepler Mission, which ...

, was found in 2011 by the Kepler Mission, specifically designed to discover Earth-size planets around other stars using the transit method. 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 twice that of the Earth) and in the habitable zone of their star. 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 A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors ( stars and brown dwarfs) and without a ...

) unbounded by any star, and free-floating in the Milky Way galaxy.

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-20b Kepler-20b is an exoplanet orbiting Kepler-20 in a system 922 light years from Earth. It is classified as a Super-Earth, as it has a radius and mass greater than that of Earth. At about 1.87 , it would most likely be a Mini-Neptune A Mini-Neptu ...

Kepler-36b Kepler-36b is an exoplanet orbiting the star Kepler-36. This planet has the closest conjunction to Kepler-36c every 97 days. Its density is similar to that of iron Iron () is a chemical element with symbol Fe (from la, ferrum) and ato ...

, Kepler-48d,

Kepler 68c Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...

Kepler-78b Kepler-78b (formerly known as KIC 8435766 b) is an exoplanet orbiting around the star Kepler-78. At the time of its discovery, it was the exoplanet most similar to Earth in terms of mass, radius, and mean density. Discovery Kepler-78b is the only ...

Kepler-89b Kepler-89 is a star with four confirmed planets. Kepler-89 is a possible wide binary star. Planetary system The discovery of four planets orbiting the star was announced October 2012 by analyzing data gathered by Kepler space telescope. F ...

Kepler-93b Kepler-93b is a Super-Earth 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 ter ...

, Kepler-97b, Kepler-99b,

Kepler-100b GSC 03549-02811 (sometimes referred to as TrES-2 A or TrES-2 parent star in reference to its exoplanet TrES-2b), also known as Kepler-1) is a yellow main-sequence star similar to the Sun. This star is located approximately 704 light-yea ...

Kepler-101c GSC 03549-02811 (sometimes referred to as TrES-2 A or TrES-2 parent star in reference to its exoplanet TrES-2b), also known as Kepler-1) is a yellow main-sequence star similar to the Sun. This star is located approximately 704 light-years ...

Kepler-102b Kepler-102 is a star in the constellation of Lyra. Kepler-102 is less luminous than the Sun. The star system does not contain any observable amount of dust. Kepler-102 is suspected to be orbited by a binary consisting of two red dwarf star ...

Kepler-102d Kepler-102 is a star in the constellation of Lyra. Kepler-102 is less luminous than the Sun. The star system does not contain any observable amount of dust. Kepler-102 is suspected to be orbited by a binary consisting of two red dwarf sta ...

, Kepler-113b, Kepler-131b, Kepler-131c,

Kepler-138c Kepler-138, also known as KOI-314, is a red dwarf located in the constellation Lyra, 219 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 p ...

, Kepler-406b, Kepler-406c,

Kepler-409b Kepler-409b is a super-Earth orbiting Kepler-409, a G-type main-sequence star. Its orbital period around the star is 69 days. Kepler-409b has a radius that is 1.199 that of Earth and a mass of 6 that of Earth. It's discovery in 2014 was made thro ...

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 A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young ...

orbiting in the habitable zones of Sun-like stars and

red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...

s within the Milky Way. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. 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-138 Kepler-138, also known as KOI-314, is a red dwarf located in the constellation Lyra, 219 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 ...

d).

Types

Several possible classifications for solid planets have been proposed. ; Silicate planet : A solid planet like Venus, Earth, or Mars, 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, such as Ceres and

10 Hygiea Hygiea (minor-planet designation: 10 Hygiea) is a major asteroid and possible dwarf planet located in the main asteroid belt. With a diameter of and a mass estimated to be 3% of the total mass of the belt, it is the fourth-largest asteroid in ...

. It is unknown if Ceres has a rocky or a metallic core. ; Iron planet : A theoretical type of solid planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other solid planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass, and is sometimes called an iron planet, though its surface is made of silicates and is iron-poor. 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. ; Icy planet Fountains of Enceladus PIA07758

: A type of solid planet with an icy surface of volatiles. In the Solar System, most planetary-mass moons (such as Titan, Triton, and Enceladus) and many dwarf planets (such as Pluto and Eris) have such a composition. Europa is sometimes considered an icy planet due to its surface ice, but its higher density indicates that its interior is mostly rocky. Such planets can have internal saltwater oceans and cryovolcanoes erupting liquid water (i.e. an internal hydrosphere, like Europa or Enceladus); they can have an atmosphere and hydrosphere made from methane or nitrogen (like Titan). A metallic core is possible, as exists on Ganymede. ; Coreless planet : A theoretical type of solid 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. Ceres and Pallas have mineral compositions similar to carbonaceous chondrites, though Pallas is significantly less hydrated.Marsset, M., Brož, M., Vernazza, P. et al. The violent collisional history of aqueously evolved (2) Pallas. Nat Astron 4, 569–576 (2020). https://doi.org/10.1038/s41550-019-1007-5 Coreless planets are thought to form farther from the star where volatile oxidizing material is more common.

References

{{DEFAULTSORT:Terrestrial Planet Types of planet

Structure

List of terrestrial exoplanets

Frequency

Types

See also

References