Terrestrial planet

Terrestrial planet

Overview
A terrestrial planet, telluric planet or rocky planet is a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 that is composed primarily of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

 rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s or metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

s. Within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, the terrestrial planets are the inner planets closest to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. The terms are derived from Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 words for Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 (Terra and Tellus), so these planets are, in a certain way, "Earth-like".

Terrestrial planets are substantially different from gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s, which might not have solid surfaces and are composed mostly of some combination of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

, and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 existing in various physical state
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

s.

Terrestrial planets all have approximately the same structure: a central metallic core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...

, mostly iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, with a surrounding silicate mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

.
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Encyclopedia
A terrestrial planet, telluric planet or rocky planet is a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 that is composed primarily of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

 rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s or metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

s. Within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, the terrestrial planets are the inner planets closest to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. The terms are derived from Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 words for Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 (Terra and Tellus), so these planets are, in a certain way, "Earth-like".

Terrestrial planets are substantially different from gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s, which might not have solid surfaces and are composed mostly of some combination of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

, and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 existing in various physical state
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...

s.

Structure


Terrestrial planets all have approximately the same structure: a central metallic core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...

, mostly iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, with a surrounding silicate mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

. The Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 is similar, but has a much smaller iron core. Terrestrial planets have canyon
Canyon
A canyon or gorge is a deep ravine between cliffs often carved from the landscape by a river. Rivers have a natural tendency to reach a baseline elevation, which is the same elevation as the body of water it will eventually drain into. This forms a canyon. Most canyons were formed by a process of...

s, craters
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

, mountain
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...

s, and volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es. Terrestrial planets possess secondary atmosphere
Secondary atmosphere
A secondary atmosphere is an atmosphere of a planet that did not form by accretion during the formation of the planet's sun. A secondary atmosphere instead forms from internal volcanic activity, or by accumulation of material from comet impacts...

s — atmospheres generated through internal volcanism or comet impacts, as opposed to the gas giants, which possess primary atmosphere
Primary atmosphere
A primary atmosphere is an atmosphere of a planet that forms by accretion of gaseous matter from the accretion disc of the planet's sun. Planets such as Jupiter and Saturn have primary atmospheres. Primary atmospheres are very thick compared to secondary atmospheres like the one found on Earth...

s — atmospheres captured directly from the original solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...

.

Theoretically, there are two types of terrestrial or rocky planets, one dominated by silicon compounds and another dominated by carbon compounds, like carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...

 asteroids. These are the silicate planets and carbon planet
Carbon planet
A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form if protoplanetary discs are carbon-rich and oxygen-poor. According to planetary science, it would develop differently from Earth, Mars and Venus, planets...

s (or "diamond planets") respectively (the Solar System does not include any carbon planets).

Solar terrestrial planets



Earth's Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 has four terrestrial planets: Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

. Only one terrestrial planet, Earth, is known to have an active hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

.

During the formation of the Solar System, there were probably many more "terrestrial" planetesimals, but they have all merged with or been destroyed by the four remaining worlds in the solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...

.

Plutoids, objects like Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, are similar to terrestrial planets in the fact that they do have a solid surface, but are composed of more icy materials.

Density trends


The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

. A greater uncompressed density indicates greater metal content. Uncompressed density is used rather than true average density because compression within planet cores increases their density (making average density depend on planet size as well as composition).

The uncompressed densities of the solar terrestrial planets, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, and the three largest asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s are shown below. Densities generally trend towards lower values as the distance from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 increases.
Object mean density uncompressed density semi-major axis
Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

5.4 g cm−3 5.3 g cm−3 0.39 AU
Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 
5.2 g cm−3 4.4 g cm−3 0.72 AU
Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 
5.5 g cm−3 4.4 g cm−3 1.0 AU
Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 
3.3 g cm−3 3.3 g cm−3 1.0 AU
Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 
3.9 g cm−3 3.8 g cm−3 1.5 AU
Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 
3.4 g cm−3 3.4 g cm−3 2.3 AU
Ceres 
2.1 g cm−3 2.1 g cm−3 2.8 AU
Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...

 
2.8 g cm−3 2.8 g cm−3 2.8 AU

The main exception to this rule is the density of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, which probably owes its lesser density to its unusual origin.

It is unknown whether extrasolar terrestrial planets in general will also follow this trend. E.g. Kepler-10b
Kepler-10b
Kepler-10b is the first confirmed terrestrial planet to have been discovered outside the Solar System. Discovered after several months of data collection during the course of the NASA-directed Kepler Mission, which aims to discover Earth-like planets crossing in front of their host stars, the...

 does: it has a density of 8.8 g cm−3, and orbits its star much closer than Mercury orbits the Sun.

Extrasolar terrestrial planets



The majority of planets found outside the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 have been gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s since they produce more pronounced wobbles
Doppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....

 in the host stars and are thus more easily detectable. However, a number of extrasolar planets are suspected to be terrestrial.

During the early 1990s, the first extrasolar planets were discovered orbiting the pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...

 PSR B1257+12
PSR B1257+12
PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a pulsar located roughly 2000 light-years from the Sun. In 2007, it was confirmed that three extrasolar planets orbit the pulsar.- Pulsar :...

 with masses of 0.02, 4.3, and 3.9 times that of Earth's. They were discovered by accident: their transit caused interruptions in the pulsar's radio emissions (had they not been orbiting around a pulsar, they would not have been found).

When 51 Pegasi b
51 Pegasi b
51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...

, the first planet found around a star still undergoing fusion
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...

, was discovered, many astronomers assumed it must be a gigantic terrestrial, as it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. However, subsequent diameter measurements of a similar extrasolar planet (HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....

), which transited its star showed that these objects were indeed gas giants.


In June 2005, the first planet around a fusing star that may be terrestrial was found orbiting around the red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

 star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 Gliese 876
Gliese 876
Gliese 876 is a red dwarf star approximately 15 light-years away from Earth in the constellation of Aquarius. As of 2011, it has been confirmed that four extrasolar planets orbit the star...

, 15 light years away. That planet, Gliese 876 d
Gliese 876 d
Gliese 876 d is an extrasolar planet approximately 15 light-years away in the constellation of Aquarius . The planet was the third planet discovered orbiting the red dwarf star Gliese 876. At the time of its discovery, the planet had the lowest mass of any known extrasolar planet apart from the...

, has a mass 7 to 9 times that of earth and an orbital period of just two Earth days. But the radius and composition of the planet is unknown.

On 10 August 2005, Probing Lensing Anomalies NETwork
Probing Lensing Anomalies Network
The Probing Lensing Anomalies NETwork collaboration coordinates a network of telescopes to rapidly sample photometric measurements of the magnification of stars in the galactic bulge undergoing gravitational microlensing by intervening foreground stars...

/Robotic Telescope Network (PLANET/RoboNet) and Optical Gravitational Lensing Experiment
Optical Gravitational Lensing Experiment
The Optical Gravitational Lensing Experiment or OGLE is a Polish astronomical project based at the University of Warsaw that is chiefly concerned with discovering dark matter using the microlensing technique. Since the project began in 1992, it has discovered several extrasolar planets as a side...

 (OGLE) observed the signs of a cold planet designated OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb
OGLE-2005-BLG-390Lb is a 'super-Earth' extrasolar planet orbiting the star OGLE-2005-BLG-390L, which is situated 21,500 ± 3,300 light years away from Earth, near the center of the Milky Way galaxy...

, about 5.5 times the mass of Earth, orbiting a star about 21,000 light years away in the constellation Scorpius. The newly discovered planet orbits its parent star at a distance similar to that of the Solar System's asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

. The planet revealed its existence through a technique known as gravitational microlensing
Gravitational microlensing
Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects ranging from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit lots of light ...

, currently unique in its capability to detect planets with masses down to that of Earth.

In April 2007, a team of 11 European scientists announced the discovery of a planet outside the Solar System that is potentially habitable, with Earth-like temperatures. The planet was discovered by the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...

's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths. Those wobbles can reveal the existence of other worlds. What they revealed is planets circling the red dwarf star, Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

. Gliese 581 c
Gliese 581 c
Gliese 581 c or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth...

 was considered to be habitable at first, but more recent study (April 2009) suggests Gliese 581 d
Gliese 581 d
Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star....

 is a better candidate. Regardless, it has increased interest in examining planets circling dimmer stars. About 80 percent of the stars near Earth are red dwarfs. The Gliese 581 (c and d) planets are about five to seven times heavier than Earth, classifying them as super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...

s.

Gliese 581 e
Gliese 581 e
Gliese 581 e or Gl 581 e is an extrasolar planet found around Gliese 581, an M3V red dwarf star approximately 20.5 light-years away from Earth in the constellation of Libra...

 is only about 1.9 Earth mass, but could have 2 orders of magnitude more tidal heating than Jupiter’s volcanic satellite Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

. An ideal terrestrial planet would be 2 Earth masses with a 25-day orbital period around a M dwarf star.

The discovery of Gliese 581 g
Gliese 581 g
Gliese 581 g , also Gl 581 g or GJ 581 g, is a hypothesized extrasolar planet proven nonexistent by the Geneva Team, orbiting the red dwarf star Gliese 581, 20.5 light-years from Earth in the constellation of Libra. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth...

 was announced in September 2010, and is believed to be the first so-called "Goldilocks planet" ever found, the most Earth-like planet, and the best exoplanet candidate with the potential for sheltering life found to date.

Using the HARPS facility, scientists discovered a Goldilocks planet named HD85512b
HD 85512 b
HD 85512 b is an extrasolar planet orbiting the K-type star Gliese 370 approximately 36 light-years away in the constellation of Vela ....

 with 3.6 times the mass of Earth and the right conditions for liquid water. The acceleration of gravity is 1.4 times that on Earth. The orange dwarf star orbited by HD85512b is 5.6 billion years old and resides 36 light years from the solar system in the constellation Vela.

The Kepler Mission
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...

 endeavours to discover Earth-like planets orbiting around other stars by observing their transits
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...

 across the star. The Kepler spacecraft was launched on 6 March 2009. The duration of the mission will need to be about three and a half years long to detect and confirm an Earth-like planet orbiting at an Earth-like distance from the host star. Since it will take intervals of one year for a truly Earth-like planet to transit (cross in front of its star), it will take about four transits for a reliable reading.

Dimitar Sasselov, the Kepler mission co-investigator, recently mentioned at the 2010 TED Conference that there have been hundreds more candidate terrestrial planets discovered since Kepler went online. If these planets are confirmed via further investigation, then it will represent the largest find of extrasolar planets to date. The Kepler science teams are, for now, keeping the initial results of any candidate planets a secret so they can confirm their results. The first public announcement of any such results is expected during the early part of 2011.

On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the "habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

." Some of these candidates were "Earth-size" and "super-Earth-size" (defined as "less than or equal to 2 Earth radii [Re]" [or, Rp ≤ 2.0 Re] - Table 5). Six of these candidates (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) are in the "habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

." A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.

A number of other telescopes capable of directly imaging extrasolar terrestrial planets are also being designed. These include the Terrestrial Planet Finder
Terrestrial Planet Finder
The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...

, Space Interferometry Mission
Space Interferometry Mission
The Space Interferometry Mission, or SIM, also known as SIM Lite , was a planned space telescope developed by the U.S. National Aeronautics and Space Administration , in conjunction with contractor Northrop Grumman...

, Darwin
Darwin (ESA)
Darwin was a suggested ESA Cornerstone mission which would have involved a constellation of four to nine spacecraft designed to directly detect Earth-like planets orbiting nearby stars and search for evidence of life on these planets...

, New Worlds Mission, and Overwhelmingly Large Telescope
Overwhelmingly Large Telescope
The Overwhelmingly Large Telescope is a conceptual design by the European Southern Observatory organization for an extremely large telescope, which was intended to have a single aperture of 100 meters in diameter...

.

Most Earth-like exoplanets


Title Planet Star Value Notes
Closest planet to 1 MEarth
Earth mass
Earth mass is the unit of mass equal to that of the Earth. 1 M⊕ = 5.9722 × 1024 kg. Earth mass is often used to describe masses of rocky terrestrial planets....

Gliese 581 e
Gliese 581 e
Gliese 581 e or Gl 581 e is an extrasolar planet found around Gliese 581, an M3V red dwarf star approximately 20.5 light-years away from Earth in the constellation of Libra...

Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

1.7 to 3.1 MEarth Close to star and potentially volcanic like Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

.
Kepler-11f
Kepler-11f
Kepler-11f is an exoplanet discovered in the orbit of the sunlike star Kepler-11 by NASA's Kepler spacecraft, which searches for planets that transit their host stars. Kepler-11f is the sixth planet from its star, orbiting one fourth of the distance of the Earth from the Sun every 47 days...

Kepler-11
Kepler-11
Kepler-11 is a sun-like star slightly larger than the Sun in the constellation Cygnus, located some 2,000 light years from Earth. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission uses to detect planets that may be transiting their stars...

2.3 MEarth Has a radius of about 2.6 REarth, implying a Saturn-like low density
Closest planet to 1 Earth Radius Kepler-10b
Kepler-10b
Kepler-10b is the first confirmed terrestrial planet to have been discovered outside the Solar System. Discovered after several months of data collection during the course of the NASA-directed Kepler Mission, which aims to discover Earth-like planets crossing in front of their host stars, the...

Kepler-10
Kepler-10
Kepler-10, formerly known as KOI-72, is a sunlike star in constellation Draco that lies 173 parsecs away from the Earth. Kepler-10 was targeted by NASA's Kepler spacecraft, as it was seen the first star identified by the Kepler mission as a possible host to a small, transiting exoplanet...

1.4 REarth Has a mass of 3.3-5.7 MEarth. The less-massive Gliese 581 e is therefore
probably smaller than Kepler-10b, unless it has a much lower density

Types



Several possible classifications for terrestrial planets have been proposed:

Silicate planet
The standard type of terrestrial planet seen in the Solar System, made primarily of silicon-based rocky mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

 with a metallic (iron) core.


Iron planet
Iron planet
]An iron planet is a type of planet that consists primarily of an iron-rich core with little or no mantle. Mercury is the largest celestial body of this type in our solar system, but larger iron-rich exoplanets may exist.-Origin:...

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
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

 in the Solar System has a metallic core equal to 60-70% of its planetary mass. Iron planets are believed to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron.


Coreless planet
Coreless planet
A coreless planet is a theoretical type of terrestrial planet that has undergone planetary differentiation but nevertheless has no metallic core, i.e...

A theoretical type of terrestrial planet that consists of silicate rock but has no metallic core, i.e. the opposite of an iron planet. The Solar System contains no coreless planets, but chondrite
Chondrite
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...

 asteroids and meteorites are common in it. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common.


Carbon planet
Carbon planet
A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form if protoplanetary discs are carbon-rich and oxygen-poor. According to planetary science, it would develop differently from Earth, Mars and Venus, planets...

 or diamond planet
A theoretical type of terrestrial planet, composed primarily of carbon-based minerals. The Solar System contains no carbon planets, but does have carbonaceous asteroids
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...

.


Super-Earth
Super-Earth
A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's gas giants. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability...

Super-Earths represent the upper-end of the terrestrial planet mass range.

See also

  • Earth analog
    Earth analog
    An Earth analog is a theoretical other planet with conditions similar to Earth....

  • Gas giant
    Gas giant
    A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

     — also known as Jovian or Giant planets.
    • Chthonian planet
      Chthonian planet
      A chthonian planet is a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star...

  • Dwarf planet
    Dwarf planet
    A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

  • Planetary habitability
    Planetary habitability
    Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

  • List of nearest terrestrial exoplanets

External links