Super-Earth
Encyclopedia
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 giant
s. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability. An alternative term "gas dwarf" may be more accurate for some examples, especially higher mass ones as suggested by MIT professor Sara Seager
.
es, (~69% of the mass of Uranus
which is the Solar System gas giant with the least mass), the lower bound varies from 1 or 1.9 to 5, with various other definitions appearing in the popular media. Some authors further suggest that the term be limited to planets without a significant atmosphere. Planets above 10 Earth masses are termed giant planets.
does not contain examples of this category of planets, as the largest terrestrial planet
in the Solar System is the Earth, and all larger planets have at least 14 times Earth's mass.
and Dale Frail
around the pulsar
PSR B1257+12
in 1992. The two outer planets of the system have masses approximately 4 times Earth, too small to be gas giants.
The first super-Earth around a main sequence
star was discovered by a team under Eugenio Rivera in 2005. It orbits Gliese 876
and received the designation Gliese 876 d
(two Jupiter-sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and a very short orbital period of just about 2 days. Due to the proximity of Gliese 876 d to its host star (a red dwarf), it may have a surface temperature of 430–650 kelvin
and may support liquid water.
with a mass of 5.5 Earth masses, which was found by gravitational microlensing
, and HD 69830 b
with a mass of 10 Earth masses.
based in Switzerland
announced the discovery of two new super-Earths around Gliese 581
, both on the edge of the habitable zone
around the star where liquid water may be possible on the surface. With Gliese 581 c
having a mass of at least 5 Earth masses and a distance from Gliese 581 of 0.073 astronomical unit
s (AU; 6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without taking into consideration effects from an atmosphere) of −3 degrees Celsius with an albedo
comparable to Venus
and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggests Gliese 581 c has likely suffered a runaway greenhouse effect
like Venus, but that its sister planet, Gliese 581 d
, does in fact lie within the star's habitable zone, with an orbit at 0.22 AU and a mass of 7.7 Earths.
was announced by astrophysicist David P. Bennett for the international MOA
collaboration on June 2, 2008. This planet has approximately 3.3 Earth masses and orbits a brown dwarf
. It was detected by gravitational microlensing.
In June 2008, European researchers announced the discovery of three super-Earths around the star HD 40307, a star that is only slightly less massive than our Sun
. The planets have at least the following minimum masses: 4.2, 6.7, and 9.4 times Earth's. The planets were detected by the radial velocity
method by the HARPS (High Accuracy Radial Velocity Planet Searcher) in Chile
.
In addition, the same European research team announced a planet 7.5 times the mass of Earth orbiting the star HD 181433. This star also has a Jupiter-like planet that orbits every three years.
, is the first super-Earth discovered that orbits a main sequence
star that is G class or larger.
The discovery of Gliese 581 e
with a minimum mass
of 1.9 Earth masses was announced on April 21, 2009. It is the smallest extrasolar planet discovered around a normal star and the closest in mass to Earth. Being at an orbital distance of just 0.03 AU and orbiting its star in just 3.15 days, it is not in the habitable zone, and may have 100 times more tidal heating than Jupiter’s volcanic satellite Io
.
Additionally, Gliese 581 d
, at 0.2 AU with a 67-day orbital period, has been confirmed to be within the habitable zone of the red dwarf star, making it the first exoplanet where the existence of liquid water is a real possibility.
A planet found in December 2009, GJ 1214 b, is 2.7 times as large as Earth and orbits a star much smaller and less luminous than our sun. "This planet probably does have liquid water," said David Charbonneau, a Harvard professor of astronomy and lead author of an article on the discovery. However, interior models of this planet suggest that under most conditions it will not have liquid water.
By November 2009, a total of 30 super-Earths had been discovered, 24 of which were first observed by HARPS.
of 4.15 Earth mass
es, is the second least massive planet detected by the radial velocity method. The only confirmed radial velocity planet smaller than this planet is Gliese 581 e at 1.9 Earth masses (see above). On August 24 astronomers using ESO’s HARPS instrument announced the discovery of a planetary system with up to seven planets orbiting a Sun-like star, HD 10180
, one of which, although not yet confirmed, has an estimated minimum mass of 1.35 ± 0.23 times that of Earth, which would be the lowest mass of any exoplanet found to date orbiting a main-sequence star. Although unconfirmed, there is 98.6% probability that this planet does exist.
The National Science Foundation
announced on September 29 the discovery of a fourth super-Earth (Gliese 581 g
) orbiting the M dwarf
star Gliese 581. The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with a period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington. However, the existence of Gliese 581 g has been questioned by another team of astronomers, and it is currently listed as unconfirmed at The Extrasolar Planets Encyclopaedia.
54 planet candidates were detected in the "habitable zone
." Six candidates in this zone were less than twice the size of the Earth [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. Based on the latest Kepler findings, astronomer Seth Shostak
estimates "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds." Also based on the findings, the Kepler Team has estimated "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.
On August 17, a potentially habitable super-Earth HD 85512 b
was found using the HARPS as well as a three super-Earth system 82 G. Eridani. On HD 85512 b, it would be habitable if it exhibits more than 50% cloud cover. Then less than a month later, a flood of 41 new exoplanets including 10 super-Earths were announced.
, calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surface gravity between 1.9g and 3.3g (19 and 32 m/s²).
Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics
due to thinner plates under more stress. In fact, their models suggested that Earth was itself a "borderline" case, just barely large enough to sustain plate tectonics. However, other studies determine that strong convection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibit plate tectonics. The planet's surface would be too strong for driving forces of magma
to break the crust into plates.
s of super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, the black-body temperature of the Earth is 254.3 K
(−19 °C or −2 °F ). It is the greenhouse gas
es that keep the Earth warmer. Venus has a black-body temperature of only 231.7 K (−41 °C or −43 °F ) even though Venus has a true temperature of 737 K (464 °C or 867 °F ). Though the atmosphere of Venus traps more heat than Earth's, NASA lists the black-body temperature of Venus based on the fact that Venus has an extremely high albedo, giving it a lower black body temperature than the more absorbent (lower albedo) Earth.
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
with a mass higher than 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...
's, but substantially below the mass of the Solar System's 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. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability. An alternative term "gas dwarf" may be more accurate for some examples, especially higher mass ones as suggested by MIT professor Sara Seager
Sara Seager
Sara Seager is a Canadian-American astronomer who is currently a professor at the Massachusetts Institute of Technology and known for her work on extrasolar planets. She was born in Toronto, Canada. In 1994, she earned the degree of Bachelor of Science in Mathematics and Physics from the...
.
Definition
In general, super-Earths are defined exclusively by their mass, and the term does not imply temperatures, compositions, orbital properties, or environments similar to Earth's. A variety of specific mass values are cited in definitions of super-Earths. While sources generally agree on an upper bound of 10 Earth massEarth 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....
es, (~69% of the mass of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
which is the Solar System gas giant with the least mass), the lower bound varies from 1 or 1.9 to 5, with various other definitions appearing in the popular media. Some authors further suggest that the term be limited to planets without a significant atmosphere. Planets above 10 Earth masses are termed giant planets.
Discoveries
The Solar SystemSolar 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...
does not contain examples of this category of planets, as the largest terrestrial planet
Terrestrial planet
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...
in the Solar System is the Earth, and all larger planets have at least 14 times Earth's mass.
First super-Earth found
The first super-Earths were discovered by Aleksander WolszczanAleksander Wolszczan
Aleksander Wolszczan is a Polish astronomer. He is the co-discoverer of the first extrasolar planets and pulsar planets.- Scientific career :...
and Dale Frail
Dale Frail
Dale A. Frail is an astronomer working at the National Radio Astronomy Observatory in Socorro, New Mexico. He was born in Canada, spent much of his childhood in Europe, and his professional career has been based in the United States.-Career:...
around 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 :...
in 1992. The two outer planets of the system have masses approximately 4 times Earth, too small to be gas giants.
The first super-Earth around a main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star was discovered by a team under Eugenio Rivera in 2005. It orbits 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...
and received the designation 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...
(two Jupiter-sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and a very short orbital period of just about 2 days. Due to the proximity of Gliese 876 d to its host star (a red dwarf), it may have a surface temperature of 430–650 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
and may support liquid water.
Other super-Earths discovered in 2006
Two further super-Earths were discovered in 2006, OGLE-2005-BLG-390LbOGLE-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...
with a mass of 5.5 Earth masses, which was found by 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 ...
, and HD 69830 b
HD 69830 b
HD 69830 b is a Neptune-mass or Super-Earth mass extrasolar planet orbiting the star HD 69830. This planet is 10 times more massive than Earth, making it the least massive in the system. It also orbits quite close to its parent star and takes 82/3 days to complete an orbit.This is likely to be a...
with a mass of 10 Earth masses.
First super-Earth in habitable zone
In April 2007, a team headed by Stephane UdryStéphane Udry
Stéphane Udry is an astronomer at the University of Geneva 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 red dwarf star Gliese 581, approximately 20 light years...
based in Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
announced the discovery of two new super-Earths around 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:...
, both on the edge of 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...
around the star where liquid water may be possible on the surface. With 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...
having a mass of at least 5 Earth masses and a distance from Gliese 581 of 0.073 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (AU; 6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without taking into consideration effects from an atmosphere) of −3 degrees Celsius with an albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
comparable to 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...
and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggests Gliese 581 c has likely suffered a runaway greenhouse effect
Runaway greenhouse effect
A runaway greenhouse effect is not a clearly defined term, but is understood to mean an event analogous to that which is believed to have happened in the early history of Venus, where positive feedback increased the strength of its greenhouse effect until its oceans boiled away...
like Venus, but that its sister planet, 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....
, does in fact lie within the star's habitable zone, with an orbit at 0.22 AU and a mass of 7.7 Earths.
Super-Earths found in 2008
The smallest super-Earth found to date is MOA-2007-BLG-192LbMOA-2007-BLG-192Lb
MOA-2007-BLG-192Lb, occasionally shortened to MOA-192 b, is an extrasolar planet approximately 3,000 light-years away in the constellation of Sagittarius. The planet was discovered orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times Earth, it is one of...
was announced by astrophysicist David P. Bennett for the international MOA
Microlensing Observations in Astrophysics
Microlensing Observations in Astrophysics is a collaborative project between researchers in New Zealand and Japan, led by Professor Yasushi Muraki of Nagoya University. They use microlensing to observe dark matter, extra-solar planets, and stellar atmospheres from the Southern Hemisphere...
collaboration on June 2, 2008. This planet has approximately 3.3 Earth masses and orbits a brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
. It was detected by gravitational microlensing.
In June 2008, European researchers announced the discovery of three super-Earths around the star HD 40307, a star that is only slightly less massive than our 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 planets have at least the following minimum masses: 4.2, 6.7, and 9.4 times Earth's. The planets were detected by the radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
method by the HARPS (High Accuracy Radial Velocity Planet Searcher) in Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
.
In addition, the same European research team announced a planet 7.5 times the mass of Earth orbiting the star HD 181433. This star also has a Jupiter-like planet that orbits every three years.
Super-Earths found in 2009
Planet COROT-7b, with a mass estimated at 4.8 Earth masses and an orbital period of only 0.853 days, was announced on 3 February 2009. The density estimate obtained for COROT-7b points to a composition including rocky silicate minerals, similar to the four inner planets of Earth's solar system, a new and significant discovery. COROT-7b, discovered right after HD 7924 bHD 7924 b
HD 7924 b is an extrasolar planet located approximately 54 light years away in the constellation of Cassiopeia, orbiting the 7th magnitude K-type main sequence star HD 7924. It was published on January 28, 2009 and is the second planet discovered in the constellation Cassiopeia...
, is the first super-Earth discovered that orbits a main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star that is G class or larger.
The discovery of 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...
with a minimum mass
Minimum mass
In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets...
of 1.9 Earth masses was announced on April 21, 2009. It is the smallest extrasolar planet discovered around a normal star and the closest in mass to Earth. Being at an orbital distance of just 0.03 AU and orbiting its star in just 3.15 days, it is not in the habitable zone, and may have 100 times 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....
.
Additionally, 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....
, at 0.2 AU with a 67-day orbital period, has been confirmed to be within the habitable zone of the red dwarf star, making it the first exoplanet where the existence of liquid water is a real possibility.
A planet found in December 2009, GJ 1214 b, is 2.7 times as large as Earth and orbits a star much smaller and less luminous than our sun. "This planet probably does have liquid water," said David Charbonneau, a Harvard professor of astronomy and lead author of an article on the discovery. However, interior models of this planet suggest that under most conditions it will not have liquid water.
By November 2009, a total of 30 super-Earths had been discovered, 24 of which were first observed by HARPS.
Super-Earths found in 2010
Discovered on January 5, 2010, a planet HD 156668 b with a minimum massMinimum mass
In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets...
of 4.15 Earth mass
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....
es, is the second least massive planet detected by the radial velocity method. The only confirmed radial velocity planet smaller than this planet is Gliese 581 e at 1.9 Earth masses (see above). On August 24 astronomers using ESO’s HARPS instrument announced the discovery of a planetary system with up to seven planets orbiting a Sun-like star, HD 10180
HD 10180
HD 10180 is a solar-type star. A research team led by Christophe Lovis of the University of Geneva discovered that the star has at least five planets, and possibly as many as seven. It is 127 light-years away, in the southern constellation of Hydrus...
, one of which, although not yet confirmed, has an estimated minimum mass of 1.35 ± 0.23 times that of Earth, which would be the lowest mass of any exoplanet found to date orbiting a main-sequence star. Although unconfirmed, there is 98.6% probability that this planet does exist.
The National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
announced on September 29 the discovery of a fourth super-Earth (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...
) orbiting the M 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 Gliese 581. The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with a period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington. However, the existence of Gliese 581 g has been questioned by another team of astronomers, and it is currently listed as unconfirmed at The Extrasolar Planets Encyclopaedia.
Super-Earths found in 2011
On February 2, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 68 candidates of approximately "Earth-size" (Rp < 1.25 Re) and 288 candidates of "super-Earth-size" (1.25 Re < Rp < 2 Re). In addition,54 planet candidates were detected 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...
." Six candidates in this zone were less than twice the size of the Earth [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. Based on the latest Kepler findings, astronomer Seth Shostak
Seth Shostak
Seth Shostak is an American astronomer. He grew up in Arlington, VA and earned his physics degree from Princeton University and a Ph.D...
estimates "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds." Also based on the findings, the Kepler Team has estimated "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.
On August 17, a potentially habitable super-Earth HD 85512 b
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 ....
was found using the HARPS as well as a three super-Earth system 82 G. Eridani. On HD 85512 b, it would be habitable if it exhibits more than 50% cloud cover. Then less than a month later, a flood of 41 new exoplanets including 10 super-Earths were announced.
Characteristics
Due to the larger mass of super-Earths, their physical characteristics differ from Earth's. A study on Gliese 876 d by a team around Diana Valencia revealed that it would be possible to infer from a radius measured by the transit method of detecting planets and the mass of the relevant planet what the structural composition of a relevant super-Earth is. For Gliese 876Gliese 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...
, calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surface gravity between 1.9g and 3.3g (19 and 32 m/s²).
Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
due to thinner plates under more stress. In fact, their models suggested that Earth was itself a "borderline" case, just barely large enough to sustain plate tectonics. However, other studies determine that strong convection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibit plate tectonics. The planet's surface would be too strong for driving forces of magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
to break the crust into plates.
Temperatures
Since the atmospheres and greenhouse effectGreenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
s of super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, the black-body temperature of the Earth is 254.3 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(−19 °C or −2 °F ). It is the greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
es that keep the Earth warmer. Venus has a black-body temperature of only 231.7 K (−41 °C or −43 °F ) even though Venus has a true temperature of 737 K (464 °C or 867 °F ). Though the atmosphere of Venus traps more heat than Earth's, NASA lists the black-body temperature of Venus based on the fact that Venus has an extremely high albedo, giving it a lower black body temperature than the more absorbent (lower albedo) Earth.
See also
- Habitable zoneHabitable zoneIn 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...
- Extraterrestrial liquid waterExtraterrestrial liquid waterExtraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life....
- Ocean planetOcean planetAn ocean planet is a hypothetical type of planet whose surface is completely covered with an ocean of water.Planetary objects that form in the outer solar system begin as a comet-like mixture of roughly half water and half rock by mass...
- Hot NeptuneHot NeptuneA hot Neptune is an extrasolar planet in an orbit close to its star , with a mass similar to that of Uranus or Neptune. Recent observations have revealed a larger potential population of hot Neptunes than previously thought...