Super-Earth

From Wikipedia, the free encyclopedia

Illustration of two hypothetical super-Earths, each with an average density exceeding Earth's. The term "super-Earth" applies to both because each has more mass than the Earth.

A super-Earth is an extrasolar planet more massive than the Earth, but theoretically less massive than a gas giant. The term super-Earth refers only to the mass of the planet and does not imply anything about the surface conditions or habitability: in particular it does not imply that the planet would have a similar temperature or environment to Earth.

[edit] Definition problems

There is some uncertainty regarding the specific mass range to which this label should apply: Valencia et al. (2007) define a super-Earth as a terrestrial planet with a mass between 1 and 10 Earth masses,^[1] while Fortney et al. (2007) use 5 to 10 Earth masses,^[2] with various other definitions appearing in the popular media.^[3][4][5] There have been several discoveries of super-Earths since the 2005 discovery of Gliese 876 d by a team led by Eugenio Rivera. The Solar System 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. Currently, the most promising technique for finding super-Earths is gravitational microlensing.^[6]

For borderline planets such as Mu Arae c (min mass 10.5 M_E) Super-Earth status has been calculated through computer models of planet formation, but there has been no confirmation through observation.

[edit] Discoveries

[edit] First super-Earth found

The first super-Earths (by mass; they are seldom called "super-Earths") were discovered by Aleksander Wolszczan and Dale Frail around the pulsar PSR B1257+12 in 1991. 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 has a high surface temperature of up to 650 kelvins and may support liquid water.^[7]

[edit] Other super-Earths discovered in 2006

Two further super-Earths were discovered in 2006, OGLE-2005-BLG-390Lb 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.^[1]

[edit] First super-Earth in habitable zone

In April 2007, a team headed by Stephane Udry based in Switzerland announced the discovery of two new super-Earths around Gliese 581,^[8] 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 units (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.

[edit] Super-Earths found in 2008

The smallest Super-Earth found to date is MOA-2007-BLG-192Lb was announced by astrophysicist David P. Bennett for the international MOA collaboration on June 2, 2008.^[9][10] 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.^[11]

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.^[12]

[edit] 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.^[13]

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,^[14] and may have 100 times more tidal heating than Jupiter’s volcanic satellite Io.^[15]

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.^[14]

A planet found in December 2009, GJ 1214b, 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.^[16]

Through November 2009, a total of 30 Super-Earths have been discovered, 24 of which were first observed by HARPS.^[17]

[edit] 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^[1] revealed 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 876 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. High surface gravity (generally higher than Neptune-, Uranus-, and Saturn-class planets, and in certain circumstances higher than Jupiter-class planets) is one of the predominant known characteristics of super-Earths.

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.^[18]

[edit] Host stars

The known super-Earth host stars are all K and M dwarfs in the mass range 0.31–0.83 Solar mass.^[19] All of the detected super-Earths orbit metal-poor stars.^[19]

[edit] See also

• Extraterrestrial liquid water
• Ocean planet
• Hot Neptune

[edit] References

v • d • e Exoplanets Classes Carbon planet • Chthonian planet • Circumbinary planet • Cold Jupiter • Cold Neptune • Desert planet • Eccentric Jupiter • Goldilocks planet • Hot Jupiter • Hot Neptune • Jovian planet • Ocean planet • Pulsar planet • Super-Earth • Terrestrial planet Systems Binary star • Hypothetical extrasolar planet • Interstellar planet • Planetary system • Extrasolar moon Missions COROT • Kepler Mission • PEGASE • Space Interferometry Mission • New Worlds Mission • Terrestrial Planet Finder • Darwin (ESA) See also: Stars with extrasolar planets • Extrasolar planet extremes • Unconfirmed exoplanets