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Super-earth |
A Super-Earth is an extrasolar planet more massive than the Earth but less massive than a gas giant. Note that the term "super-Earth" refers only to the mass of the planet and should not be taken to 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. 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.345 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.
For planets such as Mu Arae c Super-Earth status has been calculated through computer models of planet formation, but there has been no confirmation through observation.
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The first 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 that of Earth, too small to be gas giants.
The first super-Earth around a main sequence star was discovered by a team around Eugenio Rivera in 2005 and is orbiting Gliese 876 and received the designation Gliese 876 d (two Jupiter sized gas giants had been discovered in that system before). 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, it has a high surface temperature of up to 650 kelvins. 6
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
In April 2007, a team headed by Stephane Udry based in Switzerland announced the discovery of two new super-Earths around Gliese 5817, 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 or 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.
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, 2008.8, 9 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 times Earth mass, 6.7 times Earth mass and 9.4 times Earth mass. The planets were detected by the radial velocity method by the HARPS instrument in Chile. 10
In addition, the same European research team announced that they found 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. 11
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 Valencia1 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 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 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 that was just barely large enough to sustain plate tectonics.12
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