12-12-2012, 04:56 PM
Super-Earth
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Definition
In general, super-Earths are defined exclusively by their mass, and the term does not imply temperatures, compositions, orbital properties, habitability, or environments similar to that of Earth. A variety of specific mass values are cited in definitions of super-Earths. While sources generally agree on an upper bound of 10 Earth masses(~69% of the mass of Uranus, which is the Solar System gas giant with the least mass), the lower bound varies from 1 or 1.9to 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, or planets that have not just atmospheres but also solid surfaces, which the four giant planets in our solar system do not have. Planets above 10 Earth masses are termed giant planets.. By informal convention, giant planets may be subdivided into "super-Jupiters" (more than 2-3 Jupiter masses up to brown dwarf mass), "Jupiters" (like Jupiter and Saturn, greater than 30 Earth masses), and "Neptunes" (of a mass similar to Uranus and Neptune, of 10-30 Earth masses).
Discoveries
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.First super-Earth found
First super-Earth in habitable zone
In April 2007, a team headed by StéphaneUdry 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 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.
More notable super-Earth discoveries by year
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..
2008
The smallest super-Earth found as of 2008 is MOA-2007-BLG-192Lb. The planet 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.