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A super-Earth is a hypothetical planetary object which may be orbiting a star, and which may have a mass of between 2 to 5 and 10 Earth masses and is therefore in between Neptune class planets—that are normally quite hot and not icy—and Earth-sized planets.

There have been several discoveries of (mathematically calculaded, possible) 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.

Several other planets, such as Mu Arae d, are speculated to be super-Earths due to computer models of planet formation, but there has been no confirmation.

Discoveries

First super-Earth found

The first super-Earths were discovered by Wolszczan and 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 highly probable 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.

Other super-Earths discovered in 2006

Two further probable super-Earths were discovered in 2006, OGLE-2005-BLG-390Lb with a predicted mass of 5.5 Earth masses, which was found by microlensing and HD 69830 b with a mass of 10 Earth masses.

First super-Earths on fringe of 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, 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 probable mass of 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 a possible atmosphere) of −3 degrees Celsius with an albedo comparable to Venus and 40 degrees Celsius with an albedo comparable to Earth. Both these values are very speculative and under heated debate, as the values for Venus' Albedo are from an atmospher composed of 96.515% greenhouse-gas-atmosphere and no Water.

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 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.

References

1. ^ Valencia et al., Radius and structure of the first super-earth planet, September 2006, published in The Astrophysical Journal, February 2007 Cite error: The named reference "Valencia" was defined multiple times with different content (see the help page).
2. Canada's orbiting telescope tracks mystery 'super Earth', Hamilton Spectator, April 29, 2007
3. Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits, Fortney et al., April 2007
4. 7.5 M Planet orbiting the nearby star GJj 876 Rivera et al., 2005
5. Udry; et al. (2007). "The HARPS search for southern extra-solar planets, XI. An habitable super-Earth (5 M_⊕) in a 3-planet system" (PDF). Astronomy and Astrophysics. preprint: preprint. {{cite journal}}: Explicit use of et al. in: |author= (help)

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