What is super-Earth ?

A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of the Solar System's smaller ice giants Uranus and Neptune, which are 15 and 17 Earth masses respectively. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, as suggested by MIT professor Sara Seager, although mini-Neptunes is more common.In general, super-Earths are defined exclusively by their mass, and the term does not imply temperatures, compositions, orbital properties, habitability, or environments 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 1or 1.9 to 5,with various other definitions appearing in the popular media.The term Super-Earth also is used by astronomers to define planets bigger than Earth-like planets (from 0.8 Earth-radii till 1.25), but smaller than mini-Neptunes (from 2 Earth-radii till 4).

This definition was made by the Kepler Mission.  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 or oceans with a sharp boundary between liquid and atmosphere, which the four giant planets in our solar system do not have. Planets above 10 Earth masses are termed massive solid planets /mega-Earths or gas giant planets depending on whether they are mostly rock/ice or mostly gas. Due to the larger mass of super-Earths, their physical characteristics may differ from Earth's; theorical models for super-Earths provide four possible main compositions according their density: low density super-Earths are inferred to be composed mainly of hydrogen and helium (Mini-Neptunes); super-Earths of intermediate density are inferred to either have water as a major constituent (Ocean planets), or have a denser core enshrouded with an extended gaseous envelope (Gas dwarf or sub-Neptune). A super-Earth of high density is believed to be rocky and/or metallic, like Earth and the other terrestrial planets of the Solar System. A super-Earth's interior could be undifferentiated, partially differentiated, or completely differentiated into layers of different composition. Researchers at Harvard Astronomy Department have developed user-friendly online tools to characterize the bulk composition of the super-Earths. 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 d, 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).

The limit between rocky planets and planets with a thick gaseous envelope is calculated with theorical models. Calculating the effect of the active XUV saturation phase of G-type stars over the loss of the primitive nebula-captured hydrogen envelopes in extrasolar planets, it's obtained that planets with a core mass of more than 1.5 Earth-mass (1.15 Earth-radius max.), most likely cannot get rid of their nebula captured hydrogen envelopes during their whole lifetime. Other calculations point out that the limit between envelope-free rocky super-Earths and sub-Neptunes is around 1.75 Earth-radius, as 2 Earth-radii would be the upper limit to be rocky (a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 20 kbars).

If a super-Earth is detectable by both the radial-velocity and the transit methods, then both its mass and its radius can be determined; thus its average bulk density can be calculated. The actual empirical observations are giving similar results as theoretical models, as it's found that planets larger than approximately 1.6 Earth-radius (more massive than approximately 6 Earth-masses) contain significant fractions of volatiles or H/He gas (such planets appear to have a diversity of compositions that is not well-explained by a single mass-radius relation as that found in rocky planets). After measuring 65 super-Earths smaller than 4 Earth-radii, the empirical data points out that Gas Dwarves would be the most usual composition: there is a trend where planets with radii up to 1.5 Earth-radii increase in density with increasing radius, but above 1.5 radii the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles by volume overlying a rocky core. Similar results are confirmed by other studies.

Additional studies, conducted with lasers at the Lawrence Livermore National Laboratory and at the OMEGA laboratory at the University of Rochester show that the magnesium-silicate internal regions of the planet would undergo phase changes under the immense pressures and temperatures of a super-Earth planet, and that the different phases of this liquid magnesium silicate would separate into layers.