Super-Earth
Density and bulk composition
### Density and bulk composition

[thumb\|Comparison of sizes of planets with different compositions
](/wiki/File:Planet_sizes.svg "Planet sizes.svg")
Due to the larger mass of super\-Earths, their physical characteristics may differ from Earth's; theoretical models for super\-Earths provide four possible main compositions according to their density: low\-density super\-Earths are inferred to be composed mainly of hydrogen and helium ([mini\-Neptunes](/wiki/Mini-Neptune "Mini-Neptune")); super\-Earths of intermediate density are inferred to either have water as a major constituent ([ocean planets](/wiki/Ocean_planet "Ocean planet")), or have a denser core enshrouded with an extended gaseous envelope ([gas dwarf](/wiki/Gas_dwarf "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](/wiki/Gliese_876_d "Gliese 876 d") by a team around [Diana Valencia](/wiki/Diana_Valencia "Diana Valencia") revealed that it would be possible to infer from a radius measured by the [transit method](/wiki/Transit_method "Transit method") of detecting planets and the mass of the relevant planet what the structural composition 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](/wiki/Gravity "Gravity") between 1\.9[g](/wiki/Gravity_of_Earth "Gravity of Earth") and 3\.3g (19 and 32 m/s2). However, this planet is not known to transit its host star.

The limit between rocky planets and planets with a thick gaseous envelope is calculated with theoretical 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\-radii, 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 ). Whether or not the primitive nebula\-captured H/He envelope of a super\-Earth is entirely lost after formation also depends on the orbital distance. For example, formation and evolution calculations of the [Kepler\-11](/wiki/Kepler-11 "Kepler-11") planetary system show that the two innermost planets Kepler\-11b and c, whose calculated mass is ≈2 [M🜨](/wiki/Earth_mass "Earth mass") and between ≈5 and 6 M🜨 respectively (which are within measurement errors), are extremely vulnerable to envelope loss. In particular, the complete removal of the primordial H/He envelope by energetic stellar photons appears almost inevitable in the case of Kepler\-11b, regardless of its formation hypothesis.

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. Another discovery about exoplanets' composition is that about the [gap or rarity observed for planets between 1\.5 and 2\.0 Earth\-radii](/wiki/Small_planet_radius_gap "Small planet radius gap"), which is explained by a bimodal formation of planets (rocky Super\-Earths below 1\.75 and sub\-Neptunes with thick gas envelopes being above such radii).

Additional studies, conducted with lasers at the [Lawrence Livermore National Laboratory](/wiki/Lawrence_Livermore_National_Laboratory "Lawrence Livermore National Laboratory") and the [OMEGA](/wiki/Laboratory_for_Laser_Energetics "Laboratory for Laser Energetics") laboratory at the [University of Rochester](/wiki/University_of_Rochester "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.


What can be calculated from these determinations?