Could mini-Neptunes be irradiated ocean planets?

Many exoplanets known today are 'super-Earths,' with a radius 1.3 times that of Earth, and 'mini-Neptunes,' with 2.4 Earth radii. Mini-Neptunes, which are less dense, was long thought to be gas planets, made up of hydrogen and helium.

Now, scientists at the Laboratoire d'Astrophysique de Marseille (CNRS/Aix-Marseille Universite/CNES) [1] have examined a new possibility, namely that the low density of mini-Neptunes could be explained simply by the presence of a thick layer of water that experiences an intense greenhouse effect caused by the irradiation from their host star.

These findings, recently published in The Astrophysical Journal Letters, show that mini-Neptunes could be super-Earths with a rocky core surrounded by water in a supercritical state [2], suggesting that these two types of an exoplanet may form in the same way.

Another paper recently published in Astronomy & Astrophysics, involving in France scientists mainly from the CNRS and the University of Bordeaux [3], focused on the effect of stellar irradiation on the radius of Earth-sized planets containing water. Their work shows that the size of the atmospheres of such planets increases considerably when subject to a strong greenhouse effect, in line with the study on mini-Neptunes.

Future observations should make it possible to test these novel hypotheses put forward by French scientists, who are making major contributions to our knowledge of exoplanets.

Notes:[1] In collaboration with a researcher at the Laboratoire Atmospheres et Observations Spatiales (CNRS/UVSQ/Sorbonne Universite).

[2] Supercritical water exists at very high pressures and temperatures.

[3] The French scientists work at the Laboratoire d'Astrophysique de Bordeaux (CNRS/Universite de Bordeaux) and used a planetary atmosphere model developed at the Laboratoire de Meteorologie Dynamique (CNRS/ENS Paris/Ecole Polytechnique-Institut Polytechnique de Paris/Sorbonne Universite).

( With inputs from ANI )