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, were long thought to be gas planets, made up of hydrogen and helium. Now, scientists at the Laboratoire d'Astrophysique de Marseille (CNRS/Aix-Marseille Université/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 state2 , suggesting that these two types of 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 Bordeaux3 , 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.

See: Depiction of a world completely covered with ocean. ©NASA Kepler Mission/Dana Berry

• 1In collaboration with a researcher at the Laboratoire Atmosphères et Observations Spatiales (CNRS/UVSQ/Sorbonne Université)
• 2 Supercritical water exists at very high pressures and temperatures.
• 3 The French scientists work at the Laboratoire d'Astrophysique de Bordeaux (CNRS/Université de Bordeaux) and used a planetary atmosphere model developed at the Laboratoire de Météorologie Dynamique (CNRS/ENS Paris/Ecole Polytechnique– Institut Polytechnique de Paris/Sorbonne Université).