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

  • 1. In 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).

Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations. Mousis, Olivier; Deleuil Magali; Aguichine Artyom; Marcq Emmanuel; Naar Joseph; Aguirre Lorena Acuna; Brugger Bastien, and Gonçalves Thomas. The Astrophysical Journal Letters, 15 June 2020, DOI : 10.3847/2041-8213/ab9530

Revised mass-radius relationships for water-rich rocky planets more irradiated than the runaway greenhouse limit. Turbet, Martin; Bolmont Emeline; Ehrenreich, David; Gratier, Pierre; Leconte, Jérémy; Selsis, Franck; Hara, Nathan; Lovis, Christophe. Astronomy & Astrophysics, 9 June 2020, DOI : 10.1051/0004-6361/201937151


Olivier Mousis
Chercheur Aix-Marseille Université
Jérémy Leconte
CNRS researcher
Clara Barrau
CNRS Press Officer