Mini-Neptunes Could Be Irradiated Ocean Planets

Mini Neptune

Mini-Neptunes could be super-Earths with a rocky core surrounded by water in a supercritical state.

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 state[2], 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 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.


“Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations” by Olivier Mousis, Magali Deleuil, Artyom Aguichine, Emmanuel Marcq2, Joseph Naar, Lorena Acuña Aguirre, Bastien Brugger and Thomas Gonçalves, 15 June 2020, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ab9530

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


  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).

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