Low-mass stars seem to be the hotbed for habitable planets, so two astrophysicists created a sophisticated simulation to understand the properties of the exoplanets forming around these small objects.

The researchers, from the University of Bern, played with formation and composition models derived from likely evolution scenarios. The simulation suggests that most planets around low-mass stars are Earth-sized and also surprisingly rich in water, with 90 percent of the simulated objects having 500 times more water than Earth.

“While liquid water is generally thought to be an essential ingredient, too much of a good thing may be bad,” co-author Professor Willy Benz said in a statement.  

This simulation, whose results are going to be published in Astronomy & Astrophysics, is in good agreement with the discoveries of the last few months, like Proxima Centauri and Trappist-1.

“Our models succeed in reproducing planets that are similar in terms of mass and period to the ones observed recently,” added Professor Yann Alibert. “Interestingly, we find that planets in close-in orbits around these types of stars are of small sizes. Typically, they range between 0.5 and 1.5 Earth radii with a peak at about 1.0 Earth radius. Future discoveries will tell if we are correct!”

Their results are quite different from the expected planet formation around smaller stellar objects. It has been traditionally assumed that these exoplanets would be very small and very dry due to being so close to their stars.

The researchers included the potential migration of planets in their simulation, which has not often been considered for smaller planets. Their starting point involved having 10 moon-sized planetesimals and letting them evolve and interact freely.

The research shows that the exoplanets' initial properties, evolution of the disk, and their relationship with the mass of their star are the biggest discriminants in the mass, size, and composition of these simulated exoplanets.

“Habitable or not, the study of planets orbiting very low mass stars will likely bring exciting new results, improving our knowledge of planet formation, evolution, and potential habitability,” concluded Benz.

More observation campaigns are planned for Trappist-1 and Proxima, so we might soon find out how close these models are to reality.