What do the sizes of the “Kepler planets” tell us about their formation process?

During the last decade, the Kepler space observatory, launched into space by NASA in 2009, was dedicated to searching for exoplanets in the solar neighborhood and managed to discover more than 2800 worlds orbiting other stars similar to our Sun. The detection method used by the mission is that of planetary transits, which allows not only to detect new planets, but also estimating their sizes. Although these have varied sizes that go from the size of our planet to the one of Jupiter, the great majority is found in two predominant groups: the Super Earths and the mini-Neptunes, with radii of 1.3 and 2.5 times the Earth’s radius, respectively.

Recently, an international team of astrophysicists that includes two by two researchers from the Millennium Nucleus of Planetary Formation, Octavio Guilera and Paula Ronco, published a paper in the journal Astronomy & Astrophysics Letter that explains for the first time, and from the perspective of planetary formation, the existence of the size distribution observed in Kepler exoplanets.

“Since 2017, when this bimodal size distribution was discovered, there has been several works that tried to explain this property by means of planetary evolution models. Such models calculate the cooling and mass loss of the atmospheres that exoplanets can suffer due to the irradiation of their host stars. However, in order for these models to correctly reproduce the two maxima of the size distribution, it is necessary that the nuclei of the planets be completely rocky, that is to say, lacking water”, indicates Octavio Guilera, who is also an Associate Researcher of Conicet at the Institute of Astrophysics of La Plata, Argentina.

Paula Ronco indicates that the problem with these results is that they contradict the models of planetary formation that predict that many of these planets should actually form in regions where water is thought to be abundant. “To try to reconcile these results, we developed a model of planetary formation and evolution, which studies in a self-consistent way from a protoplanetary disk of gas and dust, how the nuclei of the planets are formed, what their compositions are, how they form their atmospheres, and how they evolve in the disk and in time once the gaseous component from the disk has already dissipated,” explains the astrophysicist.

Credit: NASA

This work indicates that the properties of the dust that forms the nuclei of the planets are different in regions near and far from the star. In the inner region, inward of the so-called “ice line” where the temperature in the disk is high enough to sublimate the water, the cores of the planets are formed from dry condensates. In regions beyond that position, where the temperature allows water to be in a solid state, the planets grow from condensates rich in water ice. Guilera explains that during the process of planetary formation, dry particles that collide with each other break up more easily than those that are rich in ice. In this way, the ice-rich particles favor the formation of more massive, and therefore larger, planet nuclei.

“This result naturally separates the population of planets we form with our model into dry, less massive (hence smaller) planets and water-rich, more massive (hence larger) planets, reproducing the observed bimodal distribution. In addition, once formed, the planets suffer from processes that can modify their sizes. The most important ones that we incorporate in our work are, as mentioned before, the evaporation of their atmospheres due to stellar radiation and the loss of part of their shells due to collisions between planets in the same system. Including these processes, the radii of the planets we form in our simulations are predominantly those of Super-Earths and Mini-Neptunes, and they reproduce very well the population of exoplanets observed by the Kepler mission,” concludes Ronco.



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