Julia Venturini
The existence of a Radius Valley in the Kepler size distribution stands as one of the most important observational constraints to understand the origin and composition of exoplanets with radii between that of Earth and Neptune. The goal of this presentation is to provide insights into the existence of the Radius Valley from a pure formation point of view. We run global planet formation simulations including dust growth, disc evolution, pebble and gas accretion and migration. We perform an extensive parameter study evaluating a wide range in disc properties and embryo's initial location. We find that due to the change in the dust properties at the water ice-line and the increase of the pebble isolation mass with orbital distance, rocky cores form typically with ~3 Earth masses and have a maximum mass of 5 Earth masses, while icy cores peak at ~10 Earth masses, with masses lower than 5 Earth masses being scarce. When neglecting the gaseous envelopes, the formed rocky and icy cores account naturally for the two peaks of the Kepler size distribution.
Two types of evolution models can explain the bimodality. The idea behind is that some heat source (e.g, external: photoevaporation or internal: core-powered mass-loss) produces atmospheric escape. Some planets will get so hot that will lose their atmospheres completely producing the "super-Earths", but some will retain a thin H-He atmosphere, yielding to "mini-Neptunes". Both models are able to reproduce the correct size distribution if the planets are mainly rocky in composition (Owen & Wu 2017, Gupta & Schlichting 2019 ). This led to the interpretation that these planets were formed inside the water ice line.
However, from a planet formation viewpoint, it is hard to envision scenarios producing only dry planets at short orbital periods. In general, solid accretion is enhanced beyond the ice line. I addition, super-Earth-mass planets easily undergo Type-I migration, which places water-rich objects at short orbital periods. (e.g, Alibert et al. 2013, Raymond et al. 2018, Bitsch et al. 2019).
Our study aims at shedding some light into how to reconcile theory with observations.
We performed global planet formation simulations including disc evolution, dust growth by coagulation, drift and fragmentation; icy and silicate pebble accretion, gas accretion and planet migration. We led an extensive parameter study evaluating a wide range in initial disc properties and embryo's initial location.
We find that due to the change in the dust properties at the water ice-line and the larger pebble isolation mass for larger distances form the star, rocky cores form typically with ~3 Earth masses and have a maximum mass of 5 Earth masses, while icy cores peak at ~10 Earth masses, with masses lower than 5 Earth masses being scarce. When neglecting the presence of the gaseous envelope, the formed rocky and icy cores account naturally for the two peaks of the Kepler size distribution. This suggest that some process inhibiting gas accretion or removing gas after accretion could operate efficiently at short orbital periods.