Accretion and Evolution of ~2.5 Earth-mass Planets with Voluminous H/He Envelopes

TL;DR

This paper models the formation and evolution of ~2.5 Earth-mass planets with thick H/He envelopes, comparing their final radii to Kepler observations and exploring formation scenarios including in situ formation and migration.

Contribution

It provides detailed numerical simulations of Neptune-sized planets with voluminous gaseous envelopes, analyzing their formation locations, evolution, and comparison with Kepler data.

Findings

Final planet radii match observed values at different temperatures.

Formation can occur within 2 Myr at various distances from the star.

Migration may be necessary to explain certain observed planets like Kepler-11 f.

Abstract

Formation of planets in the Neptune size range with low-mass, but voluminous, H_2/He gaseous envelopes is modeled by detailed numerical simulations according to the core-nucleated accretion scenario. Formation locations ranging from 0.5 to 4 AU from a star of 1 solar mass are considered. The final planets have heavy-element cores of 2.2--2.5 Earth masses and envelopes in the range 0.037--0.16 Earth masses. After the formation process, which lasts 2 Myr or less, the planets evolve at constant mass up to an age of several Gyr. For assumed equilibrium temperatures of 250, 500, and 1000 K, their calculated final radii are compared with those observed by the Kepler spacecraft. For the particular case of Kepler-11 f, we address the question whether it could have formed in situ or whether migration from a formation location farther out in the disk is required.