Allona Vazan - October 20, 2017

Mass-Radius Diversity of Sub-Neptune Planets: the Role of the Rocky Core

Sub-Neptune planets are very common in our galaxy and show a great scatter in
their mass-radius relation. Most studies attempt to relate this diversity to
planet composition and irradiation by the parent star. However, the thermal
evolution of the planet, and in particular the thermal evolution of the rocky
part can play a key role in this diversity. The mass-radius relation of
sub-Neptune planets with a gaseous envelope is more sensitive to planetary thermal
evolution than other planet types. In sub-Neptunes the rocky part (hereafter core)
is the dominant energy reservoir, while the gaseous envelope determines the
planet's radius. We calculate the thermal evolution of sub-Neptunes, accounting
for different core formation conditions and different heat transport rates and
examine the effects on the planetary mass-radius relation in time. We find that
the radius can substantially vary for similar planets with different core thermal
properties. We show that the timescale of core cooling can be up to billions of
years and therefore overlaps with the ages which we observe planets. We conclude
that the planet formation scenario and the thermal evolution of the core can have
a substantial and long term effects on planet radius and cannot be neglected in
evolutionary calculations of sub-Neptune planets.