In situ formation of super-Earth/sub-Neptune driven by the planetary rotation

TL;DR

This paper investigates how planetary rotation can delay gas accretion, allowing super-Earths and sub-Neptunes to form without becoming gas giants, by affecting the planet's structure and cooling processes.

Contribution

It introduces a model showing planetary rotation's role in delaying runaway gas accretion, a novel mechanism for super-Earth/sub-Neptune formation.

Findings

Rotation reduces the radiative-convective boundary temperature.

Rotational effects extend the planetary cooling timescale beyond 10 Myr.

Lower angular velocities in dusty atmospheres also promote super-Earth formation.

Abstract

Kepler's observation shows that many of the detected planets are super-Earths. They are inside a range of critical masses overlapping the core masses (2-20 $M_{\bigoplus}$), which would trigger the runaway accretion and develop the gas giants. Thus, super-Earths/sub-Neptunes can be formed by restraining runaway growth of gaseous envelopes. We assess the effect of planetary rotation in delaying the mass growth. The centrifugal force, induced by spin, will offset a part of the gravitational force and deform the planet. Tracking the change in structure, we find that the temperature at the radiative-convective boundary (RCB) is approximate to the boundary temperature. Since rotation reduces the radiation energy densities in the convective and radiative layers, RCB will penetrate deeper. The cooling luminosity would decrease. Under this condition, the evolutionary timescale can exceed the disk lifetime (10 Myr), and a super-Earth/sub-Neptune could be formed after undergoing additional mass loss processes. In the dusty atmosphere, even a lower angular velocity can also promote a super-Earth/sub-Neptune forming. Therefore, we conclude that rotation can slow down the planet's cooling and then promote a super-Earth/sub-Neptune forming.