Mass-Radius Relationships for Very Low Mass Gaseous Planets

TL;DR

This paper investigates the upper limits of radii for low-mass, gaseous exoplanets, using interior structure models and thermal evolution simulations to understand their physical characteristics and stability.

Contribution

It provides a physically grounded upper limit for planetary radii at low masses and moderate temperatures, and constructs mass-radius relationships for such planets.

Findings

Giant planet radii can correspond to planets less massive than Uranus and Neptune.

Low-mass, multi-billion year old planets with H/He envelopes can have giant-planet-sized radii.

Stability analyses suggest possible existence of low-mass, gas-dominated planets.

Abstract

Recently, the Kepler spacecraft has detected a sizable aggregate of objects, characterized by giant-planet-like radii and modest levels of stellar irradiation. With the exception of a handful of objects, the physical nature, and specifically the average densities, of these bodies remain unknown. Here, we propose that the detected giant planet radii may partially belong to planets somewhat less massive than Uranus and Neptune. Accordingly, in this work, we seek to identify a physically sound upper limit to planetary radii at low masses and moderate equilibrium temperatures. As a guiding example, we analyze the interior structure of the Neptune-mass planet Kepler-30d and show that it is acutely deficient in heavy elements, especially compared with its solar system counterparts. Subsequently, we perform numerical simulations of planetary thermal evolution and in agreement with previous studies, show that generally, 10 - 20 Earth mass, multi-billion year old planets, composed of high density cores and extended H/He envelopes can have radii that firmly reside in the giant planet range. We subject our results to stability criteria based on extreme ultraviolet radiation, as well as Roche-lobe overflow driven mass-loss and construct mass-radius relationships for the considered objects. We conclude by discussing observational avenues that may be used to confirm or repudiate the existence of putative low mass, gas-dominated planets.