Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres

TL;DR

This paper demonstrates that the mass-radius relationship for small exoplanets with thick atmospheres is highly degenerate, making it difficult to determine their composition solely from these measurements.

Contribution

It quantifies how small gas envelopes significantly alter planetary radii, highlighting the ambiguity in inferring composition from mass and radius alone.

Findings

Adding 0.1%-10% gas by mass increases radius by 5-60%.

Different compositions can produce identical mass-radius signatures.

Water worlds cannot be reliably identified without additional data.

Abstract

The bulk composition of an exoplanet is commonly inferred from its average density. For small planets, however, the average density is not unique within the range of compositions. Variations of a number of important planetary parameters--which are difficult or impossible to constrain from measurements alone--produce planets with the same average densities but widely varying bulk compositions. We find that adding a gas envelope equivalent to 0.1%-10% of the mass of a solid planet causes the radius to increase 5-60% above its gas-free value. A planet with a given mass and radius might have substantial water ice content (a so-called ocean planet) or alternatively a large rocky-iron core and some H and/or He. For example, a wide variety of compositions can explain the observed radius of GJ 436b, although all models require some H/He. We conclude that the identification of water worlds based on the mass-radius relationship alone is impossible unless a significant gas layer can be ruled out by other means.