Composition of Transiting and Transiting-only super-Earths

TL;DR

This paper examines the compositions of transiting super-Earths and transiting-only planets, using mass and radius data to infer their internal structures and volatile contents, highlighting the diversity and challenges in characterizing these exoplanets.

Contribution

It introduces methods to constrain compositions of super-Earths and transiting-only planets using observational data, addressing degeneracies and providing estimates of volatile contents.

Findings

Kepler-10b has an Earth-like to Mercury-like composition.

GJ 1214b requires a volatile envelope, likely with a gaseous atmosphere.

Kepler-9d has less than 50% water vapor or 0.1% hydrogen/helium by mass.

Abstract

The relatively recent detections of the first three transiting super-Earths mark the beginning of a subfield within exoplanets that is both fruitful and challenging. The first step into characterizing these objects is to infer their composition given the degenerate character of the problem. The calculations show that Kepler-10b has a composition between an Earth-like and a Mercury-like (enriched in iron) composition. In contrast, GJ 1214b is too large to be solid, and has to have a volatile envelope. Lastly, while three of the four reported mass estimates of CoRoT-7b allow for a rocky composition, one forbids it and can only be reconciled with significant amounts of water vapor. In addition to these three transiting low-mass planets, there are now more than one thousand Kepler planets with only measured radius. Even without a mass measurement ("transiting-only") it is still possible to place constraints on the amount of volatile content of the highly-irradiated planets, as their envelopes, if present, are flared. Using Kepler-9d as an example, we estimate its water vapor, or hydrogen and helium content to be less than 50% or 0.1% by mass respectively.