The Compositional Diversity of Low-Mass Exoplanets

TL;DR

Low-mass exoplanets exhibit a wide range of compositions, from rocky to gaseous, with ongoing advancements in measurement techniques revealing their diversity and challenging existing models.

Contribution

This paper reviews the diversity of low-mass exoplanet compositions and discusses how improved measurements are informing and challenging planetary formation and atmospheric models.

Findings

Measurement precisions are approaching theoretical uncertainties.

Some low-density low-mass planets challenge atmospheric loss models.

Planet sizes may not be strictly separated by composition.

Abstract

Low-mass planets have an extraordinarily diverse range of bulk compositions, from primarily rocky worlds to those with deep gaseous atmospheres. As techniques for measuring the masses of exoplanets are advancing the field towards the terrestrial regime, from ultra-short orbital periods to Venus-like distances, we identify the bounds on planet compositions, where sizes and incident fluxes inform bulk planet properties. In some cases, measurement precisions of planet masses and sizes are approaching the theoretical uncertainties in planet models. An emerging picture explains aspects of the diversity of low-mass planets although some problems remain; do extreme low density low-mass planets challenge models of atmospheric mass loss? Are planet sizes strictly separated by bulk composition? Why do some stellar characterizations differ between observational techniques? As the TESS mission begins, low-mass exoplanets around the nearest stars will soon be discovered and characterized with unprecedented precision, permitting more detailed planetary modeling and atmospheric characterization of low mass exoplanets than ever before.