The Exoplanet Perspective on Future Ice Giant Exploration

TL;DR

This paper argues that studying our solar system's ice giants is essential for understanding the most common exoplanets, mini-Neptunes and super-Earths, by providing ground truth data on their formation, composition, and atmospheres.

Contribution

It highlights the importance of ice giant exploration for interpreting exoplanet observations and understanding the formation and evolution of common exoplanet types.

Findings

Exoplanets between Neptune and Earth size are most common.

Atmospheric measurements of mini-Neptunes reveal clouds and molecular absorption.

Understanding ice giants in our solar system can inform exoplanet characterization.

Abstract

Exoplanets number in their thousands, and the number is ever increasing with the advent of new surveys and improved instrumentation. One of the most surprising things we have learnt from these discoveries is not that small-rocky planets in their stars habitable zones are likely common, but that the most typical size of exoplanet is that not seen in our solar system - radii between that of Neptune and the Earth dubbed mini-Neptunes and super-Earths. In fact, a transiting exoplanet is four times as likely to be in this size regime than that of any giant planet in our solar system. Investigations into the atmospheres of giant hydrogen/helium dominated exoplanets has pushed down to Neptune and mini-Neptune sized worlds revealing molecular absorption from water, scattering and opacity from clouds, and measurements of atmospheric abundances. However, unlike measurements of Jupiter, or even Saturn sized worlds, the smaller giants lack a ground truth on what to expect or interpret from their measurements. How did these sized worlds form and evolve and was it different from their larger counterparts? What is their internal composition and how does that impact their atmosphere? What informs the energy budget of these distant worlds? In this we discuss what characteristics we can measure for exoplanets, and why a mission to the ice giants in our solar system is the logical next step for understanding exoplanets.