Evolutionary Models of Super-Earths and Mini-Neptunes Incorporating Cooling and Mass Loss

TL;DR

This study models the evolution of super-Earths and mini-Neptunes considering cooling and mass loss, revealing key factors influencing their observed properties and providing insights into their formation histories.

Contribution

It introduces comprehensive models that incorporate radiative cooling and XUV-driven mass loss, analyzing their effects on planetary evolution and matching observations of specific exoplanets.

Findings

Orbital distance and initial envelope mass are critical for radius evolution.

Mass loss scenarios are necessary to match observed exoplanet properties.

Formation beyond the snow line with inward migration explains Kepler-11 planets.

Abstract

We construct models of the structural evolution of super-Earth- and mini-Neptune-type exoplanets with hydrogen-helium envelopes, incorporating radiative cooling and XUV-driven mass loss. We conduct a parameter study of these models, focusing on initial mass, radius, and envelope mass fractions, as well as orbital distance, metallicity, and the specific prescription for mass loss. From these calculations, we investigate how the observed masses and radii of exoplanets today relate to the distribution of their initial conditions. Orbital distance and initial envelope mass fraction are the most important factors determining planetary evolution, particular radius evolution. Initial mass also becomes important below a "turnoff mass," which varies with orbital distance, with mass-radius curves being approximately flat for higher masses. Initial radius is the least important parameter we study, with very little difference between the hot start and cold start limits after an age of 100 Myr. Model sets with no mass loss fail to produce results consistent with observations, but a plausible range of mass loss scenarios is allowed. In addition, we present scenarios for the formation of the Kepler-11 planets. Our best fit to observations Kepler-11b and Kepler-11c involves formation beyond the snow line, after which they moved inward, circularized, and underwent a reduced degree mass loss.