Inversion of Hydrogen-rich Atmosphere and Water Content for GJ 486b

TL;DR

This study models atmospheric escape on GJ 486b to understand how initial hydrogen-rich atmospheres and water content influence current water levels, revealing degeneracies and age dependencies.

Contribution

It introduces a method to invert for initial atmospheric and water conditions of GJ 486b using broad parameter space scans and atmospheric escape modeling.

Findings

Degeneracy between water reservoir and initial hydrogen-rich atmosphere.

Hydrogen-rich atmosphere delays water escape and reduces water inventory.

Inferred star age is approximately 2.9 Gyr, consistent with other methods.

Abstract

GJ~486b is a close-in planet orbiting an M dwarf and is therefore expected to have undergone strong atmospheric escape. Motivated by theoretical and observational studies on the constraints of its water and atmosphere, we investigate which combinations of an primordial hydrogen-rich atmosphere and water inventory could fit the current water content implied by bulk density measurements. We model the atmosphere escape using VPLanet, following the loss of an initial hydrogen-rich atmosphere and the subsequent escape of a water-dominated atmosphere. By scanning a broad parameter space across different stellar ages, we invert for the initial hydrogen-rich atmospheric mass and water inventory consistent with the current constraints. Our results reveal a strong degeneracy between the water reservoir and the initial hydrogen-rich atmosphere. Even a modest hydrogen-rich atmosphere can significantly delay early escape of the water and reduce the water inventory required to reproduce the current water content. We also find that the inferred initial conditions are also strongly age dependent. Incorporating a planet formation dataset as a prior, we derive a probabilistic constraint on the host star age, yielding an expected age of $2.90^{+2.47}_{-2.27}$~Gyr, which is consistent with the results obtained from other methods to determine M dwarf ages.