Cosmic Hydrogen and Ice Loss Lines

TL;DR

This paper explains the temperature-dependent trends in exoplanet mass-radius data by analyzing hydrogen escape mechanisms and introduces two Cosmic Hydrogen and Ice Loss Lines (CHILLs) that mark significant loss thresholds.

Contribution

It presents a new framework linking planetary composition, escape processes, and the mass-radius diagram, highlighting the role of hydrogen loss in planetary evolution.

Findings

Identification of two CHILLs in the mass-radius diagram.

Hydrogen escape significantly influences planetary radii.

Primordial H2 envelopes are gradually lost over time.

Abstract

We explain the overall equilibrium-temperature-dependent trend in the exoplanet mass-radius diagram, using the escape mechanisms of hydrogen and relevant volatiles, and the chemical equilibrium calculation of molecular hydrogen (H$_2$) break-up into atomic hydrogen (H). We identify two Cosmic Hydrogen and Ice Loss Lines (CHILLs) in the mass-radius diagram. Gas disks are well known to disperse in ten million years. However, gas-rich planets may lose some or almost all gas on a much longer timescale. We thus hypothesize that most planets that are born out of a hydrogen-gas-dominated nebular disk begin by possessing a primordial H$_2$-envelope. This envelope is gradually lost due to escape processes caused by host-stellar radiation.