Cosmic hydrogen and ice loss lines.

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. [ABSTRACT FROM AUTHOR]