Hydrogen Greenhouse Planets Beyond the Habitable Zone

TL;DR

This paper demonstrates that thick hydrogen atmospheres can create greenhouse effects beyond traditional habitable zones, potentially supporting life on planets far from their stars.

Contribution

It introduces the concept that hydrogen-dominated atmospheres can sustain habitable conditions beyond classical zones, supported by radiative-convective modeling and hydrogen escape analysis.

Findings

Hydrogen atmospheres can maintain surface temperatures above freezing beyond classical habitable zones.

40 bars of H2 can keep a 3 Earth-mass planet warm at 1.5AU from M stars.

Hydrogen escape models suggest such atmospheres can persist at several AU from stars.

Abstract

We show that collision-induced absorption allows molecular hydrogen to act as an incondensible greenhouse gas, and that bars or tens of bars of primordial H2-He mixtures can maintain surface temperatures above the freezing point of water well beyond the "classical" habitable zone defined for CO2 greenhouse atmospheres. Using a 1-D radiative-convective model we find that 40 bars of pure H2 on a 3 Earth-mass planet can maintain a surface temperature of 280K out to 1.5AU from an early-type M dwarf star and 10 AU from a G-type star. Neglecting the effects of clouds and of gaseous absorbers besides H2, the flux at the surface would be sufficient for photosynthesis by cyanobacteria (in the G star case) or anoxygenic phototrophs (in the M star case). We argue that primordial atmospheres of one to several hundred bars of H2-He are possible, and use a model of hydrogen escape to show that such atmospheres are likely to persist further than 1.5 AU from M stars, and 2 AU from G stars, assuming these planets have protecting magnetic fields. We predict that the microlensing planet OGLE-05-390L could have retained a H2-He atmosphere and be habitable at ~2.6 AU from its host M star.