天文学：水素とヘリウムに富む系外惑星は数十億年にわたって生命が存在可能な条件をもたらす可能性がある

Rocky exoplanets with atmospheres dominated by hydrogen and helium gases can sustain temperate conditions and liquid water on their surface for billions of years, according to a modelling study published in Nature Astronomy. These findings suggest that even planets with very different atmospheres from ours can be potentially habitable for long periods of their history.

As hydrogen and helium gases were readily available in the planet-forming disk of materials around young stars, all planets accumulated atmospheres that were dominated by these two elements. In our Solar System, rocky planets lost this primordial atmosphere in favour of heavier elements, such as oxygen and nitrogen on Earth. However, large rocky exoplanets at some distance from their star could retain their hydrogen and helium-dominated atmospheres.

Marit Mol Lous and colleagues investigated the evolution of such planets. The team used a numerical model to predict the duration that hydrogen and helium-rich exoplanets could host liquid water on their surface. The authors reveal that, depending on the mass of the planet and how far away it is from its star, these planets could keep a temperate surface environment for as long as 8 billion years, provided that the atmosphere is thick enough (between 100 to 1,000 times thicker than the Earth’s).

The authors highlight that although future research is needed to address many remaining questions — such as the likelihood of these planets forming or how liquid water would get there — these results suggest that habitable conditions may be very different from what we are used to on Earth. We therefore, they state, need to remain open-minded when investigating habitability on other planets.