Icarus 277, 215–236 (2016)

Earlier this year, the NASA Kepler mission announced the discovery of 1,284 new planets. Nine of these reside in the 'habitable zone', that is, liquid water can pool on them. But that criterion alone, Lena Noack and colleagues say, isn't sufficient for making an exoplanet a potential birthplace for life as we know it.

Based on a novel ocean model for water-rich planets, Noack et al. argue that the formation of high-pressure ice has to be factored in when assessing habitability. On Earth-sized planets such ice phases form for oceans with depths on the order of hundreds of kilometres. The dense ice layer can then separate the liquid water from the mineral surface at the ocean–mantle boundary, shutting off a vital source of nutrients and building blocks.

Nonetheless, Earth-size planets could still be habitable for water layers up to almost 1,000 km, provided that the submarine surface is hot enough to melt the ice. In that scenario, a second ocean layer would form below the ice — although from the outside it would be difficult to detect signs of life there.