I’m a great believer in what I might call the ‘conventional’ habitable zone; i.e., a habitable zone defined by the possibility of liquid water on the surface. The definition is offered not to exclude exotic possibilities like micro-organisms floating in the clouds of Venus or aquatic life deep inside an ice-covered moon like Europa. Rather, it acknowledges that finding life is hard enough without losing our focus. In terms of exoplanets and feasible near-term study, a warm planet with liquid water — the kind we live on — would command our immediate attention.
But as we look at much broader issues of how life forms, we may indeed learn that our kind of life is but one component of a vast continuum, as recent work out of the University of Aberdeen reminds us. In a new paper published in Planetary and Space Science, researchers tackle the question of life living deep underground. Now the habitable zone starts to broaden, because things get warmer as we go deep.
We know of life here on Earth that exists more than five kilometers below the surface, and given the difficulty of probing these regions, we probably have fragmentary knowledge at best of what’s down there at deeper levels. So if we’re talking underground microorganisms, maybe a place like Gliese 581 d, evidently just past the outer edge of its star’s habitable zone in terms of liquid water, would still qualify. The Aberdeen team thinks conditions less than two kilometers below the surface there could be clement.To read more, click here.