Exoplanets are cool. Except those which are hot. Or better still, those which are in-between, which we call “habitable” - because none of us want to be alone, the holy-grail of exoplanet research is to describe an exoplanet in the so-called ‘habitable zone’.

So, I’ll start by describing the physics of the ‘habitable zone’, classically defined via surface water. In particular, why non-linearities in the radiative properties of water and wet atmospheres give hard boundaries to the edge of the HZ

From thereon in, physics starts to fail us. At best the HZ tells us where surface liquid water is not strongly excluded by theory. If we want to know if a planet is actually going to be habitable, we need to understand its climate and thus the composition of its atmosphere. It turns out that chemistry, geology and (gasp) even biology are running the show here. Its messy - and not clear that habitability and inhabitance are separable.

Credit

NASA/JPL-Caltech/Ames

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