5 The habitable zone
Earth is currently the only place in the whole Universe we know of that harbours life. Further discoveries in our Solar System or beyond may come in the not too distant future, but for now it makes sense to assume that ‘Earth-like’ planets are the best places to look for signs of life.
From current theories about planetary formation and structure, as well as from studies of exoplanets with known densities, scientists believe that rocky, terrestrial planets must be relatively small. Planets larger than about 1.5–2 RE would be expected to accumulate a substantial gaseous envelope and so be more akin to Neptune.
As well as a planet being small and rocky, there are other things we need to consider when we’re deciding whether or not it’s likely to be habitable.
Habitability is a very complicated issue because we still don’t know exactly what conditions were necessary for life to evolve on Earth, but one thing scientists think is likely to be very important is the presence of liquid water. All living things on Earth need liquid water to survive, and the average human body is made up of around 60 per cent liquid water.