Beyond Earth, what's the most likely site of life?

Asked by: Mark Pitkin, London

That depends on what you mean by life. If you mean 'life as we know it', the best bet is Mars as it’s the most Earth-like of the other planets.

But even the Earth is home to so-called extremophiles – organisms that can cope with extraordinary temperatures or radiation – so we shouldn’t be surprised if life exists elsewhere in the Solar System.