The Mars rover, Curiosity, is the latest in a long line of missions to Mars: landers sent to scoop its soil and study its rocks, orbiters sent to map its valleys and ridges.

They are all asking the same question. Did liquid water once flow on this dry and dusty world? Did it support life in any form? And are there remnants left to find? The science that comes out of these missions may help answer a much larger, more philosophical question.

Is our planet Earth the norm, in a galaxy run through with life-bearing planets? Or is Earth a rare gem, with a unique make-up and history that allowed it to give rise to living things?