Gliese 581 g: the first of many Goldilocks planets (Image: L.Calçda/ESO)

SO CLOSE, yet so far. Gliese 581 g is the first planet discovered that is the right mass and distance from its star for the surface to be awash with liquid water and perhaps life. Chances are we’ll never know for sure without an armada of space telescopes, and their future looks uncertain. But a 2014 mission could tell us whether any habitable worlds with better viewing angles have signs of life.

Gliese 581 g is 20 light years from our solar system and three to four …