The Kepler space observatory found at 600 light years from Earth the first planet in the 'habitable zone'. The planet, Kepler-22b, is the smallest one found by the spacecraft of a star similar to the Earth's one. It's bigger than Earth and has not yet been determined whether it is rocky, gaseous or liquid. Kepler detects planets and planet candidates by measuring dips in the brightness of 150,000 stars observed to see whether they pass in front of their stars (transit).