The reason why that's exciting, is one of the properties we'd like to have on a habitable planet is liquid water on the surface.

BBC: Tiniest extrasolar planet found

Two degrees is the difference between water being liquid and water being a solid.

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But close orbits would broadly be too hot - the hunt seeks roughly Earth-sized planets orbiting at a sufficient distance that water, if it is there, can exist in liquid form - and not so distant that it freezes.

BBC: Exoplanets near red dwarfs suggest another Earth nearer

The idea of using a liquid with a lower boiling point than water to drive a turbine is nothing new it has simply not proved worthwhile in the past.

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Given the right kind of atmosphere, it is therefore reasonable to speculate, says the team, that they might be able to sustain water in a liquid state - a generally accepted precondition for life.

BBC: Kepler telescope spies 'most Earth-like' worlds to date

"The role of liquid water on Mars is of great importance for its habitability and this study using Mars Express describes a very large zone where groundwater was present for a long time, " said Olivier Witasse, a project scientist with ESA's Mars Express.

MSN: New proof that water existed underground on ancient Mars