In the constellation Libra, 20.5 light years away there is a star we call Gliese 581. This is a small rather dim star which astronomers have been studying for several years, and had found two large planets circling it. One at 15 times the mass of the earth orbiting every 5.4 days and another 8 times the mass of the earth orbiting every 84 days.
They are far too small to be detected directly by present day telescopes however they do make their presence felt. As they orbit they cause the star to wobble slightly. This movement changes the colour of the light given off due to the doppler effect which can be measured showing their presence.
Gliese 581 is a very dim star so although the closer planet is much too hot for water to be liquid the more distant planet is too cold. However Xavier Deflosse and a team from Grenoble have found another planet that is about right. This third planet orbits the star every 15 days is about 5 times the mass of the earth and is probably on average about -3°C to 40°C putting it in the middle of the habitable zone. This is the first planet we have found other than the Earth that could be in this temperature range.
However at 20 light years away we are unlikely to visit soon as this is about 20 million million km and it would take the fastest probe so far built about 37,000 years to get there.
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