In 1961 US radio astronomer Frank Drake developed an equation to help estimate the number of planets hosting intelligent life - and capable of communicating with us - in the galaxy.
The Drake equation multiplies together seven factors including: the formation rate of stars like our Sun, the fraction of Earth-like planets and the fraction of those on which life develops. Many of these figures are open to wide debate, but Drake himself estimates the final number of communicating civilisations in the galaxy to be about 10,000.
In 2001, a more rigorous estimate of the number of life-bearing planets in the galaxy - using new data and theories - came up with a figure of hundreds of thousands. For the first time, the researchers estimated how many planets might lie in the "habitable zone" around stars, where water is liquid and photosynthesis possible. The results suggest that an inhabited Earth-like planet could be as little as a few hundred light years away.
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