Over the past 15 years, scientists have discovered more than 500 planets outside our solar system. They're still looking for one that orbits a star at the right 'Goldilocks zone' - not too cold and not too hot - just like the earth and its distance to the sun.

The last milestone discovery took place just two weeks ago, when Nasa's Kepler mission found the smallest planet outside our solar system. Shostak said it was 'very likely that there is a great deal of alien life in the depths of space. There might even be some nearby.'

Places close by - astronomically speaking - such as the three moons of Jupiter and the two moons of Saturn, possibly contain liquid water. If so, they may have microbial life, or life you would need a microscope to see.

But in star systems at least tens of light years away, there could be life that has evolved to be intelligent. 'Our experiments are designed to find evidence of such life, although we haven't been successful so far,' he said.

Data from planet-hunting astronomers suggest that tens of billions of earth-like worlds could exist in our galaxy alone, and Shostak is aware of 200 billion galaxies.

According to this argument, endorsed by scientist Stephen Hawking, it would be improbable for life not to exist somewhere other than earth.

'If life formed here it could form elsewhere,' Leo Blitz, an astronomy professor at the University of California Berkeley, said. 'My bet is that a planet like the earth will be identified within the next two years.'