clipped from: news.bbc.co.uk   

The discovery of more than 330 planets outside our solar system in recent years has helped refine the number of life forms that are likely to exist.

The work is reported in the International Journal of Astrobiology.

Mr Forgan simulated a galaxy much like our own, allowing it to develop solar systems based on what is now known from the existence of so-called exoplanets in our galactic neighbourhood.

The first assumed that it is difficult for life to be formed but easy for it to evolve, and suggested there were 361 intelligent civilisations in the galaxy.

A second scenario assumed life was easily formed but struggled to develop intelligence. Under these conditions, 31,513 other forms of life were estimated to exist.

possibility that life could be passed from one planet to another during asteroid collisions
result of some 37,964 intelligent civilisations in existence
Green Bank telescope (AP)
We are likely to be listening for a long time, even if there are many worlds