Astronomers have identified a habitable zone in the Milky Way galaxy, a kind of suburb of stars with the elements needed for life, and old enough for sophisticated civilisations to have developed.
The good news for those seeking extraterrestrial intelligence is that 10% of our galaxy - perhaps 10bn stars - could be ringed by rocky planets big enough to hold on to their atmosphere and water. The bad news is that they are on average a billion years older than the sun.
'This should be a sobering result,' Charles Lineweaver, of the University of New South Wales, says today in Science. 'A billion years is a long, long time.'
As we speak our culture is blasting outward in radio and television signals at the speed of light. Assuming anyone still uses radio after a billion years, they'll be judging us by the quality of our broadcasting.
We started transmission in 1921 which means the volume where our stuff can be received is now a sphere 164 lightyears in diameter. That is not a totally encouraging thought.
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