However, aliens could now be on their way as the galactic weather has improved in the past few billion years, says Dr James Annis, an astrophysicist at the Fermilab, in Chicago.
Though the idea put forward by Dr Annis in today's New Scientist may sound fanciful, it does explain something that has mystified astronomers for decades. Our galaxy, the Milky Way, is 10 billion years old and only about 100,000 light years across. Logically, any spacefaring race able to travel even at just one-thousandth of the speed of light would pervade the galaxy in just 100,000 years.
But repeated searches for signs of extraterrestrial intelligence have all failed. That, Dr Annis suggests,could be because although life may have arisen all over the galaxy in the past, it kept being wiped out by intense radiation from "gamma ray bursters" caused by the collision of superdense stars whose radiation could sterilise an entire planet in moments.