Still waiting for little green men to make contact? _ . A leading astronomer has concluded there probably aren't any aliens out there - meaning we are absolutely alone in the universe. Even though there may be tens of thousands of other distant planets similar in size to Earth, the conditions on them are likely to be too hard to support life-forms such as ET. Dr Howard Smith, a senior astrophysicist at Harvard University, believes there is very little hope of discovering aliens and, even if we did, it would be almost impossible to make contact. So far astronomers have discovered a total of 500 planets in distant solar systems - known as extrasolar systems - although they believe billions of others exist. But Dr Smith points out that many of these planets are either too close to the sun or too far away, meaning their surface temperatures are so bad that they could not support life. Others have unusual orbits which cause vast temperature variations, making it impossible for water to exist - the most important thing for life. Dr Smith said, "We have found that most other planets and solar systems are wildly different from our own. It means it is highly unlikely there are any planets with intelligent life close enough for us to make contact." But his suggestions contradict other leading scientists who have claimed aliens almost certainly exist. Only last month Professor Stephen Hawking said the fact that there are billions of galaxies out there made it reasonable to think there were other life-forms in the universe. Researchers from the University of London have recently suggested that aliens could be living on as many as 40,000 other planets. But Dr Smith said: "Any hope of contact has to be limited to a relatively tiny space around the Earth, reaching maybe 1,250 light years out from our planet, where aliens might be able to pick up our signals or send us their own. But communicating would still take decades or centuries." Dr Smith concludes there probably aren't any aliens because other planets   _  .
are too far away from the earth are different from the Earth in size don't have rich natural resources don't have a suitable living environment