A leading U.S. astronomer says there is no hope of finding alien life in space because all other planets discovered so far are to hostile for life.

Howard Smith, a senior astrophysicist at Harvard University, says he believes humans are alone in the universe, following an analysis of the 500 planets discovered so far that shows they are all hostile to life, Britain's Daily Telegraph reported Monday.

The extreme conditions found so far on all planets discovered around suns outside the solar system are likely to be the norm and it may well be that the hospitable conditions on Earth are unique, he says.

"We have found that most other planets and solar systems are wildly different from our own," he says. "They are very hostile to life as we know it."

Other scientists remain confident life will be found, and are looking forward to the next few weeks as NASA's Kepler satellite telescope is expected to confirm the existence of hundreds of newly discovered planets.

Smith insists extrasolar planets are too different from our own and even if they did support life it would be impossible for us to contact it.

"Any hope of contact has to be limited to a relatively tiny bubble of space around the Earth, stretching perhaps 1,250 light years out from our planet, where aliens might be able to pick up our signals or send us their own," he says. "But communicating would still take decades or centuries."