A leading astronomer has said there might not be alien life "out there" and that it was wrong to assume that a habitable planet would contain life.
Prof Charles Cockell, director of the UK Centre for Astrobiology at the University of Edinburgh, said: "The pervasive nature of life on Earth is leading us to make this assumption.
"On our planet, carbon leaches into most habitat space and provides energy for microorganisms to live - there are only a few vacant habitats that may persist for any length of time on Earth, but we cannot assume that this is the case on other planets," he told members of the Royal Society.
About 800 planets outside the Solar System have been detected and scientists were analysing which of them could sustain life.
Cockell said that while habitable planets might be abundant, life might not be: "It is dangerous to assume life is common across the universe - it encourages people to think that not finding signs of life is a failure when in fact it would tell us a lot about the origins of life," he said.
He added that it was wrong to assume that we would be able to detect or even recognise extraterrestrial life even if it does exist because we are heavily influenced by our own knowledge of life.
Cockell laid out a number of assumptions scientists would use to detect life on another planet.
Odds of finding aliens in 2013 are 33:1
These are that aliens will evolve metabolisms that produce signature gases we associate with life; that these organisms will colonise the planet in high numbers; and that they will produce enough biosignatures to be detectable by mankind.
The bookies Paddy Power is giving odds 33/1 that alien life will be discovered this year. Russia and the US were most likely to have first contact, followed shortly by China, said bookmakers.
In January, Prof Chandra Wickramasinghe, of the University of Buckingham, said algae found inside a meteorite in Sri Lanka proved that aliens exist.
He said the seaweed-like algae fossils was similar to fossilised seaweed that had been dated to 55 million years ago. That proved that aliens were responsible for life on Earth, he added.
"These finds are crushing evidence that human life started outside Earth," he said.
"The algae organisms are similar to ones found in Earth fossils but the rock also has other organisms we have not identified.
"We are all aliens - we share a cosmic ancestry. Each time a new planetary system forms, a few surviving microbes find their way into comets. These then multiply and seed other planets.
"These latest finds are just more evidence to point to the overwhelming fact that life on Earth began on other worlds."