Aliens DO exist, scientists claimed this week, but the reason we have not yet had any extra-terrestrial contact is because they are already EXTINCT.
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Astro-biologists from the Australian National University say that, while there are countless habitable planets in the universe, most aliens will have died out before reaching a stage capable of contact.
Expert Aditya Chopra said in a statement: "The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens.
"Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable.
"Early life is fragile, so we believe it rarely evolves quickly enough to survive.
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Is there alien life in space?
He said the survival of life on other planets would depend on something called the 'Gaian bottleneck', in which life changes its environment in order to aid evolution - an extremely rare occurence.
For example, life on Earth thrived when tiny organisms began regulating carbon dioxide and producing oxygen, which allowed larger organisms to develop. Failure to do so results in extinction.
For example, life on Earth thrived when tiny organisms began regulating carbon dioxide and producing oxygen, which allowed larger organisms to develop. Failure to do so results in extinction.
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ANU say that, four billion years ago, Earth's relatively close neighbours Venus and Mars may have been perfectly suited for the development of life.
However, the climates on both planets became extremely hostile, as Venus "turned into a hothouse" and Mars "froze into an icebox."
However, the climates on both planets became extremely hostile, as Venus "turned into a hothouse" and Mars "froze into an icebox."
Seth Shostak, an astronomer at the SETI Institute, said: "Early Armageddon, as proposed here, may occur on some planets some of the time.
"However, while change may thin the cosmic herd, I can hardly imagine it inevitably wipes the herd out.
"Consider that in 65 million years, mammals went from being small rodents to becoming us. In other words, even if the neighbourhood deteriorates, life has time enough to adapt."
"However, while change may thin the cosmic herd, I can hardly imagine it inevitably wipes the herd out.
"Consider that in 65 million years, mammals went from being small rodents to becoming us. In other words, even if the neighbourhood deteriorates, life has time enough to adapt."