The search for extra terrestrial life on distant planets has turned up nothing because ET and others like him have probably gone extinct.
Life likely developed on many planets but then just disappeared due to runaway cooling or overheating, say a team of astrobiologists in Australia.
Astronomers have found no end of potentially habitable planets outside our solar system "so many scientists think it should be teeming with aliens", said Dr Aditya Chopra of the Australian National University.
Yet the evidence of life seen in a changed atmosphere or use of technology is just not there.
"Early life is fragile so we believe it rarely evolves quickly enough to survive," said Dr Chopra, lead author in a study published in the journal Astrobiology.
The environment of a young planet would be unstable and likely to change very quickly. If any life that formed could not keep up with the changes then it would not have survived.
But if evolving life was able to take control and begin modifying a planet's environment then the chances of survival multiply.
"To produce a habitable planet life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable," Dr Chopra said.
Four billion years ago Earth, Venus and Mars all probably harboured early life, but a billion years later Venus turned into an oven while Mars became a refrigerator. In contrast life on Earth helped to stabilise the atmosphere, allowing more complex organisms to arise.
The theory put together by Dr Chopra and colleagues offers answers about survival and whether the likes of an ET would get a chance to evolve.
“The mystery of why we haven’t found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces,” he said.
So if we manage to find fossils that confirm life on distant planets, almost all of them will be of extinct microbes.
Much rarer will be fossils of multicellular organisms such as ET or humanoids. With them will be evidence of life that damaged its atmosphere or polluted its water only to die off in a changing environment in which it could no longer live.