The reason there has never been a confirmed sighting of an alien - or a message from an alien race - could be very depressing.

The reason we haven’t heard from aliens is because they’re all already dead, a new study has suggested.

Researchers from Australian National University suggest that the environments on early planets tend to be so hostile that even when life arises, it is quickly exterminated.

‘The Universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens,’ said Aditya Chopra from the Australian National University.

‘Early life is fragile, so we believe it rarely evolves quickly enough to survive.’

‘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.’’

The researchers simulated how life might have evolved based on what we know about the ‘early years’ of Venus, Mars and Earth.

The reseachers suggest that life on Earth evolved so quickly that it ended up regulating greenhouse gases - and stabilising the environment on the planet.

What happened here was a fluke - and is highly unlikely to have happened elsewhere, the researchers say.

Charles Lineweaver of the Australian National University says, ‘The mystery of why we haven’t yet 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.’