Are aliens extinct?

Evidence of alien life continues to elude Earth’s scientists, despite an abundance of wet, rocky planets with the ingredients and energy sources required for life. But is this because such life never existed at all, or because it has already gone extinct?

Writing in the journal Astrobiology, Australian National University researchers Dr Aditya Chopra and Associate Professor Charles Lineweaver offer an alternative to the commonly held theory that there is “a low probability for the emergence of life (an emergence bottleneck), notionally due to the intricacies of the molecular recipe”.

“We present an alternative Gaian bottleneck explanation: if life emerges on a planet, it only rarely evolves quickly enough to regulate greenhouse gases and albedo, thereby maintaining surface temperatures compatible with liquid water and habitability,” the astrobiologists wrote.

The Parkes radio telescope is taking part in the search for alien civilisations. Image credit: Wayne England.

“Most early planetary environments are unstable,” explained Dr Chopra. For example, four billion years ago Earth, Venus and Mars may have all been habitable; however, one billion years or so after formation, Venus turned into a hothouse and Mars froze into an icebox.

While early microbial life on Earth “probably played a leading role in stabilising the planet’s climate”, Associate Professor Lineweaver said, such stabilisation may have been unsuccessful on Venus and Mars. Life on other planets would therefore be brief, fragile and become extinct very quickly — with Earth acting as the exception rather than the rule.

“One intriguing prediction of the Gaian bottleneck model is that the vast majority of fossils in the universe will be from extinct microbial life, not from multicellular species such as dinosaurs or humanoids that take billions of years to evolve,” Associate Professor Lineweaver said.

“[The] Gaian bottleneck suggests that (i) extinction is the cosmic default for most life that has ever emerged on the surfaces of wet rocky planets in the Universe and (ii) rocky planets need to be inhabited to remain habitable,” the researchers concluded.

Top image caption: Dr Aditya Chopra. Image credit: Stuart Hay, ANU.

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