Gaian bottleneck: Are we too late to find alien life?

Planets displaying conditions capable of supporting life dot the universe and, until now, prevailing thinking insisted other life-forms have simply never managed to emerge. A new theory suggests they probably did; they just didn't survive.

The reason for man’s loneliness in the universe is not because life has never evolved elsewhere, but rather it just went extinct before it could gain a foothold, astrobiologists suggest.

Many planets provide the right mix of ingredients for life to spring into being, say authors of a study published Wednesday in the journal Astrobiology, but there is only a small window for that to happen, and for the life-forms to then modify their environment.

During this narrow timeframe, a mere 500 million to one billion years, if the nascent beings are unable to stabilize planetary conditions, there is a high probability that their new home will become uninhabitable.

"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," lead author Aditya Chopra of Australian National University said in a press release. "Early life is fragile, so we believe it rarely evolves quickly enough to survive."

In the first 500 million years or so of a planet with the potential for life, temperatures will be too high and the universe will be bombarding it with too much debris for life to stand a chance.

Thereafter, however, temperatures cool and impact rates decline, providing conditions conducive to the emergence of life-forms.

But if they miss their billion-year window, the new creatures are likely to be snuffed out, as, among other variables, the liquid water boils away or freezes.

“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 authors write.

The authors have dubbed their hypothesis the “Gaian bottleneck,” suggesting 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.”

This is in contrast to the “emergence bottleneck” theory, hypothesizing that the apparent paucity of life in our universe is because of a low probability of life emerging in the first place, “due to the intricacies of the molecular recipe.”

Mars, Venus and Earth may all have been habitable four billion years ago, but a billion years after formation, temperatures soared on Venus and plummeted on Mars, stripping the planets of their ability to host life.

"Life on Earth probably played a leading role in stabilising the planet's climate," said co-author Associate Professor Charley Lineweaver from the ANU Planetary Science Institute in the release.

Yet, if this new theory is true, where are all the alien fossils?

The astrobiologists think they have the answer to that one, too: any life-forms that lost the battle to keep their planet habitable never grew big enough to leave identifiable remains.

"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," Professor Lineweaver told Space.com.

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