As we look deeper into our galaxy for signs of extraterrestrial life, we keep drawing a blank. Does this mean life on Earth is unique and we're the only ones out here? Or could it just mean that all the aliens are dead?
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Fresh on the heels of the recent news surrounding the increasingly dire climate forecast for our planet, comes a possible warning from the cosmos: climate change in extraterrestrial environments is inevitable and, should life on hypothetically habitable worlds not act as a stabilizer for their environments, it serves as a "sell-by" date for all burgeoning lifeforms.
In new research published in the journal Astrobiology, astronomers of The Australian National University (ANU) pondered this scenario and realized that young habitable planets can become unstable very quickly. What once was a life-giving oasis becomes a hellish hothouse or frozen wasteland very quickly.
"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," said Aditya Chopra, lead author of the paper. "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," he said.
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Unlike Earth, most worlds will likely not find this balance, ultimately succumbing to being cooked by a runaway greenhouse effect (like Venus) or frozen by a thinning atmosphere (like Mars). Life will often not be fortunate enough to win the race against environmental fluctuations to become a stabilizing factor.
Earth, which already has the stunning fortune to exist at just the right spot around a stable star, spawned life and that life had a role to play in stabilizing its atmosphere as it evolved over that last 4 billion years.
"Life on Earth probably played a leading role in stabilizing the planet's climate," said co-investigator Charley Lineweaver, also from ANU.
And this could be why we're not finding a galaxy filled with alien life - just because there's a habitable world out there, it doesn't mean it's suitable for life for long. It's yet another hurdle against life from gaining a foothold.
"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," said Chopra.
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One of the fundamental reasons for seeking out exoplanets, particularly small rocky worlds in orbit around their stars within their habitable zones, is to find planets that have similarities to Earth. And we're finding plenty of candidates that approximately fit the bill. But just because they possess some "Earth-like" features certainly doesn't mean they're Earth-like. This research underscores the uncertainty.
For decades we've been pondering our place in the universe and tried to theorize why we've uncovered no evidence for extraterrestrial intelligences. With all the stars and planets in our galaxy and all the water and prebiotic chemicals that are known to exist, there must be other intelligent lifeforms. But there's no sign of them. This problem is known as the "Fermi Paradox."
Chopra and Lineweaver suggest their new research provides some answer to this paradox and call it the "Gaian Bottleneck." If life isn't given a chance to stabilize its biosphere, then it's doomed.
Earth was given this opportunity, and life emerged from the Gaian Bottleneck to help form the life-giving oasis we take for granted today. Earth and its complex interplay of feedback cycles created what can be seen as a superorganism, where all life on its biosphere has a role to play in its evolution. (This is known as the "Gaia Hypothesis", a relatively controversial idea formulated by James Lovelock and Lynn Margulis in the 1970s.)
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But now we have an intelligent lifeform that emerged as a dominant force, interrupting and exploiting our planet's natural cycles. Humanity has inadvertently created a new bottleneck - let's call it the "Industrial Bottleneck" - by causing irreversible changes to our delicate biosphere. Now, we're seeing rapid impacts on our civilization as the balance in our climate is knocked off-kilter by the inexorable rise of greenhouse gases from industrial processes and energy needs.
Are these bottlenecks common throughout the cosmos? If an extraterrestrial lifeform "makes the grade" and survives the Gaian Bottleneck, does it then face another existential threat from their evolution into a industrial civilization?
For now, this is all speculation, but what's clear from observations of our own planet, is that the mother of all existential self-inflicted bottlenecks is on the horizon and, unless we find a way of reversing the damage we've caused to our environment, it seems we'll quickly become just another lifeform that didn't make the grade.
Source: ANU press release