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The Aliens Are Already Extinct

Life on other planets would likely be brief and become extinct very quickly, according to astrobiologists from The Australian National University who argue that new life would commonly die out due to runaway heating or cooling on their fledgling planets.

“The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens,” said Dr Aditya Chopra, who is lead author of the research published in Astrobiology (

“Early life is fragile, so we believe it rarely evolves quickly enough to survive,” he said. “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.”

About four billion years ago Earth, Venus and Mars may have all been habitable. However, a billion years or so after their formation, Venus turned into a hothouse and Mars froze into an icebox.

Any early microbial life on Venus and Mars failed to stabilise the rapidly changing environment, said co-author A/Prof Charley Lineweaver. “Life on Earth probably played a leading role in stabilising the planet’s climate,” he said.

Chopra said their theory solved a puzzle. “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,” he said.

While wet, rocky planets with the ingredients and energy sources required for life seem to be ubiquitous, no signs of surviving extraterrestrial life have been found. A plausible explanation is near-universal early extinction, which the scientists have named the Gaian Bottleneck.

“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,” Lineweaver said.