Wayne England/ANU Research School of Earth Sciences

Scientists wondering why we haven't yet found life on other planets think they have the answer: we're too late.

In a new study published in Astrobiology, researchers from ANU Research School of Earth Sciences suggest that extraterrestrial life may have emerged in our universe at some point, but is by now extinct. That theory, they write, answers a question referred to as Fermi's paradox:

We see no evidence that our galaxy has been colonized by an advanced technological civilization. Archaeological excavations have not unearthed alien spaceships, and the optical and radio searches for extraterrestrial intelligence have not been successful. If one assumes that once life emerges it evolves toward intelligence and technological civilizations, we are faced with Fermi’s paradox: Where is everybody?

According to the researchers, habitable conditions are not enough to make a planet friendly to life in the long-term, though there's little reason to think that life doesn't develop on appropriate planets at first. In order for life to last, the planet must also remain habitable for long enough for life forms to advance. The authors explain in their abstract:

Here, 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.

This theory suggests two things, they write:

Such a 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. In the Gaian bottleneck model, the maintenance of planetary habitability is a property more associated with an unusually rapid evolution of biological regulation of surface volatiles than with the luminosity and distance to the host star.

Lead author Aditya Chopra explained that "early life is fragile, so we believe it rarely evolves quickly enough to survive," adding, "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." This is something other scientists suggest early humans actually did—through the emission of greenhouse gases, our ancestors likely staved off an ice age. Good job, early humans.

Danielle Wiener-Bronner is a news reporter.