At the Astrobiology Science Conference held last week in Chicago, Charles Lineweaver and Aditya Chopra from the Australian National University presented an intriguing solution to the mystery of why we haven’t yet found signs of extraterrestrial life.
SETI researchers typically use the Drake equation to estimate the number of civilizations in our galaxy with whom radio communication might be possible. Given the vast numbers of stars, and the expectation that most of them have planets, it’s commonly believed that there should be many intelligent extraterrestrial civilizations out there. Yet, so far we have found none, a dilemma known as the Fermi Paradox. Why? Most scientists speculate that the origin of life might be very hard. Or, perhaps the development of technologically advanced intelligence is very rare. After all, it developed only once on our own planet.
A different approach is taken by Lineweaver and his graduate student, who think the problem may be with the short span of time that a planet remains habitable. They argue that even if life does emerge on a planet, it rarely evolves quickly enough to regulate greenhouse gases and albedo (the fraction of solar energy reflected back to space) and thereby keep surface temperatures compatible with liquid water and habitability.
Rocky planets need to be inhabited in order to remain habitable. On Earth, we know that early life provided feedback mechanisms to support itself, such as when early microbes exhaled methane, a greenhouse gas that helped to warm the planet at a time when Earth was frozen over. Further, it took a long time—billions of years of continuing habitable conditions—to arrive at a species that can explore space and build radio antennas. Venus, on the other hand, developed into a hothouse, and Mars into an icebox. Life may still be hanging on in some niches on those planets (for example, in the lower cloud layer of Venus or the subsurface of Mars), but without life’s feedback mechanisms, the survival of life on the surface is very unlikely, and the evolution of a technologically advanced civilization appears all but impossible.
It’s a sobering conclusion: The “Gaian bottleneck” model of Lineweaver and Chopra suggests that biological extinction is the cosmic default for most life that emerges on the surfaces of wet, rocky planets.