Quanta has a pretty interesting article up today: A Solution to the Faint-Sun Paradox Reveals a Narrow Window for Life. Our understanding of the physics of the sun indicate that it should have been only 70% as bright as it is today. But if so, early Earth should have been a snowball not really capable of sustaining life. Yet the earliest evidence for life goes back 4.1 billion years.
The article chronicles the investigation of this paradox over the decades. The current answers seem to indicate a combination of shifting factors, such as the level of carbon dioxide in the atmosphere, with variances from geological processes, and the initial close orbit of the moon and its gravitational tidal effects, both on Earth’s oceans and geology. Given those factors, if the early sun had been as bright as it is today, it would have resulted in a “steam Earth” too hot for life. Earth’s continuing habitability over the eons ends up hinging on a complex series of shifting coincidences.
The implications for extraterrestrial life aren’t promising.
For planets in other solar systems, the faint young sun problem complicates the question of extraterrestrial life. In December 2020, Tyrrell calculated that Earth’s continuing habitability is mostly due to chance. He created a computer model of 100,000 planets. Each started out as habitable. He then subjected each planet to 100 simulations of various climate feedback scenarios. For 91% of the planets, not a single simulation kept the planet habitable over geological timescales. “Earth’s success was not an inevitable outcome but rather was contingent,” he wrote. “It could have gone either way.” Thus, in order for exoplanets to have the potential to develop life, perhaps they need to have the right ingredients in just the right circumstances — like Earth.
It’s worth noting however the case of Mars, for which there’s good evidence that water existed on it in its early history.
Mars presents a trickier conundrum. According to new data from NASA’s Perseverance rover, Mars appears to have had rivers and lakes on its surface at least 3.7 billion years ago. It’s unclear how that would have been possible at its greater distance from the sun.
“On Mars the puzzle is enhanced,” said Kirsten Siebach, a planetary scientist at Rice University and a member of the science team for multiple robotic Mars missions, including Perseverance. “Ancient Mars would have required twice the greenhouse effect we have on Earth today.” Samples being collected by Perseverance, to be returned to Earth in the 2030s, could tell us if this was possible.
This, to me, indicates a couple of possibilities. One is that we don’t understand the sun’s processes and overall evolution as well as we think we do. Or two, that the conditions that made early life on Earth possible weren’t as freakish as they might seem.
Still, Mars’ biosphere, if it ever had one, is most likely long gone. That implies that while biospheres may be reasonably common in the universe, their window of existence may average a lot less than what we’ve had on Earth. It may be that the typical biosphere gets a few hundred millions years of life before the conditions that allow it dry up. Earth’s four billion year old biosphere may be a freakish event.
If so, given that it took complex life 3.5 billion years to really get going here, that type of life may be extremely rare.
The Fermi paradox is the observation that, if intelligent life is pervasive in the universe, we should have been colonized long ago, but there’s no evidence for it. The most straightforward solution to this paradox is that intelligent life is profoundly rare. Given that intelligence as we understand it depends on complex life, and complex life itself may be very rare, we might have to shift intelligent life from profoundly to sublimely rare.
Unless of course I’m missing something?