Chance played role in keeping Earth habitable

The fact that Earth has remained continuously habitable for billions of years appears contingent rather than inevitable, according to computer simulations of climate evolution for 100,000 randomly generated planets.

Toby Tyrrell, a professor of Earth system science at the University of Southampton, devised a study to examine how the planet was able to remain habitable without interruption, given the precarious balance of its climate and threats such as asteroids. Usually habitability is explained by global stabilizing feedbacks, but in this study, chance is recognized as a key player.

Tyrrell's conclusions are published in the Nature journal Communications Earth & Environment, Dec. 11, 2020.

Earth's climate remained thermally comfortable enough for life to develop from simple cells to intelligent life over the last 3 to 4 billion years. But why Earth's temperature remained livable given the variability of the sun is a puzzle. The sun is 30% more intense today than it was on the early Earth. 

What intervened to keep the early Earth from freezing and our present Earth from having boiling oceans?

Tyrrell told Current Science Daily that he became interested in the habitability puzzle through attending conferences on James Lovelock‘s Gaia hypothesis. 

"I read his books, and found it fascinating, the idea that once life becomes established on a planet, it intervenes in climate regulation to keep it stable," he said. "I looked at all the evidence, and the new information we had since then, and eventually wrote a book about it, trying to bring together all the evidence. I concluded that the hypothesis doesn't really match up against the evidence. It's a very nice idea, but the world doesn't work like that."

The question for Tyrrell became if life isn't what keeps the Earth habitable, what does? Could it be partly luck?

Tyrrell set out to find a way of simulating on a computer the long-term habitability of a planet or exoplanet. 

He told Current Science Daily: "I worked out a way with a very simple model that you could get some insights into whether it could be luck or do you have to have stabilizing mechanisms like feedbacks that keep the climate on track."

Tyrrell's model used random numbers to generate a very diverse group (a "zoo") of 100,000 artificial planets with different feedback systems. The model also made each planet experience a progressive change, such as produced by changes in its star's luminosity.

"I also included instantaneous threats to habitability (perturbations) in the model. Habitability can be lost due to an asteroid hit, or a nearby supernova with an enormous burst of energy, or flares from stars," Tyrrell said.

"I ran each randomly generated planet 100 times, with randomly different perturbations each time. There was actually only 1 out of 100,000 that stayed habitable every time. All of the rest would only occasionally stay habitable." 

"That says clearly that there's an element of chance in whether a planet will be successful in staying habitable. But it's not enough to just have luck. I made artificial planets that didn't have any stabilizing feedback, and they never stayed habitable for long enough."

Tyrrell repeated the whole simulation with different variants of the model and always got the same overall result. The simulation results showed that both chance and feedback mechanisms play a role in keeping a planet habitable.

"It's quite puzzling really how a planet can stay at a similar temperature for billions of years," Tyrrell told Current Science Daily. "For instance, small imbalances between CO2 input and output can add up quite quickly over geological time scales."

Tyrrell characterized a planet's climate as being "on a knife edge, and it has to stay on that knife edge for an enormous duration."

Does he think that there are other planets out there with conditions for habitable life? 

"It's not likely that many planets are able to do it. That's supported by the results of the simulations. There may be a number of planets out there that are habitable now, but probably only a few able to keep that habitability without interruption over billions of years."

"Planets with simple microbial life might be more common. It must be much less common for planets to have intelligent life."

As for the future, Tyrrell told Current Science Daily that there is "some work in the pipeline to take the model and look at the planets that did survive for 3 billion years and simulate them for another few million years and see what happens," he said. "It's certainly an interesting question."

A 2-minute video of Tyrrell talking about his work appears here.