Earth from orbit

Luck has played a big part in Earth’s continued habitability, research suggests

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The reason the Earth has managed to stay habitable for billions of years is simply down to good luck, Southampton University researchers have said.

Using the university’s Iridis supercomputing facility, they ran simulations looking at how 100,000 randomly different planets responded to random climate-altering events spread out across three billion years, until they reached a point where they lost their habitability.

Each planet was simulated 100 times, with different random events each time. With the results they considered whether habitability persistence was restricted to just a few planets which were always capable of sustaining life for three billion years, or instead was spread around many different planets, each of which only sometimes stayed habitable for this period.

Most of those planets which remained life-sustaining throughout the three-billion-year period only had a probability, not a certainty, of staying habitable.

Many instances were of planets which usually failed in the simulations and only occasionally remained habitable. Out of a total population of 100,000 planets, nine per cent (8,700) were successful at least once – of those, nearly all (about 8,000) were successful fewer than 50 times out of 100 and most (about 4,500) were successful fewer than 10 times out of 100.

Geological data demonstrates that Earth’s climate has remained continuously habitable for more than three billion years. However, it has been precariously balanced, with the potential to rapidly deteriorate to deep-frozen or intolerably hot conditions causing planet-wide sterility.

Professor Toby Tyrrell, a specialist in Earth system science, said the results of the study suggested chance is a major factor in determining whether planets such as Earth can continue to nurture life over billions of years.

“A continuously stable and habitable climate on Earth is quite puzzling,” he said.

“Our neighbours, Mars and Venus, do not have habitable temperatures, even though Mars once did.

“Earth not only has a habitable temperature today, but has kept this at all times across three to four billion years – an extraordinary span of geological time.

“We can now understand that Earth stayed suitable for life for so long due, at least in part, to luck.

“For instance, if a slightly larger asteroid had hit Earth, or had done so at a different time, then Earth may have lost its habitability altogether.

“To put it another way, if an intelligent observer had been present on the early Earth as life first evolved, and was able to calculate the chances of the planet staying habitable for the next several billion years, the calculation may well have revealed very poor odds.”

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