Modelling finds water-world exoplanets stay stable long enough for life to take hold

Hot on the heels of recent research suggesting “water worlds” may be abundant across the universe comes a new study that concludes about 10% of them are capable of harbouring life.

The work, led by geophysicist Edwin Kite from the University of Chicago in the US, interrogates one of the key objections raised regarding the possibility of life developing on water worlds that comprise deep subterranean oceans – with or without sealed crusts, such as that found on Saturn’s moon Enceladus.

The presence of water on Earth, while generally regarded as essential for the development of life, is not in itself sufficient for the process to kick off and sustain. Of critical importance is a stable climate – or, more accurately, a climate that fluctuates over long periods within a constrained range.

Over eons, our planet cycles minerals and gases, cooling itself by drawing greenhouse gases down and sequestering them in minerals, then releasing them into the atmosphere through volcanic eruptions.{%recommended 7451%}

It has long been assumed that this was the only type of model that would provide the long-term stability necessary for life to evolve.

Kite and his colleagues ran more than 1000 computer simulations testing the climate stability of water worlds positioned at a range of distances from a range of host stars. The simulations ran for the equivalent of billions of years.

The results were a surprise.

“This really pushes back against the idea you need an Earth clone – that is, a planet with some land and a shallow ocean,” says Kite.

The researchers found that certain types of planet hit a “sweet spot” of climatic conditions that resulted in stability for longer than a billion years.

Distance from host star was an important factor, but so too was the amount of carbon present in the system. It was also essential that the hypothetical planet did not contain too many minerals or elements that would make carbon unavailable by sequestering it, so that carbon cycling occurred primarily only between ocean and atmosphere.

“The surprise was that many of them stay stable for more than a billion years, just by luck of the draw,” Kite explains. “Our best guess is that it’s on the order of 10% of them.”

Although Kite’s team used model stars broadly similar to the sun, the findings also point to potentially similar results for planets orbiting red dwarf stars – in part because such stars change in brightness very slowly, providing stable energy inputs over an extremely long period.

“How much time a planet has is basically dependent on carbon dioxide and how it’s partitioned between the ocean, atmosphere and rocks in its early years,” says Kite.

“It does seem there is a way to keep a planet habitable long-term without the geochemical cycling we see on Earth.”

The research is published in The Astrophysical Journal.

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