Ocean worlds could be habitable for long periods of time

Climates can be kept stable by processes other than those that occur on Earth.
By Laurel Kornfeld | Sep 24, 2018
Water worlds, planets covered with oceans, could be habitable for life for more than a billion years, according to a new study that conducted over a thousand computer simulations.

Until recently, scientists thought that the only planets capable of supporting life are Earth-like worlds that have land and shallow oceans. Planets completely covered in oceans do not cycle minerals and gases, a phenomenon that keeps Earth's climate stable.

Over very long periods of time, Earth keeps itself cool by pulling atmospheric greenhouse gases into minerals, then warms itself up by releasing the gases back into the atmosphere through volcanic eruptions.

With telescopes growing more powerful, scientists have discovered numerous exoplanets orbiting stars other than the Sun. Many of these are not at all Earth-like, and some are covered in oceans with depths of up to several hundred miles.

On ocean worlds, Earth's method of maintaining a stable climate would not work, as all the rock is covered by water, which also suppresses volcanoes.

Edwin Kite of the University of Chicago and Eric Ford of Pennsylvania State University decided to research whether ocean worlds might use a different method to maintain climate stability. They turned to a computer simulation that tracked the evolution of thousands of planets over several billion years.

The computer model showed that if planets are in ideal orbits around their parent stars, have the right amount of carbon, begin their lives with sufficient water, and cycle carbon between their atmospheres and oceans in just the right amounts, they can maintain stable climates.

Additionally, the planets cannot have too many minerals and elements dissolved in their oceans, as these would remove atmospheric carbon.

"The surprise was that many of them stay stable for more than a billion years, just by luck of the draw. Our best guess is that it's on the order of 10 percent of them," Kite said.

"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. It does seem there is a way to keep a planet habitable long-term without the geochemical cycling we see on Earth," he added.

While the simulations were done for planets orbiting Sun-like stars, their results could also apply to those orbiting smaller, dimmer red dwarf stars.

Findings of the study have been published in the Astrophysical Journal.

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