Closest exoplanet to Earth could be 'highly habitable'

An artist's impression of the view from Proxima Centarui b, a newly discovered Earth-sized planet just four light-years away. It is unclear if there is intelligent life in the universe, but searches continue to find Earth-sized planets in the habitable zones of their respective stars. (Credit: NASA)

Just a cosmic hop, skip and jump away, an Earth-size planet orbits the closest star to our sun, Proxima Centauri.

Ever since the discovery of the exoplanet — known as Proxima Centauri b— in 2016, people have wondered whether it could be capable of sustaining life.

Now, using computer models similar to those used to study climate change on Earth, researchers have found that, under a wide range of conditions, Proxima Centauri b can sustain enormous areas of liquid water on its surface, potentially raising its prospects for harboring living organisms. [9 Strange, Scientific Excuses for Why Humans Haven't Found Aliens Yet]

"The major message from our simulations is that there's a decent chance that the planet would be habitable," said Anthony Del Genio, a planetary scientist at the NASA Goddard Institute for Space Studies in New York City. Del Genio is also the lead author of a paper describing the new research, which was published Sept. 5 in the journal Astrobiology.

Proxima Centauri is a small, cool red-dwarf star located just 4.2 light-years from the sun. Despite its proximity, scientists still know very little about Proxima Centauri's planetary companion, besides that its mass is at least 1.3 times that of Earth and that it goes around its parent starevery 11 days. Therefore, Del Genio and his colleagues had to make some reasonable guesses about the exoplanet Proxima Centauri b — namely, that it had an atmosphere and an ocean on its surface — for their work to proceed.

Proxima Centauri b orbits in its star's habitable zone, meaning it's at just the right distance to receive enough starlight to keep its surface above the freezing temperature of water. But this zone is extremely close to the star, Space.com, a Live Science sister site, reported. So it's likely that the planet has become tidally locked due to gravitational forces. This means that the same side of Proxima Centauri b always faces its parent star, much like how the moon always shows the same side to Earth.

Previous simulations published in a 2016 paper in the journal Astronomy & Astrophysicsmodeled a hypothetical atmosphere on Proxima Centauri b and suggested that the star-facing hemisphere of the exoplanet might be baked under an intense glare, while a space-facing ocean would be frozen over. Therefore, only a circle of warm sea might exist on Proxima Centauri b — a scenario Del Genio's team calls "eyeball Earth."

But the new simulations were more comprehensive than prior ones; they also included a dynamic, circulating ocean, which was able to transfer heat from one side of the exoplanet to the other very effectively. In the researchers' findings, the movement of the atmosphere and ocean combined so that "even though the night side never sees any starlight, there's a band of liquid water that's sustained around the equatorial region," Del Genio told Live Science.

He likened this heat circulation to our own planet's seaside climates. The U.S. East Coast is balmier than it would be otherwise, he said, because the Gulf Stream carries warm water up from the tropics. In California, by contrast, ocean currents bring cold water down from the North, and the West Coast is colder than it otherwise would be, Del Genio added.

The team ran 18 separate simulation scenarios in total, looking at the effects of giant continents, thin atmospheres, different atmospheric compositions and even changes in the amount of salt in the global ocean. In almost all of the models, Proxima Centauri b ended up having open ocean that persisted over at least some part of its surface.

"The larger the fraction of the planet with liquid water, the better the odds that if there's life there, we can find evidence of that life with future telescopes," Del Genio said.

Ravi Kopparapu, a geoscientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study, agreed.

"I think it's exciting that some of these climate outcomes can be observed," Kopparapu told Live Science. Next-generation facilities, such as the Extremely Large Telescope currently under construction in Chile, might be able to witness heat coming off Proxima Centauri b and differentiate its possible surface conditions, he added.

Originally published on Live Science.