Scientists Say Closest Exoplanet Could Be Habitable After All

It can be depressing that all the remarkable exoplanets scientists keep discovering are so mind-bogglingly far away. We obviously can’t visit them, but they’re also more difficult to study at such great distances. Proxima Centauri b offered a ray of hope when it was confirmed in orbit of our nearest neighbor star. While it was in the habitable zone, astronomers expressed skepticism it actually was habitable. Now, a new analysis of Proxima Centauri b suggests we may have been too hasty in that assessment.

Being in the habitable zone of a star doesn’t mean much else than a planet could hypothetically have liquid water on its surface. There are a lot of other variables to consider when assessing whether or not an exoplanet is hospitable, and Proxima Centauri b has issues. A red dwarf like Proxima Centauri is smaller and cooler than our sun, so the habitable zone (where Proxima b sits) is much closer and more perilous.

Existing in the Proxima Centauri habitable zone means the planet could be affected by solar flare activity that could strip away the atmosphere (assuming it has one) and water. The nearness of Proxima Centauri also means the planet would be bombarded with radiation. Intense UV and X-rays could sterilize the surface. That’s more water and atmospheric loss, and red dwarfs are usually much more active early in their lives. Proxima Centauri b could have been scorched and dehydrated early on.

So, that’s the bad news. The good news is a study led by NASA’s Anthony D. Del Genio found several ways in which Proxima Centauri b could still be a habitable world. The team created 3D simulations of the planet with variable conditions to assess its habitability. Unlike past studies, the team included a dynamic ocean that could distribute heat. That led to some interesting conclusions.

For example, the study points out that Proxima b could have started farther out in the solar system before migrating inward. That would spare it from the harsh early conditions of the red dwarf. Alternatively, it might have started with much more water or atmosphere than Earth, so it could still have plenty after coping with a nearby infant red dwarf.

After modeling various configurations of ocean and atmosphere, the team found that every case produced a planet with at least some remaining surface water. In fact, even a tidally locked planet (where one side always faces the star) resulted in a habitable planet. That’s thanks to ocean currents that distribute heat from the day to night side. With a highly saline ocean, even the night side that never sees the sun could have liquid water.

Many of our theories about Proxima Centauri b are impossible to test right now. This is one of the few exoplanets detected by analyzing its gravitational effects on its host star — Proxima b doesn’t pass in front of the star at all. That makes it harder to characterize. However, future instruments like the James Webb Space Telescope could make it possible to take a look at Proxima Centauri b and other distant worlds.

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