Potentially habitable planet found orbiting nearest stellar neighbor

Proxima Centauri, along with the more distant Alpha Centauri A and B, is seen low on the horizon of the newly discovered planet in this artist’s illustration. Proxima b has a similar mass as Earth and orbits inside the habitable zone of the red dwarf star. As such, it’s year is only 11.2 days. Additionally, the planet it is likely tidally locked to the star, making the prospect of life on the rocky world difficult.

Derek Richardson

Astronomers from the European Southern Observatory announced Aug. 24 the discovery of a planet inside the habitable zone of Earth’s closest stellar neighbor.

The new planet, Proxima b, was found orbiting Proxima Centauri, the smallest star of the triple-star Alpha Centauri system. The planet’s host star is a dim red dwarf and is located a mere 4.2 light years from Earth.

However, even a star as dim and close as Proxima Centauri is too bright to allow for current telescopes to see planets directly. As such, alternative methods have to be used. Proxima b was detected using a technique that studies how a star “wobbles” around its center of mass and measures the Doppler shift.

Proxima b’s mass is estimated to between 1.3 and three times Earth’s, making it a rocky world, and not a gas giant like Jupiter. It also orbits around the “goldilocks zone,” an area around a star that allows for water to exist as a liquid.

“I don’t like that people are saying this planet is “Earth-sized,” said Brian Thomas, professor in Washburn’s physics and astronomy department and leader of the Washburn Astrobiophysics research group. “They do not have a size, but rather a mass measurement. The mass is roughly Earth’s mass.”

However, the star is a dim red dwarf, the habitable zone is extremely close to the star – closer than Mercury is to the sun. Where Mercury orbits the sun in 88 days, Proxima b orbits its star in a speedy 11.2 days.

A consequence of this closeness means the planet is likely tidally locked. But depending on its orbital eccentricity – how elliptical the orbit is – it could also be in a 3:2 resonance. That would mean for every three planetary rotations, Proxima b would orbit the star two times.

Thomas believes both scenarios would make the possibility of life actually developing on Proxima b a challenge.

“A big problem with tidal locking is the atmosphere might freeze out on the dark side,” Thomas said. “The details of the atmospheric dynamics are complicated and depend on a lot of parameters, so it might be that the atmosphere is thick enough and has strong enough longitudinal transport to distribute heat around the whole sphere, which would help habitability.”

But one major roadblock for life would be the fact Proxima Centauri periodically undergoes unpredictable and dramatic increases in brightness – similar to solar flares. That amount of radiation hitting a planet could potentially sterilize the surface.

“The severity depends strongly on whether the planet has a magnetic field,” Thomas said. “That can’t be measured right now. If it does [have a magnetic field], it may be able to deflect a lot of the particle radiation.”

Thomas said if there is no magnetic field, then any atmosphere the planet may have had would have been blown away.

“In any case, life on the surface might be subject to intense [ultraviolet] radiation from the flares,” Thomas said.