The odds of life on Proxima b may have improved ESO/M. Kornmesser

Proxima Centauri may be an interloper from far away. The stellar system closest to Earth consists of three stars: the closely orbiting pair of Alpha Centauri A and B, and an outlier called Proxima Centauri.

A new analysis finds that Proxima, along with its planet Proxima b, may not have been born alongside its stellar siblings. If so, the planet could have a better chance at harbouring life.

Proxima Centauri’s orbit is currently bound to Alpha Centauri A and B, but it is unclear if it has always been stable or if it will stay bound, according to Fabo Feng and Hugh Jones at the University of Hertfordshire in the UK. To determine how likely that is, they ran computer simulations of 100 Proxima “clones” do-si-do-ing around their binary partners in different orbits.


Over a simulation lasting 10 billion years, Proxima Centauri ended up in an unstable orbit, no longer gravitationally bound to Alpha Centauri A and B, 26 per cent of the time. If it were born within the system, Feng says, it is unlikely the orbit would still be unstable after aeons. Therefore, this suggests that Proxima may have been born elsewhere in space and captured by the binary as it hurtled by, resulting in an unstable orbit.

Previous studies have found that Proxima contains much less metal than its binary companions, another hint that it might have been snatched up later in the stars’ lifetimes.

Catch a star

It is extremely difficult for a single star to capture a companion, but binaries have it a bit easier because of their orbit, lending some credence to the possibility that this happened to Proxima. That is because a star zipping through space has too much energy to get caught – unless something slows it down.

“To park a car in the garage, you have to stop it somehow. If you keep stepping on the gas, you’ll go right through your garage,” says Scott Kenyon at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. “You have to get rid of some of the car’s energy to slow it down, and that same thing has to happen when you’re capturing a star.”

A single star would have a tough time doing the capturing. It can’t absorb the interloper’s energy, so the passing star would just keep going. But a binary star system acts like a shock absorber, taking in more of the passing star’s energy and slowing it down enough to be captured.

Feng says this capture could be a good sign for the habitability of Proxima b. Sometimes, Alpha Centauri A and B may get close enough to Proxima b to destabilise its orbit and push it out of its star’s habitable zone, where temperatures are right for life.

But if Proxima Centauri has only been there for a short while, cosmically speaking, its orbiting planet would have had fewer chances of being knocked out of the small patch of space that is friendly to life. Instead, it would have spent a long time in a stable orbit around a single star before encountering the dangerous influence of Alpha Centauri A and B.

“Life needs time to develop and emerge. This scenario gives it more time,” says Feng.

But Kenyon says the opposite is equally possible: it may be that a capture scenario would make Proxima b’s orbit less stable than if the planet and its star were formed as part of the larger system.

“If the three stars have been orbiting each other for a few billion years, one would think they’d be a nice happy family,” he says.

Journal reference: arXiv, 1709.03560v1

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