Science & Technology

Alpha Centauri Might Include a Stolen Star

We're not alone in the universe. At least, our sun isn't. It's got a next-door neighbor in Alpha Centauri, a system that features not one, not two, but three stars and attendant planets. But as researchers have recently discovered, one of those stars might not be an original resident of this neighborhood — it might have even been stolen.

Hubble's best image of Alpha Centauri A and B

The Star on the Milk Carton

In interstellar distances, Alpha Centauri is a hop, skip, and a jump away; a distance of 4.3 lightyears is nothing if you can travel at the speed of light and have 4.3 years to kill. In all seriousness, some of the brightest minds in astrophysics are working on a solution that could cut the journey down to about 20 years — not exactly a trip you'd make on a whim, but if the current fastest vessel were to make a shot for the system today, it wouldn't make it for another 70,000 years. The question is, would we want to go if we knew that it was a solar system of thieves?

There are two stars about the same size as the sun in Alpha Centauri, and they go by the catchy names of Centauri A and Centauri B. But there's a third star as well, this one much smaller, that was discovered (relatively) recently, about a century ago. Proxima Centauri, also known as Centauri C, is just over 1/10 the size of our sun and seems a bit out of place in the system. According to a new set of gravitational simulators, that might be because it's not from the system at all.

As it turns out, in 26 percent of the simulations, Proxima Centauri ends up eventually falling out of orbit with the other two over the course of about 10 billion years. If our littlest neighbor was originally from Alpha Centauri, you'd expect its orbital relationship with its big brothers to be a lot more stable than that. In other words, this just might be a star-napping.

Alpha Centauri A (left) and B (right). The faint red star in the center of the yellow circle is Proxima Centauri.

Along for the Ride

If Proxima Centauri really is a new addition to the Alpha Centauri system, then it didn't come alone. The littlest star is orbited by something even littler: the nearest Earth-like planet in the galaxy. Proxima b is 1.3 times the size of the Earth and stays about half the distance from its star as the Earth is from the sun. Since Proxima Centauri is so much smaller than our sun, that keeps it well within range of the habitable zone.

Interestingly, if Proxima Centauri and its little planet buddy are newcomers to the system, that could be good news for the potential of life on the planet. In its current position with three stars so close by, Proxima b would be in some danger of being knocked out of the habitable zone by the multiple gravitational forces in the area. But if the little star and its planet have been partnered for longer, they might have had enough time for single-cell organisms to develop. We just hope they're happy in their new home.

The First Earth-Sized World of Alpha Centauri

Written by Reuben Westmaas October 30, 2017

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