The Cosmic Watchdog

Alpha Centauri, the nearest star system to the sun, is probably shedding comets and asteroids into our solar system — and even producing a few meteors in our sky.

Located just 4.3 light-years from Earth, Alpha Centauri consists of three stars that revolve around one another. If Alpha Centauri has an Oort cloud of distant comets as the sun does, about a million of these objects larger than a football field are now in our solar system, astronomers Cole Gregg and Paul Wiegert of the University of Western Ontario in London, Canada, estimate in work submitted February 5 to arXiv.org.

“Most of [the objects] would be in the far reaches of the solar system,” Gregg says. That puts them well beyond the orbit of Pluto, where they are mingling with the native objects in the sun’s own Oort cloud of cometary bodies.

Astronomers have only ever detected one interstellar asteroid and one interstellar comet in our solar system. But neither came from Alpha Centauri.

Just as Jupiter’s gravity catapulted the two Voyager spacecraft onto interstellar trajectories, so the stars of Alpha Centauri and their planets should do the same to some of the comets and asteroids that swing around them. A small percentage of the ejected objects — 0.03 percent — pass through our solar system, Gregg and Wiegert say, but none of the large bodies is close enough for telescopes to see.

Still, small particles from Alpha Centauri probably reach Earth’s atmosphere, where they burn up. Gregg and Wiegert estimate that up to 10 meteors worldwide come from Alpha Centauri each year.

“We expect these numbers to go up by about a factor of 10 when Alpha Centauri is closest,” Gregg says. Alpha Centauri is racing toward us at 0.007 light-years per century (80,000 kilometers per hour) and will be closest 28,000 years from now, when it will be 3.2 light-years from Earth.

But 10 or even 100 meteors a year is a pittance compared with Earth’s annual total of 7 trillion meteors. Furthermore, because Alpha Centauri lies far to the south, its meteors appear only in the far southern sky, out of sight of most people on Earth, Gregg and Wiegert say.

“Their calculations are right, but the problem hides basically in the assumptions,” says Simon Portegies Zwart, an astronomer at Leiden Observatory in the Netherlands. We don’t know the rate at which Alpha Centauri ejects material, he says, which means the actual number of interstellar objects coming from our near neighbor could be much greater or smaller than the study calculates. Nevertheless, the work demonstrates that our solar system is not an isolated object in space, he says. “We are connected to other objects — like Alpha Centauri, like other stars in the neighborhood.”