© Royal Observatory, Edinburgh/AAO/SPL
Stolen from a stellar nursery
Our sun may have stolen the vast majority of its comets from other stars. The theft could explain the puzzling profusion of objects in a huge reservoir surrounding the sun called the Oort cloud.

The Oort cloud is a collection of comets thought to orbit the sun in a roughly spherical halo about 50,000 times as far from the sun as Earth - at the outer edge of the solar system. How did the comets get there? In the standard picture, they formed much closer to the sun, then migrated outward in a two-stage process.

First, the gravity of the giant planets flung them into elongated orbits to form a population called the scattered disc. Objects in the scattered disc come about as close to the sun as Neptune, but venture dozens of times further out, to more than 1000 times the Earth-sun distance. That far from the sun, the gravitational pull of the galaxy becomes significant, so many of the scattered-disc objects get pulled out to populate the Oort cloud.

There is a problem with this picture, however. Simulations have long predicted that this process could only populate the Oort cloud with 10 times as many comets as are currently in the scattered disc, while estimates based on observed comets suggest the ratio is more like 700 to 1.

"This nice, beautiful picture we have been developing for the last 25 years or so just crashes and burns," Hal Levison of the Southwest Research Institute in Boulder, Colorado, said at a meeting of the American Astronomical Society's Division on Dynamical Astronomy in Boston last week.

Levison and his colleagues say many of the Oort-cloud objects may have been stolen from other stars born in the same stellar nursery as the sun. Most stars like the sun form in clusters of between 10 and 1000 members. According to the team's simulations, encounters between stars in this crowded environment tend to disturb their scattered discs and detach objects from them, creating a reservoir of free-floating comets.

When stars later leave the cluster, some of these objects move along with them, getting captured into wide, Oort cloud-like orbits. "They head off in the same direction together and eventually become bound," says Levison. That could explain the high number of Oort-cloud objects in our solar system. If most of the cloud's members were captured from other stars, then famous comets like Halley and Hale-Bopp, whose trajectories suggest they once resided in the Oort cloud, probably were too, Levison says.

Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was not involved in the study, says it is too soon to tell whether there really is an overabundance of Oort-cloud objects, since observations are not good enough to provide very precise estimates of the population there. But if the overabundance persists with better observations, the capture scenario could explain it, he says. "It's a very interesting idea and it might work."