A close encounter between our sun and a passing star some four billion years ago may have played a role in shaping our solar system, a new report suggests. Computer simulations published today in the journal Nature describe how a rendezvous between two young solar systems could have occurred. And one potential scenario shows our sun kidnapping a planet or smaller object from the other star's solar system.

The Kuiper belt, a region of icy objects orbiting beyond Neptune, has an abrupt outer edge. Last year, astronomers discovered an icy world far beyond the Kuiper belt called Sedna, which orbits the sun in a different plane than the major planets. Scott J. Kenyon of the Smithsonian Astrophysical Observatory and Benjamin C. Bromley of the University of Utah devised computer simulations using a NASA supercomputer to analyze scenarios that could explain Sedna's presence. The interaction of two young solar systems still in the planetary-disk phase would act like two circular saw blades crossing paths, they report. "Any objects way out in the planetary disk would be stirred up greatly," Bromley explains. Indeed, their calculations show that the flyby could have changed the orbit of planetoids already circling the sun, sheared off objects on the outer edge of the disk or allowed our solar system to capture a planet from the nearby solar system.

The results indicate that there is up to a 10 percent chance that Sedna formed in our solar system and had its orbit perturbed by an encounter with another star, and about a 1 percent chance that Sedna is a captured alien world. The predictions are hindered, because Sedna is the sole known object of its type out beyond the Kuiper belt. But "there may be thousands of objects like Sedna near the edge of our solar system," Bromley says. "So there is an even greater chance that some may be alien worlds captured from another solar system."