Accounting For Comets

A new study suggests that long-period comets like Hale-Bopp and Halley's may have originated in another star system.

Current theory holds that the Oort Cloud, home to many comets that occasionally blaze into the inner Solar System, was created when Jupiter ejected comets orbiting near it to the very edge of the Sun's influence. There seems to be two problems with that theory. First, the projected number of objects in the Oort Cloud seems much larger than any Jupiter ejection theory could produce. Second, gravity weakens substantially at huge distances from the primary, and it's unclear how so many bodies flung so far by Jupiter could simply stop rather than continuing into interstellar space.

The new study, which uses computer simulations, suggests the Sun was born in a star cluster, as most stars are. As the stars in the cluster went their own ways in space, comets born around one star were carried away under the gravitational influence of another.

Some astronomers, of course, question the new explanation. For one thing, they argue, we don't yet know how many bodies are in the Oort Cloud, so we can't say that number doesn't match current theory. Still, the new explanation is intriguing. We might be able to physically sample another solar system far sooner than scientists think.