Comets formed in other solar systems

Many well-known comets such as Halley, Hale-Bopp and McNaught, may have been born around other stars, says an international team of researchers.

Hal Levison and David Kaufmann from the Southwest Research Institute, along with Martin Duncan from Queen's University in Canada and Ramon Brasser from Observatoire de la Cote d’Azur in France, teamed up to create computer simulations that model the evolution of the Sun's early life.

Comets like McNaught may be interlopers from another solar system. Image: S. Deiries/ESO.

Although the Sun has no companion stars today, it likely formed within a cluster of hundreds of closely packed stars all sharing the same gas cloud from which they grew. Each star formed with its own disc of dust and gas which eventually spawned planets. Left over material from the planet-building phase included rock and icy debris such as asteroids and comets. Many of these comets were slung out of the newly forming solar system by the dominating gravity of the gas giants. But as gas blown out by the hottest young stars in the cluster caused the cluster to disperse, the Sun hung on to a large cloud of comets.

“The process of capture is surprisingly efficient and leads to the exciting possibility that the cloud contains a potpourri that samples material from a large number of stellar siblings of the Sun,” says Duncan.

The Oort cloud in our Solar System has long been thought to have formed from the Sun's proto-planetary disc, but models suggest that there would not be enough material to account for its population. In comes Levison et al's idea. “When it was young, the Sun shared a lot of spit with its siblings, and we can see that stuff today,” he says. “If we assume that the Sun’s observed proto-planetary disc can be used to estimate the indigenous population of the Oort cloud, we can conclude that more than 90 percent of the observed Oort cloud comets have an extra-solar origin.”

The formation of the Oort cloud has been a much debated subject for over half a century, and the new simulations present an exciting solution to this long-standing problem.

The details of the computer simulations are presented in the 10 June issue of the journal Science Express.