Many comets originally formed in other solar systems

“Anyone who has seen a long tail comet in the night sky may be looking at material from another star,” says Professor Duncan.

The researchers used computer simulations to show that the Sun may have captured small icy bodies from its sibling stars while it was in its birth star cluster, and this created a reservoir for observed comets.

Although the Sun currently has no companion stars, it is believed to have formed in a cluster containing hundreds of closely packed stars that were embedded in a dense cloud of gas. During this time, each star formed a large number of small icy bodies (comets) in a disk from which planets formed. Most of these comets were gravitationally slung out of these prenatal planetary systems by the newly forming giant planets, becoming tiny, free-floating members of the cluster.

The Sun's cluster came to an end when its gas was blown out by the hottest young stars. The researchers' computer models show that the Sun then gravitationally captured a large cloud of comets as the cluster dispersed.

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

Evidence for the team's scenario comes from the roughly spherical cloud of comets (called the Oort cloud) which surrounds the Sun. Exactly how the Oort cloud was created has been a mystery for more than 60 years.

“We have a new model of how the Oort cloud formed. We're not the first to suggest this could happen but we are the first to show it in a detailed computer simulation,” adds Professor Duncan.

The research team also included Hal Levison and David Kaufmann (both of Southwest Research Institute in Boulder, CO) and Ramon Brasser (Observatoire de la Cote d'Azur, France). Their findings, “Capture of the Sun's Oort Cloud from Stars in its Birth Cluster” was published today in the online journal Science Express.