Comet Halley, and other famous comets, could have been "stolen" from other stars systems.

• Comets formed within a cluster of stars
• Sun captured comets when the stars dispersed
• 90% could be from beyond the Solar System

Many of our Solar System’s most well known comets—including Halley, Hale-Bopp and, most recently, McNaught—could have been stolen from other stars, according to a new idea by an international team of astronomers.

The team used computer simulations to show that the Sun may have captured small icy bodies from its sibling stars when it was still young.

While the Sun currently has no companion stars, it is believed to have formed in a star cluster containing hundreds of closely packed stars that were embedded in a dense nebula of gas.

During this time, each star formed a large number of small icy bodies (comets) in a cloud from which its planets also formed.

Most of these comets were slung out of these young systems by the gravity of the newly forming planets, becoming tiny, free-floating ice balls in deep space.

The Sun’s star cluster eventually scattered, the individual stars going their own ways. The new computer models show that the Sun gravitationally captured a large cloud of comets as the cluster dispersed.

“When it was young, the Sun shared a lot of ‘spit’ with its siblings, and we can see that stuff today,” says lead author Dr Hal Levison of the Southwest Research Institute.

An artist's impression of the Oort cloud, a swarm of comets that surrounds the Solar System.

90% of comets are foreigners

The scientists say this leads to the exciting possibility that the Sun’s current comet cloud contains a potpourri that includes material from a large number of the Sun’s infant stellar siblings.

Evidence for the scenario comes from the roughly spherical cloud of comets, known as the Oort cloud, that surrounds the Sun, extending halfway to the nearest star.

It has been commonly assumed this cloud formed from the flattened gas and dust cloud that surrounded the young Sun. However, because detailed models show that comets that formed as part the Solar System would have produced a thinner cloud than that known today, an extra source of comets is required.

Dr Levison says that “we can conclude that more than 90 percent of the observed Oort cloud comets” came from beyond the Solar System.

“The formation of the Oort cloud has been a mystery for over 60 years and our work likely solves this long-standing problem,” says Dr Ramon Brasser of the Observatoire de la Côte d’Azur, France.

Adapted from information issued by Southwest Research Institute / NASA.