Most comets may have extra-solar origin

By Katia Moskvitch
Science reporter, BBC News

Hale-Bopp comet
Image caption,
Hale-Bopp comet is believed to have formed in the Oort cloud

Many famous comets may have formed in other Solar Systems, a new theory proposes.

Astronomers now believe that when our Sun was still a young star, it may have gravitationally captured the "dusty" Oort cloud comets formed elsewhere in the galaxy.

This contradicts the earlier theory that most comets were born in the Sun's protoplanetary disk.

The scientists described their findings in the journal Science.

The formation of the Oort cloud has long been a mystery.

Up until now, astronomers thought that this spherical cloud of comets lying at the outermost edge of the Solar System might have formed in the Sun's protoplanetary disk - a cloud of gas and matter that gave birth to planets, some 4.6 billion years ago.

But this hypothesis has been challenged by an international group of astronomers led by Dr Harold Levison from the Southwest Research Institute in Colorado, US.

Sun's cluster

A member of the team, Dr Ramon Brasser from the University of Nice-Sophia Antipolis, explained to BBC News that the Sun was not born alone.

Instead, he said, it is believed to have formed in a cluster of about a thousand of other stars, all packed together.

"Imagine that you have a very large cloud of gas composed of mostly hydrogen that is sitting around in our galaxy.

"From some disturbances inside it, the cloud slowly starts to collapse, it shrinks, becoming more compact.

"It then forms lumps and those lumps compress even further - that is how stars are born," said Dr Brasser.

He explained that each young star then creates a huge number of small icy bodies around it in a disk from which planets gradually form.

In our galaxy's early times, many of these icy objects got "ejected" from the planetary systems and eventually became comets.

But a few stayed near the Sun, affected by strong interstellar forces. They formed, astronomers used to believe, what became known as the peculiar "dusty" Oort cloud, about a light-year from the Sun.

It was assumed to be the birthplace of the majority of the famous comets, including Halley, Hale-bopp and McNaught.

Mystery 'solved'

When the Sun's cluster dispersed, exploding from inside out, the star was left all alone.

Image caption,
The Oort cloud was thought to have formed around the Sun

And the new study showed that its gravitational field may have been so strong that it pulled in a large cloud of comets originally formed in other solar systems.

The idea of the Oort cloud comets being extra-solar was suggested before, in the early 1990s. But back then, the methods used were not precise enough to prove the theory and it was abandoned.

Dr Levinson said that his team picked up on the same thought and used computer simulations to construct a model of a star cluster and comets - and had some interesting results.

"If we assume that the Sun's observed proto-planetary disk can be used to estimate the indigenous population of the Oort cloud, we can conclude that more than 90% of the observed Oort cloud comets have an extra-solar origin," commented the astronomer.

His colleague Martin Duncan from the Queen's University in Ontario, Canada, said that the findings lead "to the exciting possibility that the [Oort] cloud contains a potpourri that samples material from a large number of stellar siblings of the Sun".

Dr Brasser concluded that the recent findings may be an important missing link to explain the formation of the Universe.

"For 60 years we have not known how the Oort cloud formed and for 60 years people have been looking for an answer. It has been a missing piece and it might help understand the evolution and the formation of our Solar System," he said.

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