[meteorite-list] The Sun Steals Comets from Other Stars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 23 Nov 2010 14:54:05 -0800 (PST)
Message-ID: <201011232254.oANMs5tR029602_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/science-news/science-at-nasa/2010/23nov_aliencomets/

The Sun Steals Comets from Other Stars
NASA Science News

Nov. 23, 2010: The next time you thrill at the sight of a comet
blazing across the night sky, consider this: it's a stolen pleasure.
You're enjoying the spectacle at the expense of a distant star.

Sophisticated computer simulations run by researchers at the Southwest
Research Institute (SWRI) have exposed the crime.

"If the results are right, our Sun snatched comets from neighboring
stars' back yards," says SWRI scientist Hal Levison. And he believes
this kind of thievery accounts for most of the comets in the Oort Cloud
at the edge of our solar system.

"We know that stars form in clusters. The Sun was born within a huge
community of other stars that formed in the same gas cloud. In that
birth cluster, the stars were close enough together to pull comets away
from each other via gravity. It's like neighborhood children playing in
each others' back yards. It's hard to imagine it not happening."

According to this "thief" model, comets accompanied the nearest star
when the birth cluster blew apart. The Sun made off with quite a
treasure - the Oort Cloud, which was swarming with comets from all over
the "neighborhood."

The Oort cloud is an immense cloud of comets orbiting the Sun far beyond
Pluto. It is named after mid-20th century Dutch astronomer Jan Oort, who
first proposed such a cloud to explain the origin of comets sometimes
seen falling into the inner solar system. Although no confirmed direct
observations of the Oort cloud have been made, most astronomers believe
that it is the source of all long-period and Halley-type comets.

The standard model of comet production asserts that our Sun came by
these comets honestly.

"That model says the comets are dregs of our own solar system's
planetary formation and that our planets gravitationally booted them to
huge distances, populating the cloud. But we believe this kind of
scenario happened in all the solar systems before the birth cluster
dispersed."

Otherwise, says Levison, the numbers just don't add up.

"The standard model can't produce anywhere near the number of comets we
see [falling in from the Oort Cloud]. The Sun's sibling stars had to
have contributed some comets to the mix."

Comets in the Oort Cloud are typically 1 or 2 miles across, and they're
so far away that estimating their numbers is no easy task. But Levison
and his team say that, based on observations, that there should be
something like 400 billion comets there. The "domestic" model of comet
formation can account for a population of only about 6 billion.

"That's a pretty anemic Oort Cloud, and a huge discrepancy - too huge to
be explained by mistakes in the estimates. There's no way we could be
that far off, so there has to be something wrong with the model itself."

He points to the cometary orbits as evidence.

"These comets are in very odd orbits - highly eccentric long-period
orbits that take them far from our Sun, into remote regions of space. So
they couldn't have been born in orbit around the Sun. They had to have
formed close to other stars and then been hijacked here."

This means comets can tell us not only about the early history of the
Sun - but also about the history of other stars.

"We can study the orbits of comets and put their chemistry into the
context of where and around which star they formed. It's intriguing to
think we got some of our 'stuff' from distant stars. We're kin."

Author: Dauna Coulter
Editor: Dr. Tony Phillips
Credit: Science at NASA
Received on Tue 23 Nov 2010 05:54:05 PM PST