Many famous comets originally formed in other solar
systems
Thursday, June 10, 2010
Many of the most well known comets, including Halley, Hale-Bopp
and, most recently, McNaught, may have been born in orbit around
other stars, according to a new theory by an international team of
astronomers led by a scientist from the Southwest Research
Institute (SwRI) in Boulder, Colo.
Dr. Hal Levison (SwRI), Dr. Martin Duncan (Queen's University,
Kingston, Canada), Dr. Ramon Brasser (Observatoire de la Côte
d'Azur, France) and Dr. David Kaufmann (SwRI) 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,
thereby creating a reservoir for observed comets.
While 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 a violent end, however, when its gas was
blown out by the hottest young stars. These new models show that
the Sun then 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 Levison.
"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 co-author Duncan.
Evidence for the team's 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 Sun's proto-planetary disk. However,
because detailed models show that comets from the solar system
produce a much more anemic cloud than observed, another source is
required.
Levison says, "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 percent of the observed
Oort cloud comets have an extra-solar origin."
"The formation of the Oort cloud has been a mystery for over 60
years and our work likely solves this long-standing problem," says
Brasser.
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Thanks to Southwest Research Institute for this article.