[meteorite-list] Stray Star May Have Jolted Sedna

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jul 27 17:29:27 2004
Message-ID: <200407272117.OAA18897_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/news/news.jsp?id=ns99996204

Stray star may have jolted Sedna
Maggie McKee
New Scientist
July 27, 2004

Sedna, the most distant planetoid ever seen in the Solar System,
probably got kicked into its orbit when a star swept past the Sun more
than four billion years ago, suggest the first detailed calculations of
the object's origins.

The research supports the leading theory of Sedna's origins but also
leaves open more outlandish possibilities.

The planetoid, about three-quarters the size of Pluto, was discovered in
November 2003. It takes about 12,000 years to traverse an elongated
orbit that stretches from 74 to 900 times the distance from the Sun to
the Earth. And its journey around the Sun is thought to take Sedna from
its present location in the shadowy Kuiper Belt out towards the Oort
Cloud at the Solar System's outer edges.

The Kuiper Belt is a mysterious band of rock and ice leftover from the
birth of the Solar System, which lies beyond Neptune. The remote Oort
Cloud forms a spherical shell of icy bodies around the Solar System and
its edges lie many thousands of times Pluto's distance from the Sun.

Sedna's orbit is so extreme researchers say it could not have formed
simply from the gravitational kicks of the giant planets, which are
responsible for the eccentric orbits of the comets and Pluto.

"If this thing was scattered out by a planet, something else had to
change the orbit, something we don't see," says study co-author Hal
Levison, an astronomer at the Southwest Research Institute in Boulder,
Colorado. "That's why Sedna and 2000 CR 105 [the next most-distant
object] are so cool. They tell us something was different back when they
formed."

Cluster of stars

Levison and colleague Alessandro Morbidelli of the Observatoire de la
Cote d'Azur in Nice, France, used computer simulations to study five
different scenarios for how Sedna and 2000 CR 105 got their orbits.

The most likely scenario supports one of the theories put forward by
Sedna's discoverers. They believe the Sun was born in a cluster of
stars, and that one or more of those siblings passed by the Sun in the
stars' first 100 million years.

The new study recreates Sedna's orbit using this scenario. "I still
strongly favour that hypothesis," Sedna's co-discoverer Michael Brown of
the California Institute of Technology in Pasadena told New Scientist.

But the new study discounts Brown and his colleagues' other main theory
- that a planet lying at about 75 times the Sun-Earth distance is
responsible for Sedna's orbit. "It's still a possibility, but we haven't
found anything there so we don't believe it so much these days," Brown
concedes.

Brown dwarf

The study also quashes other theories, including the hypothesis that
Neptune and Uranus, thought to have been in more eccentric orbits in the
past, could have pushed Sedna and other bodies outward. Those planets
are not massive enough to have done the job in their short eccentric
phases, Levison says.

But the researchers thought up another improbable scenario that managed
to explain Sedna's orbit remarkably well. Sedna could have been born
around a brown dwarf about 20 times less massive than the Sun and
captured by our Solar System when the brown dwarf approached.

"What's striking about this idea is how efficient it is," says Levison,
whose calculations suggest about half of the material orbiting the dwarf
would have gone into orbit around the Sun. "Even if it's wrong it's a
cool idea."

"It just seems implausible, but that doesn't mean it's not true," agrees
Brown.

The study is scheduled for publication in November 2004 in the
Astronomical Journal.
Received on Tue 27 Jul 2004 05:17:53 PM PDT