[meteorite-list] Strange New Object Found At Edge of Solar System (2004 XR190)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Dec 13 16:37:16 2005
Message-ID: <200512132135.jBDLZal24005_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8455-strange-new-object-found-at-edge-of-solar-system.html

Strange new object found at edge of Solar System
Maggie McKee
New Scientist
13 December 2005

A large object has been found beyond Pluto travelling in an orbit tilted
by 47 degrees to most other bodies in the solar system. Astronomers are
at a loss to explain why the object's orbit is so off-kilter while being
almost circular.

Researchers led by Lynne Allen at the University of British Columbia in
Vancouver, Canada, first spotted the object in observations made with
the Canada-France-Hawaii Telescope in December 2004. Since October 2005,
they have made follow-up observations that have revealed the object's
perplexing path.

Tentatively named 2004 XR190, the object appears to have a diameter of
between 500 and 1000 kilometres, making it somewhere between a fifth and
nearly half as wide as Pluto. It lies in a vast ring of icy bodies
beyond Neptune called the Kuiper Belt, most of which orbit in nearly the
same plane as Earth.

But at 47 degrees, 2004 XR190's orbit is one of the most tilted, or
inclined, Kuiper Belt Objects known. That suggests it was flung out of
the solar system's main disc after a close encounter with another object
- such as Neptune or perhaps a star that passed by the Sun billions of
years ago.

Scatter effect

Neptune has been blamed for scattering many other KBOs into tilted
paths. But these tend to show other signs of a past interaction with the
giant planet, such as moving in elliptical paths and having one part of
their orbit pass near Neptune's at 30 astronomical units (AU) from the
Sun (1 AU is the distance between the Earth and the Sun).

2004 XR190, however, follows a nearly circular path. And it is too
distant to have come into direct contact with Neptune, travelling
between 52 and 62 AU from the Sun. Its orbit is also too circular - and
too small - to have been tilted by a passing star, says Allen.

These traits make the object, nicknamed "Buffy" after the US television
series about a vampire slayer, hard to explain. "Maybe Buffy is going to
be a bit of a theory slayer," Allen told New Scientist.

But she suggests one theory that might account for the space rock's
strange orbit. It involves a commonly held notion that early in the
solar system, Neptune itself moved outward into its present orbit, from
around Uranus's current location.
          
Gravitational kick

As it did so, its gravitational reach extended outwards, as well. This
reach comes in the form of zones, or resonances, where an object's
orbital period happens to be an integer multiple of Neptune's. So when
one of these outward-expanding resonances swept past 2004 XR190, it
could have kicked the object out of a fairly circular, flat orbit into a
more elongated, tilted one.

Then, over time, the orbit might have grown more circular as the tilt
increased. "These interactions can cause some Kuiper Belt Objects to
circularise and tilt," says Allen. But she remains cautious: "We don't
know if Buffy's orbit really was created in this manner - because it
could be too far away from a resonance or the resonance could not be
strong enough - but this seems like the best shot."

Renu Malhotra, a planetary scientist at the University of Arizona in
Tucson, US, says most resonances simply elongate an object's orbit. She
says a few objects could "trade off" some of their elongation, or
eccentricity, for a higher tilt, but the effect would be small. "I find
it hard to see how you would get a large inclination out of a modest
eccentricity," Malhotra told New Scientist. "There's a limit on how much
inclination you can trade off."

Hal Levison, a planetary scientist at the Southwest Research Institute
in Boulder, Colorado, US, says he and others have produced objects like
Buffy using models of such special resonances. "However, I do have some
problems with the idea," he admits.

Hidden objects

He points out that this object was found when it happened to be passing
through the plane of the solar system - where it spends just 2% of its
orbit. That suggests many more such objects remain undiscovered, tilted
at orbits where most surveys do not search for them. "I just don't think
these mechanisms can deliver that much stuff," Levison told New Scientist.

He ventures another possible explanation - that the Sun had a twin and
that both stars followed circular orbits around each other. "That could
excite inclinations without exciting the eccentricities," he says.
"However, this idea creates more problems than it solves, by far."

Allen and her colleagues will continue to observe the new object to pin
down its orbit more accurately. And following the convention for naming
bodies in the Kuiper Belt, they have suggested several Inuit names for
the object to the International Astronomical Union.
Received on Tue 13 Dec 2005 04:35:36 PM PST