[meteorite-list] Discovery of a Large Kuiper Belt Object with an Unusual Orbit (2004 XR190)

[Ron Baalke]

http://www.cfeps.astrosci.ca/4b7/index.html

Canada France Hawaii Telescope
Kamuela, Hawaii

EMBARGOED
Print media: Publication no earlier than Tuesday Dec 13th
Electronic Media: No publication before 12:00 GMT Dec 13th

Discovery of a Large Kuiper belt object with an Unusual Orbit

A team of astronomers working in Canada, France and the United States have 
discovered an unusual small body orbiting the Sun beyond Neptune, in the 
region astronomers call the Kuiper belt. This new object is twice as far 
from the Sun as Neptune and is roughly half the size of Pluto. The body's 
highly unusual orbit is difficult to explain using previous theories of 
the formation of the outer Solar System.

Currently 58 astronomical units from the Sun (1 astronomical unit, or AU, 
is the distance between the Earth and the Sun), the new object never 
approaches closer than 50 AU, because its orbit is close to circular. 
Almost all Kuiper belt objects discovered beyond Neptune are between 30 AU 
and 50 AU away. Beyond 50 AU, the main Kuiper belt appears to end, and 
what few objects have been discovered beyond this distance have all been 
on very high eccentricity (non-circular) orbits. Most of these 
high-eccentricity orbits are the result of Neptune "flinging" the object 
outward by a gravitational slingshot. However, because this new object 
does not approach closer than 50 AU, a different theory is needed to 
explain its orbit. Complicating the problem, the object's orbit also has 
an extreme tilt, being inclined (tilted) at 47 degrees to the rest of the 
Solar System.

The Discovery and Follow-up

The object, which received the official designation 2004 XR 190 in the 
International Astronomical Union's official announcement, was discovered 
during routine operation of the Canada-France Ecliptic Plane Survey 
(CFEPS) running as part of the Legacy Survey on the Canada France Hawaii 
Telescope. For now, the discoverers are using the temporary nickname 
"Buffy" to identify the new object, although they have proposed a 
different official name in keeping with normal procedures for naming such 
objects.

Buffy was extracted from the mountain of Legacy Survey data (about 50 
gigabytes per hour of operation) by powerful computers combing through the 
telescopic images and producing hundreds of candidates. Astronomers then 
sift through the candidates to identify the distant comets.

Astronomer Lynne Allen of the University of British Columbia was the first 
to lay eyes on the new object, as she completed the initial identification 
in the course of processing CFEPS data from December 2004. "It was quite 
bright compared to the usual Kuiper belt objects we find", said Dr. Allen, 
"but what was more interesting was how far away it was."

The object's brightness implies it is likely between 500 and 1000 
kilometers (300 to 600 miles) in diameter. Buffy is thus a very large 
Kuiper belt object, but about half a dozen are larger.

"We immediately realized that the object was about twice as far as Neptune 
from the Sun and that its orbit was potentially nearly circular," said UBC 
professor Brett Gladman, who noticed the unusual nature of the object when 
determining its orbit, "but further observations were required."

One to two years of observations of a Kuiper belt object are required 
before their orbits can be precisely measured. The first additional 
observations of Buffy came in October 2005 when Gladman and Phil Nicholson 
of Cornell University used the Hale 5-meter telescope to re-observe the 
object.

Measurement of Buffy's new position proved that the orbit was not only 
extremely tilted, inclined (tilted) at 47 degrees to the plane of the 
planetary system (essentially tying the record for a Kuiper belt object) 
but confirmed that Buffy was unlike any other previously-known object 
because it was on a nearly circular orbit while at a very large distance.

More measurements of Buffy's position on images from telescopes at Kitt 
Peak National Observatories in Arizona by team members Joel Parker 
(Southwest Research Institute), as well as JJ Kavelaars (National Research 
Council of Canada, Herzberg Institute of Astrophysics) and Wes Fraser 
(University of Victoria), through November 2005 refined the estimate for 
Buffy's closest approach to the Sun. Additional observations, to further 
confirm the orbit, where then provided by the CFHT Legacy Survey project. 
Astronomers will need to wait until February 2006 to measure the fine 
details of the Buffy's orbit.

The team have reported their find to the Minor Planet Center, the 
clearinghouse for astronomical measurements of new minor planets. "To find 
the first known object with a nearly circular orbit beyond 50 AU is indeed 
intriguing," reacted Brian Marsden, director of the MPC.

Challenging Theories

Although it is neither the smallest, largest, nor most distant object 
discovered in this region, the new Kuiper belt object has a highly unusual 
orbit which challenges theories of the evolution of the Solar System.

Why is Buffy's orbit considered so unusual? Only one other detected 
object, Sedna, remains further than 50 astronomical units (AUs) from the 
Sun throughout its entire orbit. However, Sedna is on a very elliptical 
orbit, swooping in to 76 AU before traveling back out beyond 900 AU. In 
contrast, Buffy spends all of its time in the narrow range between 52 and 
62 AU from the Sun. Combined with the tilt in its orbit, this new object 
challenges current theories about the history of the early Solar System.

Astronomers have detected other Kuiper belt objects that spend most of 
their time beyond 50 AU. These are on very elliptical orbits, and almost 
all approach within 38 AU of the Sun. That close approach places those 
objects within the reach of the gravitational influence of Neptune. These 
objects are generally thought to have been scattered out to their present 
orbits by a gravitational slingshot with Neptune. This group of objects 
was thus called the "Scattered Disk".

Prior to the discovery of Buffy, a few other Kuiper belt objects were 
discovered which spend much of their time beyond 50 AU like those in the 
"Scattered Disk", yet did not approach within the gravitational reach of 
Neptune. This group has been named the "Extended Scattered Disk". Two of 
its members are 1995 TL8 and 2000 YW134, which approach to 40 AU of the 
Sun but have fairly elliptical orbits that take them back out beyond 60 
AU. Two more extreme examples of the "Extended Scattered Disk" are 2000 
CR105, which approaches to 44 AU, and Sedna, which never comes closer to 
the Sun than 76 AU.

Due to their large eccentricities, these objects are likely to have been 
strongly perturbed by something, although it could not have been Neptune 
because they do not come close enough to be scattered by that planet's 
gravitational force. As both Sedna and 2000 CR105 also travel beyond 500 
AU from the sun, one theory is that after being scattered by Neptune, a 
passing star could have pulled their closest approaches away from the Sun.

Buffy is clearly a member of the "Extended Scattered Disk". However, 
Buffy's almost circular orbit makes it stand out from the other members. 
In addition, Buffy's large orbital tilt is not so easily explained by the 
passing star idea. If a star could have affected Buffy so strongly, it 
should also have disrupted much of the main Kuiper belt as well. Since 
astronomers do not detect that strong disruption, a more complex theory is 
needed to explain Buffy's orbit.

The elusive explanation may lie in side-effects from rearrangements of the 
Solar System early in its history. One possibility is that as Neptune's 
orbit slowly expanded in the young Solar System, complex gravitational 
interactions could have caused some Kuiper belt orbits to circularize and 
tilt. While Buffy's orbit could have been created this way, this theory 
would not seem to explain 2000 CR105 and Sedna. This new discovery is 
exciting because it causes us to rethink our understanding of how the 
Kuiper belt formed.

The Future

Over the last half decade, theories about the formation of our outer Solar 
System have been pushed to their limits: unusual Kuiper belt objects, like 
Buffy, which never come close to Neptune yet have high inclination must be 
explained.

Although theories that explain individual objects exist, reproducing the 
entire ensemble of known objects with one process poses a difficult 
challenge to current solar system models. Because the unusual objects, 
like Buffy, are very rare, astronomers are still scratching the surface of 
the dark corners of the Kuiper belt. Future large-scale surveys that 
systematically explore the Kuiper belt are the only way unlock the 
mysteries of what happened early in the history of our Solar System.

LINKS

* 3D simulation of Buffy's orbit, copyright dunnbypaul.net. Please note 
this is an 11MB MPEG movie.
  http://www.cfeps.astrosci.ca/4b7/Buffy-simulation.mpg

* A black and white, EPS version of the face-on view of the Solar System 
(first figure above).
  http://www.cfeps.astrosci.ca/4b7/4b7orb_bw.eps

* A black and white, EPS version of the side-on view of the Solar System 
(second figure above).
  http://www.cfeps.astrosci.ca/4b7/4b7inc_bw.eps

* A slightly more technical web page on Buffy, with some more information 
and details.
  http://www.cfeps.astrosci.ca/4b7/techie.html

* Also, orbital element diagrams, a pair of technical figures describing 
the orbital parameters of 2004 XR190 (in red, with uncertainty estimate) 
relative to other known Kuiper belt objects whose orbits have been 
observed for more than two years.
  http://www.cfeps.astrosci.ca/4b7/plotelem.gif

For reference: The first link on this web page 
<http://www.astro.ubc.ca/people/gladman/pub.html> goes to a only 
slightly-technical recent review paper in Science about orbital population 
of the outer Solar System.