BIRMINGHAM, Ala.-Planetary scientists at the California Institute of
Technology have discovered a spherical body in the outskirts of the
solar system. The object circles the sun every 288 years, is half
the size of Pluto, and is larger than all of the objects in the
asteroid belt combined.

The object has been named “Quaoar” (pronounced KWAH-o-ar) after the
creation force of the Tongva tribe who were the original inhabitants
of the Los Angeles basin, where the Caltech campus is located.
Quaoar is located about 4 billion miles from Earth in a region beyond
the orbit of Pluto known as the Kuiper belt. This is the region
where comets originate and also where planetary scientists have long
expected to eventually find larger planet-shaped objects such as
Quaoar. The discovery, announced at the meeting of the Division of
Planetary Sciences of the American Astronomical Society in
Birmingham, Alabama, today, is by far the largest object found so far
in that search.

Currently detectable a few degrees northwest of the constellation
Scorpio, Quaoar demonstrates beyond a doubt that large bodies can
indeed be found in the farthest reaches of the solar system.
Further, the discovery provides hope that additional large bodies in
the Kuiper belt will be discovered, some as large, or even larger
than Pluto. Also, Quaoar and other bodies like it should provide new
insights into the primordial materials that formed the solar system
some 5 billion years ago.

The discovery further supports the ever-growing opinion that Pluto
itself is a Kuiper belt object. According to recent interpretations,
Pluto was the first Kuiper belt object to be discovered, long before
the age of enhanced digital techniques and charge-coupled (CCD)
cameras, because it had been kicked into a Neptune-crossing
elliptical orbit eons ago.

“Quaoar definitely hurts the case for Pluto being a planet,” says
Caltech planetary science associate professor Mike Brown. “If Pluto
were discovered today, no one would even consider calling it a planet
because it’s clearly a Kuiper belt object.”

Brown and Chad Trujillo, a postdoctoral researcher, first detected
Quaoar on a digital sky image taken on June 4 with Palomar
Observatory’s 48-inch Oschin Telescope. The researchers looked
through archived images taken by a variety of instruments and soon
found images taken in the years 1982, 1996, 2000, and 2001. These
images not only allowed Brown and Trujillo to establish the distance
and orbital inclination of Quaoar, but also to determine that the
body is revolving around the sun in a remarkably stable, circular
orbit.

“It’s probably been in this same orbit for 4 billion years,” Brown says.

The discovery of Quaoar is not so much a triumph of advanced optics
as of modern digital analysis and a deliberate search methodology.
In fact, Quaoar apparently was first photographed in 1982 by
then-Caltech astronomer Charlie Kowal in a search for the postulated
“Planet X.” Kowal unfortunately never found the object on the
plate-much less Planet X-but left the image for posterity.

Because the precise location of Quaoar on the old plates is highly
predictable, the orbit is thought to be quite circular for a solar
system body, and far more circular than that of Pluto. In fact,
Pluto is relatively easy to spot-at least if one knows where to look.
Because Pluto comes so close to the sun for several years in its
248-year eccentric orbit, the volatile substances in the atmosphere
are periodically heated, thereby increasing the body’s reflectance,
or albedo, to such a degree that it is bright enough to be seen even
in small amateur telescopes.

Quaoar, on the other hand, never approaches the sun in its circular
orbit, which means that the volatile gases never are excited enough
to kick up a highly reflective atmosphere. As is the case for other
bodies of similar rock-and-ice composition, Quaoar’s surface has been
bathed by faint ultraviolet radiation from the sun over the eons, and
this radiation has slowly caused the organic materials on the body’s
surface to turn into a dark tar-like substance.

As a result, Quaoar’s albedo is about 10 percent, just a bit higher
than that of the moon. By contrast, Pluto’s albedo is 60 percent.

As for spin rate, the researchers know that Quaoar is rotating
because of slight variations in reflectance in the six weeks they’ve
observed the body. But they’re still collecting data to determine
the precise rate. They will also probably be able to figure out
whether the spin axis is tilted relative to the ecliptical plane.

Inclination is about 7.9 percent, which means that the plane of
Quaoar’s orbit is tilted by 7.9 degrees from the relatively flat
orbital plane in which all the planets except Pluto are to be found.
Pluto’s orbital inclination is about 17 degrees, which presumably
resulted from whatever gravitational interference originally thrust
it into an elliptical orbit.

Quaoar’s orbital inclination of 7.9 degrees is not particularly
surprising, Brown says, because the Kuiper belt is turning out to be
wider than originally expected. The Kuiper belt can be thought of as
a band extending around the sky, superimposed on the path of the sun.
Brown and Trujillo’s research, in effect, is to take repeated
exposures of a several-degree swath of this band and then use digital
equipment to check and see if any tiny point of light has moved
relative to the stellar background.

Brown and Trujillo are currently using about 10 to 20 percent of the
available time on the 48-inch Oschin Telescope, which was used to
obtain both the Palomar Sky Survey and the more recent Palomar
Digital Sky Survey. The latter was completed just last year, thus
freeing up the Oschin Telescope to be refitted by the Jet Propulsion
Laboratory for a new mission to search for near-Earth asteroids.
About 80 percent of the telescope time is now designated for the
asteroid survey, leaving the remainder for scientific studies like
Brown and Trujillo’s.

Siuce the discovery, the researchers have also employed other
telescopes to study and characterize Quaoar, including the Hubble
Space Telescope (related news release available at link below) and
the Keck Observatory on Mauna Kea, Hawaii. Information derived from
these studies will provide new insights into the precise composition
of Quaoar and may answer questions about whether the body has a
tenuous atmosphere.

But the good news for the serious amateur astronomer is that he or
she doesn’t necessarily need a space telescope or 10-meter reflector
to get a faint image of Quaoar. Armed with precise coordinates and a
16-inch telescope fitted with a CCD camera-the kind advertised in
magazines such as Sky and Telescope and Astronomy-an amateur should
be able to obtain images on successive nights that will show a faint
dot of light in slightly different positions.

As for Brown and Trujillo, the two are continuing their search for
other large Kuiper-belt bodies. Some, in fact, may be even larger
than Quaoar.

“Right now, I’d say they get as big as Pluto,” says Brown.