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10/07/2002 07:00:00

Caltech scientists find largest object in solar system since Pluto's discovery

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 1983, 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.

Since 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.


Written by elisabeth nadin