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Mini-planet found
Discovery 4 billion miles away near Pluto hints other celestial bodies may be in same neighborhood

Carl T. Hall, Chronicle Science Writer
Tuesday, October 8, 2002

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Astronomers have discovered the biggest thing out there since Pluto was found -- a rocky, icy mini-planet circling the sun far beyond Neptune.

Tentatively named "Quaoar" (pronounced "kwah-oh-wahr") after a creation god of Southern California's Tongva Indian tribe, the newfound world is about 4 billion miles away in an outer fringe of the solar system known as the Kuiper Belt, a birthing zone of comets.

The discovery sets a new outer limit to Earth's celestial neighborhood and strongly hints that other hidden "minor planets" may exist in the same general region as Pluto.

These objects, generally too small and probably too numerous to be counted officially as "planets," are just coming into view with the aid of the powerful Hubble Space Telescope and sophisticated, computer-guided scans of the heavens.

"We all went to school and were taught there were nine planets orbiting the sun," said astronomer Lucy McFadden at the University of Maryland. "Now we know there are different regions with many smaller objects. So the solar system turns out to be a much more complicated place, and much more populated, than we thought."

Quaoar is only about 800 miles across, or one-tenth the diameter of Earth. But that's big enough to make it roughly half the size of Pluto and a little larger than Charon, Pluto's mysterious moon, which was discovered in 1978.

The measurements were taken from Hubble images after initial discovery with earthbound instruments by astronomers at the California Institute of Technology.

"This heralds a new era of finding objects in the outer Kuiper Belt," said NASA spokesman Donald Savage. "Our capabilities are better. The Hubble is the only telescope capable of achieving this kind of measurement."

Caltech planetary scientist Michael Brown and post-doctoral researcher Chadwick Trujillo announced the discovery Monday in Birmingham, Ala., during a meeting of the American Astronomical Society's division for planetary sciences.

A STABLE PATH

Unlike Pluto's elliptical orbit, Quaoar follows a remarkably stable, circular path around the sun, taking 288 Earth years to make one trip. The same orbit appears likely to have persisted for about 4 billion years.

"My first reaction was, 'This thing is really bright,"' Trujillo said during an interview. "We knew its distance, so we knew it had to be big."

"Our initial guess was it might be larger than Pluto, in fact, because it was so bright," he said. "It turned out to be half the size of Pluto -- but that's still big."

Although it's not highly compacted, for sheer volume the newfound object is larger than all the asteroids combined. Some even bigger Kuiper Belt objects may be out there. But for now, space experts said Quaoar ranked as the biggest discovery in the solar system since Pluto was found 72 years ago.

It appears in the night sky in the summer constellation Ophiuchus as an 18. 5-magnitude dot -- about a 100,000th as bright as the faintest star visible to an unaided human eye. Amateur stargazers with the exact coordinates, a 16-inch telescope and a CCD camera should be able to spot it moving on successive nights, according to the Caltech astronomers. (Coordinates and other details are at www.gps.caltech.edu/~chad/quaoar/).

FIVE-HOUR TRIP

Located 1 billion miles past Pluto, Quaoar is the most distant object in the solar system to be resolved and measured by telescope. It takes sunlight five hours to get there. It would take us about 25 years at space shuttle speeds.

As for what we'd find, scientists can only speculate: An unearthly jumble of primordial rock and ice, including frozen water, methane, methanol, carbon dioxide and carbon monoxide. Almost certainly no atmosphere.

Perhaps there would be a scarred, cratered surface, judging from the look of other objects of the same basic kind -- and a somewhat fanciful artist's rendering issued by NASA. "We really have no idea what the surface is like," Trujillo said.

Scientists refer to the icy sphere as 2002 LM60, the mini-planet's only official name until the International Astronomical Union considers whether to go along with the choice of its discoverers, who researched local Southern California Indian lore for a suitable moniker.

In Tongva legend, Quaoar is the name of a creation god or force that "came down from heaven, and after reducing chaos to order, laid out the world on the back of seven giants."

PIP-SQUEAK OF A PLANET

But on a planetary scale, the newfound Quaoar turns out to be a pip-squeak - - probably not a "planet" at all, depending on how the term is used.

Technically speaking, both Pluto and Quaoar are merely the largest known Kuiper Belt objects, deep-frozen relics of the early formation of the universe. About 650 such objects, nearly all tiny, have been cataloged so far. The new discovery suggests Pluto was merely the first, and so far still the largest, to be noticed.

The latest find was tagged first in June as a faint traveling pinpoint of light on a digital image of the sky made at the Palomar Observatory near San Diego.

Later observations with the Hubble telescope brought much clearer images and the first direct measurements. Researchers then examined old archived sky pictures, finding Quaoar was lurking undetected on images taken as far back as 1982.

Brown and Trujillo said they had found Quaoar after looking at less than 10 percent of the sky. They expect to find many more objects similar to Quaoar -- some perhaps bigger than Pluto -- as they continue to systematically sweep the heavens with the aid of a computer program specifically designed to pick out big Kuiper Belt objects.

E-mail Carl T. Hall at chall@sfchronicle.com.

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