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.
