Donald Savage
Headquarters, Washington October 7, 2002
(Phone: 202/358-1547)
Ray Villard
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4514)
Robert Tindol
Caltech, Pasadena, Calif.
(Phone: 626/395-3631)
RELEASE: 02-190
HUBBLE SPOTS AN ICY WORLD FAR BEYOND PLUTO
NASA's Hubble Space Telescope has measured the largest
object in the solar system seen since the discovery of Pluto
72 years ago.
Approximately half the size of Pluto, the icy world 2002
LM60, dubbed "Quaoar" (pronounced kwa-whar) by its
discoverers, is the farthest object in the solar system ever
to be resolved by a telescope. It was initially detected by a
ground-based telescope as simply a dot of light, until
astronomers aimed Hubble's powerful telescope at it.
Quaoar is about 4 billion miles away from Earth, well over a
billion miles farther away than Pluto. Unlike Pluto, its
orbit around the Sun is circular, even more so than most of
the planetary-class bodies in the solar system.
Although smaller than Pluto, Quaoar is greater in volume than
all the asteroids combined (though probably only one-third
the mass of the asteroid belt, because it's icy rather than
rocky). Quaoar's composition is theorized to be largely ices
mixed with rock, not unlike the makeup of a comet, though 100
million times greater in volume.
This finding yields important new insights into the origin
and dynamics of the planets, and the mysterious population of
bodies dwelling in the solar system's final frontier: the
elusive, icy Kuiper Belt beyond Neptune.
Michael Brown and Chadwick Trujillo of the California
Institute of Technology, Pasadena, Calif. are reporting the
findings today at the 34th annual meeting of the Division for
Planetary Sciences of the American Astronomical Society in
Birmingham, Ala.
Earlier this year, Trujillo and Brown used the Palomar Oschin
Schmidt telescope to discover Quaoar as an 18.5-magnitude
object creeping across the summer constellation Ophiuchus
(it's less than 1/100,000 the brightness of the faintest star
seen by the human eye). Brown had to do follow-up
observations using Hubble's new Advanced Camera for Surveys
on July 5 and August 1, 2002, to measure the object's true
angular size of 40 milliarcseconds, corresponding to a
diameter of about 800 miles (1300 kilometers). Only Hubble
has the sharpness needed to actually resolve the disk of the
distant world, leading to the first-ever direct measurement
of the true size of a Kuiper Belt Object (KBO).
Like Pluto, Quaoar dwells in the Kuiper Belt, an icy debris
field of comet-like bodies extending 7 billion miles beyond
Neptune's orbit. Over the past decade more than 500 icy
bodies have been found in the Kuiper Belt. With a few
exceptions all have been significantly smaller than Pluto.
Previous record holders are a KBO called Varuna, and an
object called 2002 AW197, each approximately 540 miles across
(900 kilometers). Unlike dimensions derived from Hubble's
direct observations, these diameters are deduced from
measuring the objects' temperatures and calculating a size
based on assumptions about the KBOs' reflectivity, so the
uncertainty in true size is much greater.
This latest large KBO is too new to have been officially
named by the International Astronomical Union. Trujillo and
Brown have proposed naming it after a creation god of the
Native American Tongva tribe, the original inhabitants of the
Los Angeles basin. According to legend, Quaoar "came down
from heaven; and, after reducing chaos to order, laid out the
world on the back of seven giants. He then created the lower
animals, and then mankind."
Quaoar's "icy dwarf" cousin, Pluto, was discovered in 1930 in
the course of a 15-year search for trans-Neptunian planets.
It wasn't realized until much later that Pluto actually was
the largest of the known Kuiper Belt objects. The Kuiper Belt
wasn't theorized until 1950, after comet orbits provided
telltale evidence of a vast nesting ground for comets just
beyond Neptune. The first recognized Kuiper Belt objects were
not discovered until the early 1990s. This new object is by
far the "biggest fish" astronomers have snagged in KBO
surveys. Brown predicts, within a few years, even larger KBOs
will be found, and Hubble will be invaluable for follow-up
observations to pin down sizes.
Electronic images, illustrations, animation, and additional
information are available at:
http://oposite.stsci.edu/pubinfo/pr/2002/17
- end -
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