Say Hello To Quaoar
Astronomers have dubbed it "Quaoar" (pronounced kwa-whar) after
a Native American god. It lies a billion kilometers beyond Pluto
and moves around the Sun every 288 years in a near-perfect circle.
Until recently it was just a curious point of light. That's all
astronomers could see when they discovered it last June using a
ground-based telescope.
But now it's a world.
NASA's Hubble Space Telescope has measured Quaoar and found it
to be 1300 km wide. That's about 400 km wider than the biggest
main-belt asteroid (Ceres) and more than half the diameter of Pluto
itself. Indeed, it's the largest object in the solar system seen
since the discovery of Pluto 72 years ago.
Quaoar is greater in volume than all known asteroids combined.
Researchers suspect it's made mostly of low-density ices mixed with
rock, not unlike the makeup of a comet. If so, Quaoar's mass is
probably only one-third that of the asteroid belt.
Michael Brown and Chadwick Trujillo of the California Institute
of Technology, Pasadena, CA are reporting these 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 48-inch
telescope to discover Quaoar as an 18.5-magnitude object creeping
across the summer constellation Ophiuchus. Although Quaoar was
relatively bright (by the feeble standards of such distant objects)
its disk was too small for the Palomar telescope to resolve.
Brown followed-up their discovery using the Hubble Space
Telescope. Hubble's new Advanced Camera for Surveys revealed 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 such a distant
world.
Like the planet Pluto, Quaoar dwells in the Kuiper Belt, an icy
debris field of comet-like bodies extending 5 billion kilometers
beyond Neptune's orbit. Over the past decade more than 500 icy
bodies--Kuiper-Belt Objects or "KBOs" for short--have been found
there. 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). Those diameters were deduced by measuring the objects'
temperatures and calculating a size based on assumptions about the
KBOs' reflectivity. Such estimates are less certain than Hubble's
direct measurements.
Quaoar (also known as 2002 LM60) hasn't been officially named
yet. It's too new. The International Astronomical Union will make
the final decision. Trujillo and Brown suggested "Quaoar" after a
creation god of the Native American Tongva tribe -- the original
inhabitants of the Los Angeles basin where Caltech is located.
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."
Eventually, predicts Brown, KBOs even larger than Quaoar will be
found, and Hubble will be invaluable for follow-up observations to
pin down their sizes. Meanwhile, Quaoar is the record-holder -- a
tantalizing glimpse of perhaps bigger things to come.