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The Hubble Space Telescope has measured the diameter of a distant world more than half the size of Pluto.

by Ron Koczor

Astronomers have dubbed it "Quaoar" (pronounced kwa-whar) after a Native American god. It lies a billion kilometres 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 2002 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, California, reported these findings at the 34th annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Birmingham, Alabama in Oct 2002.

Quaoar's size compared to that of Earth, Earth's moon and Pluto.

Earlier in 2002, Trujillo and Brown had 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 milliarc seconds, corresponding to a diameter of about 800 miles (1300 kilometres). 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 kilometres 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.

An artist's concept of Quaoar.

Previous record holders are a KBO called Varuna, and an object called 2002 AW197, each approximately 540 miles across (900 kilometres). 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.

The name Quaoar has now been voted on, and approved by The International Astronomical Union (IAU). The body which makes the final decision on such matters.

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.

Note: Pluto is both a planet and a member of the Kuiper Belt. Quaoar is merely a KBO. It's too small to merit automatic planethood. If you ask a dozen astronomers how big something has to be to be called a planet, you might get a dozen different answers. The definition of planets is a topic of lively discussion and size is only one factor. For now, the solar system has 9 planets. Quaoar is not the 10th. It is, nevertheless, an impressive and intriguing new world.


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