Quaoar - Hubble Space Telescope Measures

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.