A New Kuiper Belt Kingpin

Barely resolved in this Hubble Space Telescope image, the object designated 2002 LM60 (and named, provisionally, Quaoar) looks slightly elongated due to motion during the 120-second-long exposure. It travels in a roughly circular orbit about 43 astronomical units (6.5 billion kilometers) from the Sun.
Courtesy Chad Trujillo and Michael Brown (Caltech) and NASA.
Caltech astronomers Chad Trujillo and Michael E. Brown have announced sighting a large object beyond Pluto in the distant Kuiper Belt — a body larger than all numbered minor planets combined and perhaps the biggest solar-system find since Clyde Tombaugh spied Pluto in 1930.

The object was first spotted on June 2nd as an 18½-magnitude blip in electronic images acquired with the 48-inch Oschin Schmidt telescope on Palomar Mountain. But Trujillo and Brown kept their discovery a secret — even from the International Astronomical Union's Minor Planet Center — until they could get a better handle on its size. Many researchers suspect that Kuiper Belt objects are very dark, coated with the ultrablack carbon-rich compounds that have been seen on cometary nuclei. If so, they should reflect only a few percent of the sunlight striking them and thus would need to be quite large to appear as bright as they do.

Above is the discovery image of Quaoar. The observations were made from Palomar Observatory with the 48-inch Oschin Telescope. Click on image for an animation.
Courtesy Chad Trujillo and Michael Brown (Caltech)

Trujillo and Brown suspected they might have something rivaling Pluto in size. But no such luck: images taken on July 5th and August 1st with the Hubble Space Telescope's new Advanced Camera for Surveys resolved the object's disk and proved that it's 1,255 ± 190 kilometers across, making it only the size of Charon, Pluto's satellite. Radio-wavelength observations from the IRAM (Institut de Radio Astronomie Millimétrique) telescope in Spain matched Hubble observations nicely. It's inherently a half magnitude brighter than another Kuiper Belt giant, 28978 Ixion, which was discovered last year.

Quaoar Size
Courtesy NASA and A. Feild (STScI).

Something the size of Pluto may yet await discovery in the Kuiper Belt, but Trujillo and Brown's find isn't it. Far from being disappointed, however, the two astronomers have deduced that this distant body has a higher-than-expected albedo (reflectivity) of 9 or 10 percent. Such determinations are rare for Kuiper Belt objects; two other big ones, 20000 Varuna and 2002 AW197, are comparably reflective. The higher the albedo, the more likely it is that these distant bodies are partly covered with relatively fresh ice or frost.

For now, the Kuiper Belt's new kingpin carries the designation 2002 LM60, and it's been tracked down on several old images — including plates dating from May 1983. Ironically, Caltech astronomer Charles Kowal took the latter with the same telescope as part of a years-long search for a 10th planet beyond Pluto. "I can't figure out how Kowal missed it," Brown says. "The Kuiper Belt should have been discovered 20 years ago!"

Above is an artist's impression of Quaoar.
Courtesy NASA and G. Bacon (STScI).

All these observations give dynamicists confidence that 2002 LM60 circles the Sun every 288 years at an average distance of 43.37 astronomical units (6.5 billion km). Consequently, it will not have to carry its temporary designation for long. Trujillo and Brown have proposed the name Quaoar (pronounced kwah-o-wahr), the creation god of the Tongva, or San Gabrelino, tribe that inhabited Southern California before the arrival of Spanish explorers and Caltech astronomers.

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