Dec 1, 2000

Possibly 2nd Brightest Known Trans-Neptunian Object Discovered

TUCSON, Ariz. -- The Spacewatch Project at the University of Arizona has discovered a minor planet in the outer reaches of the solar system which appears to be the brightest known such object other than Pluto.

During routine scanning with the Spacewatch 36-inch telescope on 2000 November 28, observer R. S. McMillan was manually blinking the displayed scans in real time and noticed this relatively slow-moving object. Its rate of motion is too slow for the real-time software to detect; normally the slower objects such as this one are found with another computer program that processes the data off-line.

The web link to images of the object is

The target was subsequently reobserved by J. A. Larsen, whose observing shift followed McMillan's. With their 12 observations spanning three days, The International Astronomical Union's Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts determined a preliminary orbit by assuming the orbit is circular. As of 2000 December 1, the MPC's orbit suggests that this object is 43 times farther from the Sun than the Earth is, and is presently 42 times farther from Earth than the Earth is from the Sun. With an apparent magnitude of 20 at those distances, the object would be the brightest of all 346 known Trans-Neptunian Objects other than Pluto.

Further observations of the object's position will be made in the coming weeks and months to improve knowledge of the orbital parameters and thereby the object's absolute magnitude, a parameter describing the object's brightness at a standard distance. Since solar system objects shine by reflected sunlight, this brightness depends on the object's size as well as its reflectivity. Therefore it is not possible to give a definitive diameter for 2000 WR106. However, if it has a reflectivity comparable to other minor planets, its diameter would be between 330 and 750 miles. This can be compared to the diameters of the largest known asteroid (1) Ceres of 570 miles or (4) Vesta of 320 miles. Pluto is at a distance comparable to that of 2000 WR106, and is 1,470 miles in diameter.

Brightness equates with ease of measuring the object's reflection spectrum, so 2000 WR106 will no doubt be the target of spectroscopy and measurements of its thermal infrared flux (heat output). That additional information will allow a determination of its diameter.

The Spacewatch Project is a survey of the whole solar system, from the vicinity of the Earth's orbit all the way out to beyond the orbit of Neptune. It was begun in 1980 by Tom Gehrels and Robert S. McMillan at the Lunar and Planetary Laboratory of The University of Arizona. Spacewatch uses the 36-inch telescope of the University of Arizona's Steward Observatory on Kitt Peak mountain in the Tohono O'odham Nation, Arizona. This project is one of four or five around the world that regularly scan the sky electronically for asteroids and comets. More information about Spacewatch can be found on the web site: