THE objects that lurk in the dark outer reaches of the Solar System are
growing larger and more numerous.
Before last year, astronomers had observed 39 objects lurking beyond the
orbit of Neptune in a region called the Kuiper Belt. New observations have now
added another two, which are intriguingly different from the rest of the
The first object, called 1996 TL66, was spotted last October by David Jewitt
of the University of Hawaii in Honolulu, and Jane Luu of the Harvard-Smithsonian
Center for Astrophysics in Cambridge, Massachusetts. It is 500 kilometres
across, and calculations of its trajectory from subsequent observations have
revealed that its orbit takes it between four and six times farther from the Sun
The second object, spotted by Eleanor Helin’s group at the Jet Propulsion
Laboratory in Pasadena, California, is called 1996 RQ20. It is slightly smaller
than TL66, and its orbit appears to reach two to three times farther out from
the Sun than that of Neptune.
The Kuiper Belt is thought to be the source of short-period
comets—those that complete an orbit of the Sun in less than 200 years. The
39 objects found in the Kuiper Belt before last year measure between 100 and 200
kilometres across, and most have nearly circular orbits that lie in the plane of
the Solar System. In contrast, the new pair have irregular orbits inclined at
least 20 degrees from the plane in which the planets orbit.
According to Brian Marsden of the Harvard-Smithsonian Center, many similar
objects could lie in the outer reaches of the Solar System, too faint to be seen
using existing telescopes. Marsden described the new discoveries in an
electronic circular distributed to planetary scientists two weeks ago.