New crowd at the solar system's edge.

When astronomer Jane Luu and her colleagues discovered a mammoth, icy body at the fringes of the solar system last October, they were thrilled but not surprised. The team's large-format detector, she notes, was designed to net just such an object--a 490-kilometerwide miniplanet that ranks as the biggest and brightest body ever detected beyond Pluto.

Although the object, dubbed 1996 [TL.sub.66], was unexpectedly large, initial observations painted a familiar picture: a nearly circular orbit in a known reservoir of comets called the Kuiper belt.

Follow-up observations reveal, however, that 1996 [TL.sub.66] belongs to a new class of objects that roam a no-man's land. It follows a highly elliptical orbit that takes it far deeper into the reaches of the outer solar system than any of the 36 or so known denizens of the Kuiper belt. In fact, the body ventures as far away as 130 astronomical units (AU), or 130 times the distance from Earth to the Sun. That's more than twice as far as the most extensively studied residents of the Kuiper belt, yet not nearly as far as another proposed storehouse of comets, the Oort cloud, estimated to reside at 50,000 AU (SN: 6/7/97, p. 352).

Luu, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her collaborators suggest in the June 5 Nature that 1996 [TL.sub.66] is only the tip of the iceberg. They estimate that 800 frozen bodies of similar size litter this uncharted region of the outer solar system. "We had no idea what kind of objects were in this region until 1996 [TL.sub.66] came along," says Luu.

For now, the total mass of this mysterious population., as well as its origin, is a matter of debate. One intriguing possibility comes from computer simulations by Martin J. Duncan of Queen's University in Kingston, Ontario, and Harold F. Levison of the Southwest Research Institute in Boulder, Colo. They traced the evolution of a group Of planetary wanna-bes--frozen objects in the early solar system that were potential building material for Uranus and Neptune but were instead hurled outward. The researchers report in the June 13 Science that 1 percent 44 these objects could have survived and should reside in the kind of orbit typified by 1996 [TL.sub.66] and another object, 1996 [RQ.sub.20], recently discovered by a team led by Eleanore F. Helin of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Such objects, says Duncan, were intimate partners in the dance of planetary formation and may reveal much more about the origin of planets than members of the Kuiper belt, thought to have remained aloof from such activity.

The new population of objects may merit another distinction, say Duncan and Levison. The unstable orbits of these bodies could allow them to migrate relatively easily into the inner solar system, where they would become short-period comets, orbiting the sun every 200 years or less. In the standard picture, the Kuiper belt supplies such comets, despite the stability of orbits in that reservoir.