Our Solar System Is Getting Crowded

Astronomers have discovered a new class of the icy bodies left over from the formation of the solar system, inhabiting a region of space once thought to be barren. A paper in this week's issue of Nature describes the first such object, and a computer model appearing in next week's Science explains how they came to populate the space far beyond Neptune.

When the solar system formed 5 billion to 6 billion years ago, some of the leftover material coagulated into cometlike balls of ice tens to hundreds of kilometers in diameter. Over millions of years, these fragments settled into two major groups: the spherical Oort cloud, orbiting well beyond Pluto at some 7.5 trillion kilometers (50,000 astronomical units) from the sun, and the Kuiper belt, a disklike region of ice chunks extending from Neptune's orbit to about 7.5 billion kilometers (50 AU) away. Astronomers had little idea what lay in between.

Until now, that is. Martin Duncan of Queens University in Kingston, Ontario, and Harold Levison of the Southwest Research Institute in Boulder, Colorado, have developed a computer model implying that the gravity of the outer planets stirs up the Kuiper belt, kicking some of its objects into long-lasting orbits that take them billions of kilometers deeper into space. "Originally, the objects were in the Kuiper belt," explains Levison, whose group's model will appear in Science. "But there's a subtle chaos ... [when they get too close to Neptune] their orbits just go nuts, and they rattle around the solar system."

In independent work, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, the University of Hawaii, the University of Arizona, and an amateur in New Mexico have now spotted a wanderer like the ones in the model: an unusually large chunk of ice 490 kilometers across lying just beyond Neptune. That's close enough for the object, which they dubbed 1996TL66, to look at first like an ordinary member of the Kuiper belt. "We were complacent," says CfA's Brian Marsden. But the researchers soon discovered that TL66 has an eccentric orbit that takes it far beyond the Kuiper belt. The team concludes in tomorrow's Nature that TL66 is the first of a new cloud of particles that stretches from the Kuiper belt to the Oort cloud. "It's the first evidence that the huge volume of space between 50 and 50,000 AU is populated," says CfA's Jane Luu.

Other experts are impressed. "It's certainly different from what's been seen so far," says Renu Malhotra, an astrophysicist at the Lunar and Planetary Institute in Houston. "It's telling us quite a lot about the dynamic process that is happening in the solar system--and that the Kuiper belt is much more perturbed than what was previously thought."