With a quick vote last month, the International Astronomical Union decreed that Pluto was no longer the ninth planet, but just a dwarf planet — and not even the largest dwarf — orbiting in a distant ring of icy debris.

Cornell Capa/Time Life Pictures, via Getty Images

Gerard Kuiper, shown in 1948, speculated about the belt's existence in 1951.

But perhaps that should not be seen as a slight to Pluto.

For many astronomers, that ring of icy debris, known as the Kuiper Belt, has become an exciting spot for innovative research and has changed how they view the solar system.

“It’s a lot bigger now,” said Marc W. Buie, an astronomer at the Lowell Observatory in Flagstaff, Ariz. “For me, it’s like somebody invented a new field of science.”

 An Expanding View of the Solar System

Harold F. Levison of the space studies department in the Southwest Research Institute in Boulder, Colo., said, “The more we learn, the weirder it looks.”

More than 1,100 Kuiper Belt objects have been found so far. Astronomers estimate that half a million bodies larger than 20 miles wide are floating out there. At least one appears to be mostly rock with a coating of ice. Some are mostly ice. Some are less dense than ice, indicating a Swiss-cheese-like structure. A surprising number of them have moons.

Some move in clockwork with Neptune; Pluto, for example, is in what is called a 3:2 resonance, taking 1.5 times as long as Neptune to loop the Sun. Many Kuiper Belt objects have been flung into orbits crazily tilted to the rest of the solar system.

“This is really a very exotic zoo out there,” said S. Alan Stern, executive director of the space science and engineering division at the Southwest Research Institute and principal investigator of NASA’s New Horizon spacecraft, which is currently heading to Pluto.

The distribution of Kuiper Belt objects has already provided decisive evidence that Neptune was once perhaps nearly a billion miles closer to the Sun and was then gravitationally nudged outward. Astronomers also hope that the Kuiper Belt preserves a frozen record of the earliest building materials of the solar system.

“It’s kind of like the solar system’s attic,” Dr. Stern said. “It’s like an archaeological dig into the history of our solar system.”

Scientists had initially expected a simple structure for the belt: a thin disk of objects traveling in circular orbits in the plane of the solar system. Some Kuiper Belt objects do fit that profile, and those are called the classical Kuiper Belt objects. (One mystery is why there appears to be a sharp edge at about 4.5 billion miles, with no classical Kuiper Belt objects beyond that distance. Some think a passing star did that.)

Other Kuiper Belt objects share orbits similar to Pluto’s, in resonance with Neptune. Those in the same 3:2 resonance as Pluto have been called the Plutinos.

Still others are called the scattered-disk Kuiper Belt objects. These appear to have been tossed into highly elliptical orbits, often at a sharp angle to the rest of the solar system. Surprisingly, these include some of the larger Kuiper Belt objects, including 2003 UB313, nicknamed Xena, which is larger than Pluto.

An estimated 15 percent of Kuiper Belt objects are binaries — pairs of bodies of similar size and mass. Among some classical Kuiper Belt objects, that fraction may be as high as 30 percent — possibly higher, because even the Hubble Space Telescope cannot distinguish two separate objects if they are too close to each other.

Theorists puzzled about how such small bodies, with weak gravitational pull, could have paired up so often. The answer, it turns out, is that as two objects flew past each other, the gravitational drag generated by many other much smaller Kuiper Belt objects slowed them enough to capture each other.

That mechanism requires a fairly dense Kuiper Belt with a total mass of at least 10 Earths. But while Kuiper Belt objects are many, they do not amount to much today. Adding the masses of Pluto, Xena and the half million other objects, even those not yet seen, gives an estimate of just one-tenth the mass of Earth.

“The mass we measure is pathetic,” said David C. Jewitt, a professor of astronomy at the University of Hawaii.

That, in turn, produces a quandary. Where has 99 percent of the Kuiper Belt gone? This is, as the planetary scientists quaintly put it, the cleanup problem. (One popular idea: repeated collisions smashed most of them to dusty bits, and the bits were blown away by solar radiation.)

Just 15 years ago, the Kuiper Belt was not on the map at all. The known solar system essentially ended at Neptune, except for the occasional comet interloper from far away. (Fifteen years ago, Pluto was traveling along the inner part of its eccentric orbit, closer to the Sun than Neptune.)         Continue>>