The Dwindling Kuiper Belt

2002 AW197
This far-infrared (70-micron) view of Kuiper Belt object 2002 AW197, taken by the Spitzer Space Telescope, revealed it to be much smaller, and thus much more reflective, than previously believed.
Courtesy NASA/JPL/Caltech and J. Stansberry (University of Arizona).
Objects in the (telescope) mirror are smaller than they've appeared — at least in the case of the small, icy bodies beyond Neptune forming the debris zone known as the Kuiper Belt. Observations by several teams indicate that Kuiper Belt objects (KBOs) are generally smaller than previously thought. This, in turn, lowers the estimated total mass of the belt by a factor of 5 or 10.

About 1,000 objects have been discovered so far in this distant realm, the most famous being Pluto and its moon Charon. These frozen mini-worlds are leftover planetesimals from the solar system's early days that never stuck together to form larger bodies. Based on the numbers and estimated sizes of KBOs, researchers estimated that the entire zone contains about one-tenth of an Earth mass.

But there was a lingering uncertainty. With the exception of Pluto and Charon, astronomers could judge the sizes of KBOs only from their brightnesses, since they are too small to resolve in telescopes. But to estimate size based on brightness, you have to assume what percentage of sunlight the object's surface reflects. This percentage is known as the albedo. Astronomers assumed that most KBOs have the same very dark albedo as comet nuclei: about 4 percent.

Three recent studies show that this assumption is incorrect. Most KBOs have much higher albedos, meaning that most of them are considerably smaller than previously estimated.

A team led by John A. Stansberry (University of Arizona) recently used the Spitzer Space Telescope to measure both the temperature and the total heat emission of 2002 AW197, one of the largest KBOs known. These two measurements together allowed Stansberry and his colleagues to determine the object's diameter: 700 kilometers, or 435 miles. Comparing this with its observed brightness (the amount of reflected sunlight) gave its albedo: 18 percent. This is dark gray rather than coal-black like a comet nucleus.

"That's considerably smaller and more reflective than expected," says Stansberry. If 2002 AW197 did have an albedo of 4 percent, it would have to be 1,500 kilometers across, about two-thirds the diameter of Pluto.

Based on his team's Spitzer observations of seven other KBOs, Stansberry finds that the average albedo is about 12 percent, which means "KBOs are about half as large as previously thought." But, Stansberry adds, "there is quite a bit of albedo variation from object to object. This makes sense because these objects have so much color variation."

Stansberry and his team will use Spitzer to measure the albedos of about 20 other KBOs. "We'll know a lot more about how big and bright these things are by this time next year," he says.

Meanwhile, two teams led by Jean-Luc Margot (Cornell University) and Keith S. Noll (Space Telescope Science Institute) have reached the same conclusion by a very different method for measuring KBO albedos. Using the Hubble Space Telescope and ground-based observatories, the two teams have characterized the orbits of six of the 13 known Kuiper Belt binary objects, which in turn allowed them to determine the mass of each component. If their albedos were only 4 percent, the objects would have to be relatively large. Both teams find that the resulting densities would, in most cases, be much too low to be realistic.

When assigned plausible densities, the KBOs in the binaries have a wide range of albedos ranging from 4 to 41 percent. By lowering the size of these KBOs, the total estimated mass of the Kuiper Belt falls by a factor of 5 to 10, perfectly in line with the Spitzer results.

The Stansberry, Margot, and Noll teams presented their results this week at the American Astronomical Society's Division of Planetary Sciences conference in Louisville, Kentucky.

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