Where have all the little Plutos gone?

By Irene Klotz

updated 1:36 p.m. ET Oct. 10, 2008

An ambitious project to track down small, icy bodies circling in the far reaches of the solar system has gone cold. Apparently, Kuiper Belt objects (KBOs) between two and 17 miles in diameter don't exist.

That's the conclusion researchers made after spending two years watching for tiny flickers of light as small KBOs eclipsed background stars, a technique scientists thought would reveal objects too tiny to be spotted directly.

But the Taiwanese-American Occultation Survey, called TAOS, didn't see any flickers. Scientists accumulated more than 200 hours of data using three telescopes in Taiwan that studied light from between 200 and 2,000 stars simultaneously.

Scientists suspect the dearth of small objects may be because bodies of that size have already clumped together to form larger bodies, or because frequent collisions have broken down the objects into even smaller bodies that fall below the survey's detection level.

"The main thing that's needed is just more time on the sky," said Tim Axelrod, an astronomer with University of Arizona's Steward Observatory in Tucson and a co-author of a paper appearing in this month's Astrophysical Journal Letters.

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Scientists hope additional studies with larger telescopes and faster cameras will further define what the KBO population looks like.

The Kuiper Belt, which is the area of the solar system beyond Neptune's orbit, includes a few large bodies, such as the dwarf planet Pluto and the recently discovered Haumea and Makemake, and about 1,000 smaller objects that orbit between 30 and 55 times as far from the sun as Earth.

The size distribution is uncertain because most reflect too little sunlight to be seen with today's technology. That may change as new, large observatories, such as the Large Synoptic Survey Telescope, are put into service, said Axelrod, who is developing software systems for the observatory.

"TAOS is one of the first surveys of what will probably be several generations of experiments," Axelrod told Discovery News.

Scientists are eager to learn more about objects in the Kuiper Belt in hopes of improving understanding of how the solar system came into existence.

"The objects that are way out in the Kuiper Belt are of interest largely because they have not evolved very much dynamically like things in the inner solar system," Axelrod said. "They tell what the nature of the disk out of which all the planets formed."

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