INTERLUDE 15-2   The Kuiper Belt and the Search for a Tenth Planet
No one has ever observed any comets in the faraway Oort cloud—they are just too small and dim for us to see from Earth. But in the 1990s such faint objects began to be inventoried in the relatively nearby Kuiper belt, just beyond Neptune's orbit. (Sec. 14.2) Ground-based telescopes have led the way in the painstaking work to capture the meager amounts of sunlight reflected from dozens of such dark objects orbiting in the outer Solar System. The largest of these objects is only a few hundred kilometers across, yet there must be many much smaller icy fragments in cold storage. In many ways, the Kuiper belt must resemble the asteroid belt, the collection of rocky debris found mainly between Mars and Jupiter. (Sec. 14.1)

As of mid-1998, the current count of trans-Neptunian, Kuiper belt objects was 62. They range in diameter from 100 to 400 km, which is considerably smaller than either Pluto (2300 km) or its moon Charon (1100 km). (Sec. 13.9) Estimates of the total number of such objects larger than 100 km run into the tens of thousands, so the combined mass of all the debris in the Kuiper belt could be hundreds of times larger than the inner asteroid belt (though still less than the

mass of Earth). Even with large ground-based telescopes equipped with sensitive CCD detectors, it is unlikely that objects much smaller than 100 km (which almost certainly are out there, and probably number in the billions) will be found in the near future. (Astronomers using the Hubble Space Telescope have reported finding numerous such smaller objects, but none has been confirmed after repeated tries.)

Much of this observational work on the Kuiper belt began as a search for a tenth planet. In the process, astronomers have begun to wonder if the Kuiper reservoir may be the source of more than just short-period comets. For example, Pluto, its moon Charon, and the Neptunian moon Triton all reside in or near the Kuiper belt. Furthermore, as we saw in Chapter 13, these three oddballs have very similar physical properties and differ greatly from their neighboring planets and moons. Could this peculiar trio be the last survivors of a much larger set of big Kuiper belt objects? If so, then Pluto doesn't deserve the status of a genuine planet. Ironically, work that began as a search to find a tenth planet may yet succeed in reducing the number of true solar system planets to eight.