Around 1950, Gerard Kuiper and Kenneth Edgeworth proposed that in the region beyond Neptune there are no planets capable of ejecting the leftover planetesimals. There should be a zone, the two astronomers said— now called the Kuiper Belt — filled with small, icy bodies. Despite many years of searching, the first such object was not found until 1992. Since then, astronomers have discovered nearly 1,000 from ground-based telescopes. Most astronomers now believe that Pluto, discovered in 1930, is in fact a member of the Kuiper Belt.
Astronomers now use the Kuiper Belt to learn about the history of the solar system, much as paleontologists use fossils to study early life. Each event that affected the outer solar system — such as possible gravitational disturbances from passing stars or long-vanished planets — is frozen into the properties of the Kuiper Belt members that astronomers see today.
If the Hubble telescope could search the entire sky, it would find perhaps a half million planetesimals. If collected into a single planet, however, the resulting object would be only a few times larger than Pluto. The new Hubble observations, combined with the latest ground-based Kuiper Belt surveys, reinforce the idea that Pluto itself and its moon Charon are just large Kuiper Belt members. Why the Kuiper Belt planetesimals did not form a larger planet, and why there are fewer small planetesimals than expected, are questions that will be answered with further Kuiper Belt studies. These studies will help astronomers understand how planets may have formed around other stars as well.
The new Hubble results were reported by Bernstein and David Trilling (University of Pennsylvania Renu Malhotra (University of Arizona); Lynne Allen (University of British Columbia); Michael Brown (California Institute of Technology); and Matthew Holman (Harvard-Smithsonian Center for Astrophysics). The results have been submitted to the Astronomical Journal for publication, and a preliminary report is available on the [link url=http://arxiv.org/abs/astro-ph/0308467]Web[/link]._z_space_z_);