PHILADELPHIA -- Astronomers using NASA's Hubble Space Telescope have discovered three of the faintest and smallest objects ever detected beyond Neptune.  Each lump of ice and rock is roughly the size of Philadelphia and orbits just beyond Neptune and Pluto, where they may have rested since the formation of the solar system 4.5 billion years ago.  The objects reside in a ring-shaped region called the Kuiper Belt, which houses a swarm of icy rocks that are leftover building blocks, or "planetesimals," from the solar system's creation.

The results of the search were announced by a group led by Gary Bernstein of the University of Pennsylvania at today's meeting of NASA's Division of Planetary Sciences in Monterey, Calif.

The study's big surprise is that so few Kuiper Belt members were discovered.  With Hubble's exquisite resolution, Bernstein and his co-workers expected to find at least 60 Kuiper Belt members as small as 10 miles in diameter -- but only three were discovered.

"Discovering many fewer Kuiper Belt Objects than was predicted makes it difficult to understand how so many comets appear near Earth since many comets were thought to originate in the Kuiper Belt," said Bernstein, associate professor of physics and astronomy at Penn.  "This is a sign that perhaps the smaller planetesimals have been shattered into dust by colliding with each other over the past few billion years."

Bernstein and his colleagues used Hubble to look for planetesimals that are much smaller and fainter than can be seen from ground-based telescopes.  Hubble's Advanced Camera for Surveys was pointed at a region in the constellation Virgo over a 15-day period in January and February.  A bank of 10 computers on the ground worked for six months searching for faint moving spots in the Hubble images.

The three small objects the astronomers spotted -- given the prosaic names 2003 BF91, 2003 BG91 and 2003 BH91 -- range in size from 15 to 28 miles and are the smallest objects ever found beyond Neptune.  At their current locations, these objects are a billion times fainter than the dimmest objects visible to the naked eye.  But an icy body of this size that escapes the Kuiper Belt to wander near the sun can become visible from Earth as a comet as the wandering body starts to evaporate and form a surrounding cloud.

Astronomers are probing the Kuiper Belt because the region offers a window on the early history of our solar system.  The planets formed more than 4 billion years ago from a cloud of gas and dust that surrounded the infant sun.  Microscopic bits of ice and dust stuck together to form lumps that grew from pebbles to boulders to city- or continent-sized planetesimals.  The known planets and moons are the result of collisions between planetesimals.  In most of the solar system, all of the planetesimals have either been absorbed into planets or ejected into interstellar space, destroying the traces of the early days of the solar system.

Around 1950, Gerard Kuiper and Kenneth Edgeworth proposed that in the region beyond Neptune there are no planets capable of ejecting the leftover planetesimals, so there should be a zone, now called the Kuiper Belt, filled with small, icy bodies.  Despite many years of searching, the first was not discovered until 1992; nearly 1,000 have since been discovered from telescopes on the ground.  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 we see today.

If the Hubble telescope could search the entire sky, it would find perhaps a half-million planetesimals, but, if collected into a single planet, they 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 study of the Kuiper Belt.  This will help to understand how planets might have formed around other stars as well.

The new results from Hubble were reported by Bernstein and David Trilling of Penn; Renu Malhotra of the University of Arizona; Lynne Allen of the University of British Columbia; Michael Brown of the California Institute of Technology; and Matthew Holman of the Harvard-Smithsonian Center for Astrophysics.  The results have been submitted to the Astronomical Journal for publication.

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