Astronomers using NASA's Hubble Space Telescope have discovered three of the faintest and smallest objects ever detected beyond Neptune. Each object is a lump of ice and rock â€" roughly the size of Philadelphia â€" orbiting beyond Neptune and Pluto, where the icy bodies may have dwelled since the formation of the solar system 4.5 billion years ago. They 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 astronomer Gary Bernstein of the University of Pennsylvania at today's meeting of the 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 (15 km) in diameter â€" 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," Bernstein says. "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 2003. A bank of 10 computers on the ground worked for six months searching for faint-moving spots in the Hubble images.
The search netted three small objects, named 2003 BF91, 2003 BG91, and 2003 BH91, which range in size from 15-28 miles (25-45 km) across. They are the smallest objects ever found beyond Neptune. At their current locations, these icy bodies are a billion times fainter (29th magnitude) 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.
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