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 — roughly the size of Philadelphia —
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 — 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.

Astronomers are probing the Kuiper Belt because the region offers a
window on the early history of our solar system. The planets formed over
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. 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
Web at http://arxiv.org/abs/astro-ph/0308467.

Electronic images and additional information are available at

The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).