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News Release 577 of 1048

September 6, 2003 03:00 PM (EDT)

News Release Number: STScI-2003-25

Farthest, Faintest Solar System Objects Found Beyond Neptune

September 6, 2003: 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 biggest surprise of the Hubble search is that so few small 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 found. Two snapshots, taken 12 hours apart, were combined to produce this Hubble Space Telescope image of a Kuiper Belt object (named 2000 FV53) moving across the sky. Hubble's Advanced Camera for Surveys tracked the object on Jan. 26, 2003. Like all the planets, this solar-system member appears to move relative to the fixed stars and galaxies in the background. This particular object was discovered from Hawaii in March 2000 and used to help target the Hubble observations.

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Q & A: Understanding the Discovery

  1. 1. Why didn't the astronomers find more than three small Kuiper Belt objects?

  2. The astronomers think that perhaps the smaller Kuiper Belt objects have been shattered into dust by colliding with each other over the past few billion years.

  3. 2. What are the sizes of the Kuiper Belt objects that were discovered?

  4. The three small objects range in size from 15-28 miles (25-45 km) across. They are the smallest objects ever found beyond Neptune.

  5. 3. How faint are these objects?

  6. 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.

  7. 4. How did the Hubble telescope find them?

  8. 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, hunting for Kuiper Belt objects. Astronomers then fed that data to a bank of 10 computers on the ground. The computers worked for six months searching for faint-moving spots in the Hubble images, the telltale signatures of Kuiper Belt objects.

  9. 5. Why do astronomers study the Kuiper Belt?

  10. 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. The Kuiper Belt is one region where these planetesimals still exist.

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Credit: NASA and G. Bernstein (University of Pennsylvania)