When we think of our solar system, we normally picture the Sun ringed round by nine planets and assorted moons, comets, and asteroids. It wasn't until 1992 that astronomers started to discover planetesimals, giant ice-crusted rocks adrift just beyond Neptune in a region known as the Kuiper Belt. Last January a team of scientists, led by astronomy and physics professor Gary Bernstein, spotted three of them, using NASAís Hubble Space Telescope. The city-size chunks (named 2003 BF91, 2003 BG91, and 2003 BH91) are the smallest ever found in the frozen fringe of debris, three to nine billion miles from the Sun. Astronomers believe the planets were built up from collisions among planetesimals more than 4 billion years ago. By studying the leftover fragments in the Kuiper Belt, researchers hope to gain a better understanding of the solar systemís early history just as paleontologists study fossils to learn about prehistoric life on Earth.
Bernstein and his colleagues, who include physics and astronomy postdoc David Trilling, had expected the Hubble's orbiting optics to detect at least 60 small fossils over 15 days of observation. Nearly 1,000 bigger ones had already been charted by earthbound telescopes less acute than Hubble. "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 remarked. "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."
