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8-Oct-2009

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American Association for the Advancement of Science

Asteroid 2 Pallas, a wannabe planet



Artist's concept of an impact event on Pallas.
[Image courtesy of B. E. Schmidt and S. C. Radcliffe]

One of the largest members of the main asteroid belt, a grapefruit-shaped rock called 2 Pallas, has more in common with planets than asteroids, researchers report.

These findings put 2 Pallas in company with two other large asteroids in the main asteroid belt, Ceres and Vesta, which are also considered "protoplanets."

The solar system formed as small bits of dust and rock clumped together into larger pieces, which clumped into larger pieces, and then larger ones. In some cases, these bodies became large enough, and therefore hot enough, that they started to undergo chemical reactions as the result of this heating.

The largest bodies became the planets that we know today, but other medium-sized bodies became "protoplanets" not large and hot enough to go through all the chemical changes required to become planets, but not cold, small asteroids, either.

Britney Schmidt of the University of California, Los Angeles and her colleagues have now taken a close look at 2 Pallas using images from the Hubble Space Telescope.

They found that 2 Pallas also appears to be a protoplanet, based on its size, shape and density. A large crater probably marks the site of an impact that ejected the rocky fragments that orbit along with 2 Pallas. These small asteroids are known as the "2 Pallas orbital family," and their origins have been a mystery until now.

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This research appears in the 9 October issue of the journal Science.