Asteroid Looks Like Planet Wannabe
Artist’s concept of an impact event on Pallas. This artwork was created using the 3D shape model published by Schmidt, et al. 2009 in Science.
Credit: B. E. Schmidt and S. C. Radcliffe

A large asteroid in our solar system called 2 Pallas is actually a protoplanet - a moon-sized body that might have formed into a full-sized planet under different circumstances, scientists report.

2 Pallas, with a diameter of 165 miles (265 km), is one of the largest bodies in the asteroid belt between Mars and Jupiter. A recent study of the object's surface and shape based on data from the Hubble Space Telescope suggests that 2 Pallas is more dynamic than the chunks of rock that make up most of the asteroid belt.

"When you think asteroid, you think a small rubble pile, and this isn't that," said study leader Britney Schmidt, a planetary physics graduate student at UCLA. "These objects are transitions between planets and asteroids. They're almost like mini planets."

Although not perfectly round, 2 Pallas has enough gravity to have become almost a sphere shape, much rounder than most of the rocky asteroids around it.

2 Pallas now joins two other large asteroids, Ceres and Vesta, which have also been called protoplanets.

The researchers think that objects like this might have developed into regular planets if it weren't for the particular gravitational tugs of Mars and Jupiter, which kept most of the material in the asteroid belt in the form of rubble.

"Because of where they are in the solar system, in that region the gravity is such that those asteroids could never quite conglomerate into a larger planet," Schmidt said.

This fluke of fate means these objects offer a peek into the formative era of the solar system.

"They were certainly like the building blocks of the larger planets," Schmidt told "They allow us to almost take a photograph of how planet formation was happening in the early stages of the solar system."

In fact, the researchers think that 2 Pallas has remained intact and largely unchanged since its birth long ago. One of its intriguing features is the fact that the protoplanet seems to have hosted water in its past.

"Pallas' surface is covered by hydrated material that seems to indicate that the object had a lot of water at one point in time," Schmidt said. "It's possible that it still does."

This water would likely be in the form of ice buried under the surface today, she said, though there is a chance 2 Pallas may have once had liquid water oceans.

The new study is detailed in the Oct. 9 issue of the journal Science.

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