Image: 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. DCL

Oct. 8, 2009 — A protoplanet is a planetary embryo, a baby planet undergoing accretion by smaller pieces of space rock in protoplanetary disks. Although the Solar System's bodies are fully evolved planets, dwarf planets and asteroids, there is a very exclusive group of large asteroids with a protoplanetary flavor.

This exclusive group just added another member.

In research using the Hubble Space Telescope, a team headed by UCLA scientists have deduced that an asteroid measuring 265 km (165 miles) in diameter is a protoplanet, joining asteroids 1 Ceres and 4 Vesta as only the third intact protoplanet. 2 Pallas is one of the largest asteroids in the asteroid belt (orbiting the sun at a distance of between 3.4 AU and 2.1 AU), accounting for 7 percent of the total mass of the entire belt.

This new 2 Pallas study (to be published in the Oct. 9 issue of Science) identified color variations and features in the asteroid's surface that can be linked to the asteroid's thermal evolution, indicating that this planetary embryo had the potential to grow. The researchers proposed that 2 Pallas formed from water-rich material and that internal alteration (i.e. the differentiation of elements common in the interior of planets) may have occurred.

In addition, analysis of an impact crater on the asteroid has indicated that 2 Pallas was hit by something large in the past, generating lots of debris. It is thought that this impact may have produced what are known as "Pallas family" objects that continue to orbit with the asteroid to this day.