Between Asteroids and Planets: A New Category

Our current classification schemes for celestial objects are becoming more and more inadequate as new discoveries are being made. An article published in the October 9, 2009 issue of Science presents compelling evidence that Pallas, located in the asteroid belt between Mars and Jupiter and considered an asteroid since its mid-19th-century demotion from planethood, is not an asteroid at all, but an object of an intermediate category between asteroid and planet, described by some as a "protoplanet," a "planetary embryo," and/or a "baby planet."Protoplanets are usually thought of as the precursors to full planets during the early formation of solar systems. They are objects in the process of accreting in a protoplanetary disk around a protostar (a star that has not yet ignited through hydrogen fusion). Our early solar system was much more active and violent than today's, as objects in the protoplanetary disk regularly impacted one another and some began the runaway growth that would eventually transform them into full blown planets.The asteroid belt between Mars and Jupiter is composed of objects that never accreted into a single large planet, mostly because Jupiter, with its strong gravitational field, gobbled up most of the material in that region for itself.Researchers ...