If a second-grader asks if they are really planets, instead of boring him with committee recommendations and pedantic debate points, we give him a scientific answer: “We don’t know yet; we need to send a spacecraft there in order to find out.” Our planetary system deserves nothing less.
Hear, hear. Also interesting was his mention of recent research which suggested that Ceres is in fact differentiated, and therefore possibly worthy of planetary status (another interesting fact: Ceres comprises 25 percent of the asteroid belt's total mass). Hubble observations published in Nature last year [doi] by Thomas et al. indicate that Ceres is a flattened spheroid. As we all know by now, Ceres is large enough to be compressed into a spherical shape by its own gravity; however, its rotation will result in an equatorial bulge, meaning that the diameter measured across the equator is larger than the diameter measured from pole to pole. Thomas et al. measured the difference as about 32 km; however, if Ceres was a homogeneous body, we’d expect a body with its average density to have a difference closer to 40 km (just for reference, the average diameter is about 950 km). This discrepancy can be explained if Ceres is not homogeneous, but is instead differentiated with high density material (rock) concentrated at the core and lower density material (ice) on the surface. Depending on what density is assumed for the core material, modelling indicates that an icy ‘mantle’ of the order of 100 km thick would fit the observations.
These results were a rapid confirmation of a hypothesis published earlier last year by Thomas McCord and Christophe Sotin in Geophysical Research Letters [doi]. They tried to model the thermal evolution of Ceres since it formed (similar to the modelling done for Titan I’ve talked about in the past) and found that given its size and likely composition, internal heating due to radioactive decay was overwhelmingly likely to cause differentiation. They also commented that the best estimates of flattening at the time (which Thomas et al. refined but did not radically revise – it seems a little unfair that a later and less comprehensive paper probably got more publicity just because it was published in Nature) were consistent with a differentiated internal structure. They even remembered to throw in the obligatory reaction to the presence of liquid water (in the past, at least):
This suggests that chemical reactions could have existed and formed molecules of interest to exobiology studies.
More background is available here. Ceres is potentially quite interesting, people – good thing NASA decided to undo their decision to cancel the probe they’d already largely paid for; Dawn is off to visit it next year, arriving at Ceres in 2015.