I’m sure there are people who can keep things straight in the shifting world of planetary definitions, but given the fact that I’m still not used to Pluto’s demotion, I have to look twice before I write anything on the subject. After checking, then, I confirm that Haumea, the interesting outer system object recently considered as the target of a fast orbiter mission (see this earlier post, and its sequel), is called a ‘dwarf planet.’ Orbiting in the Kuiper Belt, Haumea joins Eris, Pluto and Makemake in this category, the fourth largest dwarf planet now known in the Kuiper Belt. Dwarf planet Ceres is a main-belt asteroid, and thus not, like the others, a KBO as well.
Image: Composite image of computer model frames showing Haumea’s red spot as the dwarf planet rotates. Credit: P. Lacerda .
What’s special about Haumea? Its shape, for one thing. The distant world rotates in 3.9 hours, faster than any other large object in the Solar System. That spin seems to account for Haumea’s unusual ellipsoidal shape, which is itself thought to be the result of an ancient impact. Whatever the case, the ellipsoid measures 2000 kilometers by 1600 by 1000 kilometers, balancing gravitational and rotational accelerations.
And now we have new information: Haumea has a spot. The findings were presented by Pedro Lacerda at the European Planetary Science Congress in Potsdam on Wednesday. Lacerda (Queen’s University Belfast) has this to say about the find:
“Our very first measurements of Haumea told us there was a spot on the surface. The two brightness maxima and the two minima of the light curve are not exactly equal, as would be expected from a uniform surface. This indicates the presence of a dark spot on the otherwise bright surface. But Haumea’s light curve has told us more and it was only when we got the infrared data that were we able to begin to understand what the spot might be.”
Further observations planned for 2010 on ESO’s Very Large Telescope should tell us more. Haumea is thought to be covered in water ice, based on spectroscopic observations, but its density (2.5 times that of water) implies that its interior is rocky. The spot is evidently an area richer in minerals and organic compounds than its surroundings. One possibility is that the spot is the scar of an impact, one that left traces of the impactor on the surface of Haumea, possibly mixing with materials from within the dwarf planet. Changes in brightness flagged this area, which is redder in visible light and bluer at infrared wavelengths. I’m thinking the light curve of Haumea must be a thorny thing to untangle, given the speed of rotation and oddball shape.
Two papers cover this discovery. The first is Lacerda et al., “High Precision Photometry of Extreme KBO 2003 EL61,” Astronomical Journal Vol. 135 (May 2008), pp. 1749-1756 (abstract). The second is Lacerda et al., “Time-Resolved Near-Infrared Photometry of Extreme Kuiper Belt Object Haumea,” Astronomical Journal Vol. 137 (February 2009), pp. 3404-3413 (abstract). Look here for links to full text, and also for an interesting animation of these findings.