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Kuiper Belt Worlds Under Scrutiny

“Santa,” “Easterbunny,” and “Xena” may be odd names, but they beat the official designations given these objects by the International Astronomical Union — 2003 EL61, 2005 FY9, and 2003 UB313. All three are Kuiper Belt Objects (KBOs) discovered with the 48-inch Samuel Oschin Telescope at Palomar Observatory. The last of the three is the now famous 10th planet, but the other two KBOs are close to Pluto-size themselves, and like that world, are in elliptical orbits that take them out of the plane of the Solar System.

Did we say ’10th planet?’ Centauri Dreams realizes the designation is controversial, especially at the IAU, but cannot resist the urge to editorialize (if only obliquely) on behalf of a planetary designation for Xena. The rule seems simple: Pluto-size and up means planet.

So how do such orbits happen? Mike Brown, a professor of planetary astronomy at Caltech and leader of the discovery team, says these exotic objects may have actually formed in a much warmer environment. “We think that these orbital characteristics may mean that they were all formed closer to the sun, and then were tossed around by the giant planets before they ended up with the odd orbits they currently have,” Brown said.

We’ve discussed Xena before, but much remains to be said about its size. For we don’t know how reflective its surface is — assume a completely reflective, mirrored surface and you get an object that, at Xena’s current 97 AU from the Sun, would be about the size of Pluto. Kuiper Belt Objects are not, of course, perfect reflectors, and that implies Xena is actually larger than Pluto. It will take data from the Spitzer Space Telescope, gathered in late August, and the 30-meter IRAM instrument in Spain to help confirm Xena’s size. Spitzer’s infrared observations should help establish an upper limit.

Of the other two objects, Santa is the more curious. It seems to be cigar-shaped, with the longer axis of its body being about the diameter of Pluto in length. Moreover, Santa rotates in four hours, the fastest rotation of any large body in the Solar System. A tiny moon, nicknamed Rudolph, circles Santa every 49 days.

Easterbunny, like Santa some 52 AU from the Sun at present, is also about 3/4 the size of Pluto, and like that planet (and Xena as well) seems to be covered in frozen methane. For more, check Brown’s active and interesting Web site. A California Institute of Technology press release is here.