Image Credit: NASA, ESA, and A. Schaller (for STScI)

In a previous entry this February, I described the ongoing study of 2003 UB313, a large Kuiper Belt object which might be deemed the 10th member of our solar system. Discoverer Mike Brown and his colleagues were then in the process of refining size estimates for this far-off, frozen world using a variety of observations. This is no small task given the object’s nearly unimaginable distance of 10 billion miles.

Just over two months ago, there appeared to be conflicting reports surfacing (see prior article) about “Xena’s” apparent diameter, in the wake of observations performed with the Hubble Telescope this past December. Brown urged patience at the time, emphasizing that he and his team were still in the preliminary stages of poring over that data.

Today’s press release from HST yields the current results of the ongoing investigation:

NASA’s Hubble Space Telescope has resolved the “tenth planet,” nicknamed “Xena” for the first time, and has found that it is only just a little larger than Pluto.

Though previous ground-based observations suggested that Xena was about 30 percent greater in diameter than Pluto, Hubble observations taken on Dec. 9 and 10, 2005, yield a diameter of 1,490 miles (with an uncertainty of 60 miles) for Xena. Pluto’s diameter, as measured by Hubble, is 1,422 miles.

“Hubble is the only telescope capable of getting a clean visible-light measurement of the actual diameter of Xena,” said Mike Brown, planetary scientist at the California Institute of Technology in Pasadena, Calif. Brown’s research team discovered Xena, and their results have been accepted for publication in the Astrophysical Journal.

It only required a couple of Hubble images to nail Xena’s diameter. Located 10 billion miles away, but with a diameter that is a little more than half the width of the United States, the object is 1.5 pixels across in Hubble’s view. That’s enough to precisely make a size measurement.

Because Xena is smaller than earlier thought, but comparatively bright, it must be one of the most reflective objects in the solar system. The only object more reflective is Enceladus, a geologically active moon of Saturn whose surface is continuously recoated with highly reflective ice by active geysers.

There are a pair of corresponding updates on Brown’s page as well:

1. Hubble and 2003 UB313
2. Global warming (and cooling and warming and cooling) on 2003 UB313

Here’s a timeline of the size refinements (via press):

(Discovery announced) July 2005: ~2900 km
(Discovery of “Gabrielle”, the KBO’s moon announced) October 2005: ~2700 km
(via Spitzer & IRAM observations) February 2006: 3000 +/- 400 km
(via Hubble observations) April 2006: 2400 +/- 100 km

It appears we have a sound revision, after employing a veritable arsenal of top-notch astronomical equipment (the 48-inch Samuel Oschin Telescope at Palomar Observatory, the 8-meter Gemini North telescope, the Laser Guide Star Adaptive Optics system on Keck II, the Spitzer Space Telescope, the IRAM 30-meter radio telescope, the Hubble Space Telescope … whew!) Even on the pessimistic side of this newly revised range, 2003 UB313 indeed has an edge over Pluto’s diameter of 2274 km.

Originally it was thought that 2003 UB313 reflected roughly the same amount of sunlight as Pluto. As it turns out, “Xena” has a significantly higher albedo than the 9th planet, reflecting possibly 20-30% more of the Sun’s rays (or what’s left of them at that phenomenal distance). Current calculations also indicate that this KBO has a surface temperature of -405° F (-242° C). Brr! That makes chilly Titan look downright tropical in comparison.

I have to say, though — even this amount of work looks relatively simple compared to answering the burning question on many minds: so is 2003 UB313 a planet, or not? I’ll leave that to the astronomical powers that be. As reflected in my previous article, I’m more interested in the science than the nomenclature. Surely there will be a great deal more to come.