The Planetary Society Blog
By Emily Lakdawalla
OK, 2003 UB313 is big after all
Feb. 1, 2006 | 16:01 PST | Feb. 2 00:01 UTC
Amir Alexander has just posted a story on a new measurement of the size of 2003 UB313 based on observations by a radio telescope, and these observations suggest a diameter of 3,100 kilometers, which is roughly 30% larger than Pluto's diameter of 2,300 kilometers. Also, Michael Brown has publicly stated that the remarks he made at that public talk that were quoted by Robert Irion were reported prematurely, and that he considers the radio telescope work by Frank Bertoldi and his colleagues to be a solid one.
One thing that I thought was interesting about the radio astronomy measurement is the sources of uncertainty in their estimate of 2003 UB313's size. Both Brown's and Bertoldi's measurements are subject to basic observational uncertainties having to do with the accuracy and precision of their instruments. After that, there are uncertainties having to do with the actual physical properties of 2003 UB313, which are mostly unknown. For Brown's measurement, the main source of observational uncertainty is that they don't know what the albedo (or brightness) of the surface is, and knowing the albedo is necessary for estimating the size of an object from its brightness. For Bertoldi's measurement, which is based on the temperature of (or, more accurately, the amount of heat being radiated from) 2003 UB313, albedo has only a very weak effect on the diameter estimate. What has a much stronger effect is the orientation of 2003 UB313 in space and whether it is rotating quickly or slowly, both of which are completely unknown at this time.
Here's why; there are two related reasons. The surface of a rotating body is heated when it sees the Sun, and then it cools when it doesn't see the Sun. If the body were rotating so that its axis happened to be pointing toward the Sun -- like Uranus' axis is during its summer and winter seasons -- then one hemisphere gets heated continuously. Because it's so far away, we would essentially see that hemisphere all the time, so if that were true then 2003 UB313 would be hotter than average, which would make it seem bigger to Bertoldi's measurement. Alternatively, if the rotation axis were exactly perpendicular to the Sun's rays, then it would be cooler than average, because every spot on the surface would experience daily heating and cooling, which would make it seem smaller. The second reason has to do with how fast it is spinning. If it rotates very slowly, then no matter what the orientation of its rotation axis, it would be hotter than average, because any point on its surface that we can see has been heated for a long time by the Sun.
If you want to read more about the radio study, here's Bertoldi's UB313 site.
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