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The Planetary Society Weblog

By Emily Lakdawalla

2003 UB313 is smaller than they thought

Weblog Archive

According to a report by Robert Irion in ScienceNOW, the so-called "10th planet," 2003 UB313, is significantly smaller than they first estimated. Michael Brown, UB313's discoverer, just gave a public talk at which he mentioned that recent Hubble observations have permitted them to directly measure the size and found out it was much smaller, which means that it has to be really really reflective, as bright as freshly fallen snow, reflecting 92% of the light that hits it; this is in contrast to Pluto, which reflects only 60% of the light that hits it on average. (This is significant because if an astronomer's telescope can't resolve a distant body, they'll estimate its diameter by assuming a reflectivity, or albedo, and then calculating its diameter from how bright it is.)

The bottom line is that the diameter of 2003 UB313 is likely to be very similar to Pluto's. In fact, if you extrapolate from a table on Brown's website, 2003 UB313 should be around 1% larger than Pluto but since even the diameter of Pluto has only been measured to an uncertainty of about 0.4%, and since they can't possibly know UB313's diameter more accurately than that without stellar occultations, I don't think you can say for sure anymore that UB313 is definitely larger! But there's been no scientific paper published on this yet, which will be the key event that everybody will be looking for; until then, nothing is official.