Another scientific clarification: Vanth probably not half the mass of Orcus
Posted By Emily Lakdawalla
2011/01/14 12:16 CST
So here's my other correction for today: earlier this week I got all excited about the Orcus-Vanth system, a Kuiper belt binary system containing two bodies of very different color and therefore likely different albedo. It turns out there was a math error in the version of the paper that I read, which resulted in the notion that Vanth could be nearly as big as Orcus. When the math error was corrected for the final published version of the paper (which is available here), the calculations indicate that while Vanth is still a large moon, it's not likely that big. When Brown et al. assumed that Vanth's albedo was half that of Orcus', they came up with an estimated diameter for Orcus and Vanth of 860 and 380 kilometers respectively, which isn't enormously different from the estimate assuming identical albedo (900 and 280 kilometers), and a mass ratio of 12:1. For context, Pluto and Charon have a mass ratio of 8:1.
Here's a diagram comparing the sizes of Pluto, Charon, Orcus, and Vanth to each other. The dashed lines indicate the range of diameters possible given the current level of certainty in the cited measurements.