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Emily LakdawallaJanuary 14, 2011

Another scientific clarification: Vanth probably not half the mass of Orcus

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.

Pluto, Charon, Orcus, and Vanth compared

Emily Lakdawalla

Pluto, Charon, Orcus, and Vanth compared
Pluto-Charon and Orcus-Vanth are two binary systems in the Kuiper belt. The diagrams represent the current best estimates of their size, with uncertainty. Charon's diameter is known very precisely from stellar occultation measurements. Stellar occultations have not yet been observed for Orcus and Vanth. While they have been observed for Pluto, its atmosphere acts like a lens, bending starlight, and changing the timing of occultation events; until we understand the structure of its atmosphere better (which should happen during the New Horizons flyby) its diameter will remain uncertain by a few tens of kilometers.

Read more: dwarf planets beyond Neptune

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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