The Orcus-Vanth system

[ajtribick]

The size, density, and formation of the Orcus-Vanth system in the Kuiper belt

Incidentally it seems that the value for the radius of Orcus given in Celestia is actually a diameter measurement: unfortunately the info URL is a dead link. Not sure what constitutes a good reference for physical sizes of TNOs, it seems that several of the objects are referencing initial discovery press releases: in many cases subsequent observations have resulted in a downgrade of the size estimate.

[AVBursch]

Orcus will be the next object in the solar system to be recognized as a dwarf planet by the IAU. The mass, 6.32 x 10e20 kg, is squarely within the mass range for a dwarf planet. Even though Orcus turns out to have a smaller mass than Ceres, the world still has more than enough mass to achieve hydrostatic equilibrium. The IAU will hopefully (officially) grant Orcus dwarf planet status.

[ajtribick]

Next dwarf planet... maybe.

If you merge the List of Trans-Neptunian Objects with the list of Centaurs and Scattered-Disk Objects, and sort it by absolute magnitude H, it seems that confirmation of dwarf planet status in the outer solar system is proceeding pretty much in order down the list, with Eris and Pluto first, followed by Makemake and then Haumea.

If this continues, we've still got Sedna and 2007 OR10 to go before Orcus gets a look in.

[AVBursch]

Orcus and Quaoar both have a moon, so their mass can be measured. No moon has been found around either Sedna or 2007 OR10, so their masses cannot be directly measured. Also, Sedna and 2007 OR10 are too small and too far away to have their size directly measured by HST. It will take the upcoming James Webb Telescope for astronomers to be able to nail down the size of Sedna and 2007 OR10.

Orcus and Quaoar would be next, because dwarf planet status for both of those worlds can be confirmed by a mass measurement.

[ajtribick]

Well, maybe. Then again, Makemake doesn't have any satellites, so lack of mass measurement is evidently not a showstopper. As far as I am aware there is no direct size measurement either, but I may have missed that one.

Then again, this is politics, so who knows...