Celestia Forums

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.

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.

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.

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.

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...

I've now updated outersys.ssc with Orcus-Vanth and a rough rendering of Quaoar-Weywot.
I haven't committed as yet, because I still need to shake down what sort of objects we want in outersys.ssc.
Robert Johnston maintains a useful and regularly updated archive of TNOs here, which I've used for reference.
I'd think we want to have at least those objects that match the size of current dwarf planets: so I plan to add 2007 OR10 and 2002 TC302.
I also plan to discard 2004 XR190, which is an undistinguished object that only slipped into the file by error anyway.

Adding five numbered objects would (at present) give us everything Johnston lists as bigger than 750km diameter, and leave only Ixion as a smaller outlier in the current file. Keep Ixion or discard? Do we want to use a size cut-off, and if so is 750km reasonable at present?

Another possibility would be to maintain the file with only named objects bigger than Ixion and smaller than the current smallest dwarf planet. This would avoid cluttering the place up with numbered objects: the numbered objects in the file would then be only those that look like excellent dwarf planet candidates in the near future.

Let me know!

Grant

I am just working on an add-on with TNO's ? 400km diameter.
At the moment i have 8 cubewanos, 6 plutions and 3 SDO's.

More details at Board index ‹ Celestia Content ‹ Add-on development ‹ TNO Add-on


Maybe you would like to take data from there. I could do the the same with the TNO's whitch are already in Celestia. They are in the same papers.

400 km diameter corresponds roughly to the dimensions of Mimas which is the smallest currently known object that is gravitationally rounded. Seems like a fairly natural cutoff to me.

I think it is only a good cutoff for an Add-on. Johnston' s Archive contains 60 TNO's greater 400 km diameter.

There are already 11 in Celestia 1.6.0 and 18 at the moment in my Add-on. For the other ones I couldn't find good scientific data.

60 TNO's in the Standard-Celestia would be realy much. But on the other hand there are really much moons (jovian, saturnian, ...),too ... hmmm, its difficult ...

ajtribick wrote:400 km diameter corresponds roughly to the dimensions of Mimas which is the smallest currently known object that is gravitationally rounded. Seems like a fairly natural cutoff to me.

As Gauss says, it seems rather unwieldy for the distribution Celestia. I'd personally be keen to keep outersys.ssc relatively small, featuring large named objects and just a couple of large numbered objects that currently look like hot dwarf planet candidates. The file will of course grow, both by new discoveries of large objects, and by the natural recruitment of smaller objects to dwarf planet status. But I recognize that's just my personal predisposition to avoid filling the screen and browser with KBO orbits and numerical designations. There is, however, a precedent in the form of asteroids.ssc, which has a very high size cut-off and consists otherwise of "interesting" objects of various kinds.
The "scientific" ethos of Celestia would certainly drive us towards using a rigorous size cut-off (which is why I featured that as my first option, above), but I'm definitely uneasy with the natural cut-off you mention just because it's going to generate a lot of clutter which will only increase with time.

Grant

The question is what you define as an "interesting" object. E.g. 2004 XR190 which you described above as "an undistinguished object that only slipped into the file by error anyway" is notable for its high-inclination, low-eccentricity orbit which is quite an unexpected configuration and unlike any of the other known scattered disk objects, and as a result there were press releases at the time of its discovery. On the other hand, it is significantly smaller than the dwarf planets.

Then there's the issue that the trans-Neptunian population has far more large objects than the asteroid belt - a uniform cutoff that lets in more than 1 or 2 asteroids will admit tens of TNOs.

Tricky, tricky.