I received the June 2012 issue of Icarus in the mail yesterday. This is the peer-reviewed scientific journal most closely associated with the main planetary science professional organization: the Division of Planetary Sciences of the American Astronomical Society. Reading the Table of Contents on the back, I came across "Physical properties of trans-neptunian binaries (120347) Salacia-Actaea and (42355) Typhon-Echidna," by John Stansberry and several coauthors. Stansberry's is one of very few groups of researchers doing the extremely difficult work of following up on the discoveries of distant trans-Neptunian objects by trying to characterize them as more than just moving dots of light.
So I expected an interesting paper about some odd little objects; there's so much variety beyond Neptune that there are pleasing surprises everywhere you look. I didn't expect to discover one of the biggest objects in the Kuiper belt! Salacia, it turns out, is one of the largest known objects out there. How was Salacia's large size not appreciated in the past? It turns out to have a very low albedo, 3 to 4 percent. Stansberry and his coauthors used Hubble optical data and Spitzer infrared data to constrain the size of Salacia. I've described this process before, in a discussion of Orcus and Vanth.
Getting diameters of two members of a binary pair requires a lot of assumptions. Given some reasonable assumptions, Salacia's diameter is 905 ± 103 kilometers. For context, this very similar to the largest asteroid Ceres' diameter of 975 × 909 kilometers. Salacia has a companion, Actaea, that is also good-sized: 303 ± 35 kilometers. That's bigger than Hyperion but smaller than Mimas. The two are separated by about 5600 kilometers, with a period of 5.5 days. The orbit gives you a measurement of the mass of the whole system, about 4.6 · 1020 kilograms, and that gives you a density, about 1.2 grams per cubic centimeter, only slightly denser than water ice.
It's interesting to compare this system to Orcus and Vanth. In terms of size, they're quite similar; the total system mass of Salacia-Actaea is three-quarters that of Orcus and Vanth. Vanth is likely more than a third Orcus' diameter; the Salacia-Actaea pair is similar. But Salacia and Actaea orbit each other much more closely than Orcus and Vanth, which are separated by 9000 kilometers.
How does Salacia fit in the size ranking of trans-Neptunian objects? Based on what we know now, it's somewhere between seventh and twelfth. The objects that are definitely larger than Salacia are (in order of absolute magnitude) Eris, Pluto, Haumea, Makemake, Sedna, and 2007OR10. Stansberry lists objects whose size is similar (within uncertainty) to Salacia: these include Orcus, Quaoar, 2002MS4, 2003 AZ84, and 2002TC302. There may be more of these that I'm not aware of. (Aside: are there any professionals out there keeping up with TNO size estimates being published in the literature? The only online list I'm aware of is Mike Brown's, but he doesn't seem to have kept up with the zillions of papers published in 2012 already. Someone should start a TNO size wiki.)
Salacia's dark surface is unusual among large trans-Neptunian objects, but not unheard of. Stansberry mentions that 2002MS4 and 2003 AZ84 are similarly sized and also dark, with albedo of 5.1 and 6.5 percent, respectively. (This is dark, but not as dark as Salacia.) Still, most larger objects are also brighter; Orcus has an albedo of 27%, and the biggest ones like Makemake, Haumea, Pluto, Eris, and Triton (which is often lumped among the Kuiper belt objects because it likely originated there and looks like them) are icy white. The bright icy surfaces of the objects bigger than 1000 kilometers in diameter suggest past surface geologic activity. The wide variety of surface properties among intermediate-sized objects 700 to 1000 kilometers in diameter suggests to Stansberry that "evolutionary processes can lead to either high or low albedo, with the outcome depending on the details of the history of individual objects."
In the conclusion, Stansberry and coworkers list a number of ways in which Salacia is exceptional. It's one of the largest trans-Neptunian objects. It's the darkest one in this (intermediate to large) size range. It is also the least dense of the intermediate to large-sized objects whose densities are known (although it should be pointed out that the range of possible densities significantly overlaps the range for Orcus). After that, they allow themselves to speculate a bit. They suggest that the low density and dark color might mean that Salacia looks more like a primordial trans-Neptunian object, one of the planetesimals from which the solar system's planets were constructed.
The "history of individual objects" can have a profound effect on their appearance. Just look at similarly sized Io, Europa, Ganymede, and Callisto, all of which formed at the same distance from the Sun from similar materials to see how individual history can make objects unique. How I wish we could visit all these distant moon-sized things beyond Neptune to see their unique faces and learn their individual histories!
Postscript: It seems like every time I mention Pluto, people who are obsessed with the definition of the word "planet" take over the comments. That discussion is not relevant here. Don't be Troll #4. I will delete any comments on that topic.