We don't have any detailed images of these objects yet, although I'd like to note here that New Horizons has, finally, resolved Charon as an object separate from Pluto, in another milestone announced last week. So it's hard to visualize the Pluto system. I thought I'd try out a visualization exercise that uses objects in the Saturn system -- for which we have very beautiful images -- as stand-ins for the things we can't really see yet in the Pluto system. Here's the result:
NASA / JPL-Caltech / SSI / chart by Emily Lakdawalla
Relative sizes of objects in the Pluto system represented by objects from the Saturn system (annotated)
The Pluto system contains two large worlds (Pluto and Charon) as well as at least four small moons (Nix, Hydra, Kerberos, and Styx). We will not have high-resolution images of any of them until the New Horizons flyby in 2015. In the meantime, this graphic uses objects photographed by Cassini in the Saturn system to represent the relative sizes of the objects in the Pluto system.
The diameters of Pluto and Charon are known with reasonable accuracy; I picked the two things in the Saturn system closest in size to each (Rhea and Dione) and scaled them to represent the sizes of the principal objects in the Pluto system. The sizes of Pluto's smaller moons are much less certain, so I just picked approximately similar-sized moons of Saturn and didn't bother scaling them up or down. I think the main point of this exercise is to show how much smaller Nix, Hydra, Kerberos, and Styx are than Pluto and Charon. As such, they're very likely to be lumpy, not round, bodies. But their exact shapes and sizes will not be revealed until New Horizons gets much, much closer to Pluto.
Here's an alternative version of the graphic, with the stand-in objects arranged in their correct orbital order out from Pluto: Pluto, Charon, Styx, Nix, Kerberos, and Hydra, as represented by Rhea, Dione, Telesto, Prometheus, Helene, and Pandora, respectively.
NASA / JPL / SSI / Emily Lakdawalla
Relative sizes of objects in the Pluto system represented by objects from the Saturn system
While I was preparing this article for posting, NASA and the Space Telescope Science Institute announced the discovery of a 14th moon at Neptune. S/2004 N1 is a regular moon -- meaning that it has a circular orbit in the plane of the ring system -- but it's orbiting outside the ring system. It's located between the orbits of Larissa and Proteus. That makes it an analog to the little moons Methone, Anthe, or Pallene at Saturn, though it's slightly larger than those little guys, at maybe 15 or 20 kilometers across (as opposed to about 2-5 for the Saturnian analogs). You can tell from S/2004 S1's name that it was located in images that are nine years old, spotted by Mark Showalter during a hunt for Neptunian ring arcs.
NASA, ESA, and M. Showalter (SETI Institute)
S/2004 N1, a moon of Neptune
S/2004 N1 is the fourteenth known moon of Neptune. It was discovered by Mark Showalter in 2013 during a search for ring arcs at Neptune in archival Hubble Space Telescope images.
I fired off an email to Mark about whether this thing could've been seen by Voyager. He replied quickly: "I did extrapolate the orbit back to the Voyager era and search the images that were supposed to have captured it. Apparently it is just too small for the Voyager cameras. I seem to recall that their quoted detection limit was comparable to the size of Naiad, the smallest moon that they did find. This object is quite a bit smaller than Naiad. By the way, the Uranian moon Cupid, discovered in Hubble images in 2003, was also too small for Voyager to detect."