So Pluto has a fifth moon in orbit, temporarily designated P5, to accompany Charon, Nix, Hydra, and um…P4. (I guess someone will get around to naming P4 and P5 one day. I nominated “Fred” and “George”.) P5 is so tiny and faint that even our best instruments can only give it a maximum size: it’s no bigger than 25 kilometers across, and likely smaller. All five moons appear to be regular satellites, orbiting Pluto in the same direction as Pluto’s rotation, and in the same plane. That indicates they formed together, possibly when a rogue object slammed into Pluto in the early Wild-West days of the Solar System.
Obviously we don’t know a lot about P5, or for that matter P4, Nix, or Hydra—they’re too small and faint. The New Horizons spacecraft, when it arrives in 2015, will provide a wealth of information about this miniature moon system, and I for one can hardly wait. It’s obvious that Pluto’s place in the Solar System rather than its size has dictated the satellite system surrounding it. After all, Mercury, Venus, Earth, and Mars are all much larger than Pluto, but they have a total of three satellites between them!
On the other hand, many known Kuiper Belt objects (KBOs) have moons: Eris has Dysnomia, Orcus has Vanth, Haumea has Hi’iaka and Namaka, etc.—and those are just the known satellites. Mike Brown, co-discoverer of Eris and other KBOs, told me that he’s pretty sure we’ll find more moons of Pluto, and I see no reason to doubt him. If each moon is a fragment from the impact that made Charon, then there might be dozens of tiny moonlets. Haumea (the subject of a future Moonday post, I promise!) appears also to have been shaped by a major impact early in its life, so it’s likely to have more than the two known satellites. Pluto is a busy little iceball, and again, I look forward to what more we’ll discover about it in future years.