Busy Little Iceball

Pluto and its five moons: Charon, Nix, Hydra, P4, and newly discovered P5. The image is from the Hubble Space Telescope. There’s a mask over the central part of the image to keep bright Pluto from washing out the tinier moons, so don’t be fooled into thinking Nix and Hydra are brighter than Pluto and Charon! Credit: NASA; ESA; M. Showalter, SETI Institute

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.

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6 Responses to “Busy Little Iceball”


  1. 2 Laurel Kornfeld July 12, 2012 at 20:03

    Pluto is not an “iceball,” as it is estimated at 70 percent rock. Eris, being marginally smaller but 27 percent more massive, is likely even more rocky. Calling Pluto an “iceball” is therefore misleading. Also, Eris was discovered by a team of three, not by Brown alone. Please credit Dr. Chad Trujillo and Dr. David Rabinowitz who co-discovered Eris with Brown. Interestingly, Rabinowitz signed the petition of 300 professional astronomers rejecting the IAU definition of planet.


  1. 1 Space Oddities | Sci-ence Trackback on July 13, 2012 at 10:49
  2. 2 I’ve Been Nixed! « Galileo's Pendulum Trackback on July 13, 2012 at 11:48
  3. 3 Moonday: Haumea Help You? « Galileo's Pendulum Trackback on July 16, 2012 at 21:01
  4. 4 Scientific Debates Are (Mostly) a Waste of Time « Galileo's Pendulum Trackback on December 28, 2012 at 16:06
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