That’s Not a Moon, It’s a … Wait, It Is a Moon After All

The Hubble Space Telescope team announced the discovery of a fourth moon in orbit around Pluto. For those keeping track at home, Pluto’s previously-known satellites are Charon (discovered in 1978), Nix, and Hydra (discovered in 2005). Charon is quite large compared to Pluto, so much so that they mutually orbit a point somewhere in between the two objects — they are arguably a binary system. Nix, Hydra, and the new moon are tiny by comparison, which explains how hard it was to spot them.

Does this change anything about Pluto’s status? Probably not. An object doesn’t have to have moons to be a planet (or Mercury and Venus would fail that criterion), nor again is the number of moons relevant (since Pluto passes both Earth and Mars in number of satellites). Some asteroids also have moons, and Pluto’s fellow dwarf planets Eris and Haumea have known satellites. Pluto, Eris, and friends live in a region of the Solar System known as the Kuiper Belt; a standard theory is that a collision during the early history of the Solar System broke pieces off Pluto to make its satellites. Since there are so many little chunks of icy rock in the Kuiper Belt, that idea seems very plausible (at least to someone like me with no planetary science credentials!).

In any case, we’ll learn a lot more about Pluto and its moons — however many there might be — when the New Horizons probe arrives in 2015.


8 Responses to “That’s Not a Moon, It’s a … Wait, It Is a Moon After All”

  1. 1 Michael Roberts July 20, 2011 at 11:44

    2015 seems like a long time to wait, but it would have been worth it! I do feel sorry for Pluto tho, being junked as a planet. :(

    • 2 Matthew R. Francis July 20, 2011 at 12:24

      Well, if it makes you feel any better — Pluto isn’t conscious, so it doesn’t care what we call it either way!

    • 3 Laurel Kornfeld September 13, 2011 at 16:22

      Pluto isn’t a “junked” planet. The media has done a terrible job reporting this debate, and the reality is, it is an ongoing debate. Only four percent of the IAU voted on the demotion, and most are not planetary scientists. Their decision was immediately opposed by hundreds of professional astronomers in a formal petition led by New Horizons Principal Investigator Dr. Alan Stern. To geophysicists like Stern, dwarf planets are simply a subclass of planets, large enough to be in hydrostatic equilibrium but not large enough to gravitationally dominate their orbits.

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