Pluto has another moon!

This makes six, with the most recent discovery, named Plutino 15810, an especially unusual one. From the paper’s [pdf] conclusion, published on the Cornell University Library astro-ph website:

Pluto’s system continues being a source of controversy, unanswered questions and surprises more than 80 years after its discovery. Pluto’s planethood demotion in August 2006 still stirs debate today and the recent finding of a fifth moon orbiting Pluto by the [Hubble Space Telescope] just confirms the unexpectedly complex nature of the system. In our work, we show that 15810 currently follows a quasi-satellite orbit relative to Pluto; therefore and besides having 5 regular satellites, Pluto has at least one quasi-satellite. This makes 15810 the first minor body found moving in a 1:1 mean motion resonance with Pluto and the first quasi-satellite found in the trans-Neptunian region of the solar system. It also makes Pluto the second dwarf planet, besides Ceres, to host a quasi-satellite. Our finding also confirms that the quasi-satellite resonant phase is not restricted to small bodies orbiting major planets but it is possible for dwarf planets/asteroids too. We also provide a new and somewhat unexpected mechanism to land minor bodies into the quasi-satellite dynamical state. On the other hand, Plutino 15810 is a natural candidate for a spacecraft rendezvous mission in the framework of NASA’s Pluto-Kuiper Belt Mission New Horizons that is going to complete a flyby with Pluto in 2015 and then continue to explore one or more nearby trans-Neptunian objects in the time frame 2016-2020.

The orbit is actually temporary. Plutino will no longer be a satellite of Pluto in about 350,000 years.