Pluto's New Posse: Nix and Hydra.

The International Astronomical Union made it official: Pluto’s got two new moons named Nix and Hydra.

Until last year, scientists thought Pluto was accompanied by only one moon, Charon. But the Hubble Space Telescope spotted the two satellites more than twice as far away as Charon and many times fainter.

The duo had been known by the tongue-twisting names S/2005 P 2 and S/2005 P 1. Earlier this year, the moons’ discoverers, led by Alan Stern of the Southwest Research Institute in Boulder, Colo., submitted their choices to the IAU.

The names, with roots in Greek mythology, were selected in part because their first letters, "N" and "H," were a tribute to the New Horizons spacecraft, Stern said Wednesday.

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