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.
New Horizons blasted off earlier this year on a nine-year mission to study Pluto, the last unexplored planet in the solar system. Stern is the mission's principal investigator.
Nix was originally spelled "Nyx" by Stern's group. Nyx is the Greek goddess of darkness and Hydra is the nine-headed serpent that guarded the underworld. Pluto is the Roman god of the underworld.
But since a near-Earth object was already called Nyx, the IAU decided to tweak the spelling to "Nix" to avoid confusion.
Stern said he wasn't disappointed by the spelling change because the pronunciation and significance of the names were still intact.
"The joke was that they nixed Nyx," Stern said.
This summer, the IAU will debate whether Pluto should remain a planet. The discovery of an icy object slightly larger than Pluto in the Kuiper Belt last year reinvigorated the argument over whether to demote Pluto or add other planets.