The recently discovered fourth and fifth moons of Pluto now have official names: Kerberos and Styx.
The International Astronomical Union (IAU), charged with official name designations, stipulates that names derive from Greek or Roman mythology.
The names - referring to a three-headed dog and a river separating the living from the dead, ranked second and third in an international public vote.
The winning submission, Vulcan, was vetoed by the IAU.
The two moons - each just 10-25km across, were formerly known simply as P4 and P5. They were only discovered in July 2011 and July 2012, respectively.
Shortly after the first new moon was found, another followed, and the IAU was in the market for two appropriate new names - and the Pluto Rocks! online vote was begun - receiving over half a million votes.
"I was overwhelmed by the public response to the naming campaign," Dr Showalter said.
Vulcan was a suggestion made by William Shatner of the television show Star Trek - the name of the home planet of character Mr Spock.
Although it was a clear winner among the votes, the IAU opted not to use it, on the grounds that it has been used elsewhere in astronomy and not sufficiently associated in mythology with Pluto, the ruler of the underworld.
