Astronomers estimate that the tiny fourth moon is between 8 and 21 miles (13 to 34 kilometers) wide. By contrast, Pluto's largest moon, Charon, is 648 miles (1,043 kilometers) across.
The dwarf planet's other moons, Nix and Hydra, are both in the range of 20 to 70 miles (32 to 113 kilometers) wide. (Related: "Pluto to Make a Star 'Wink Out' Twice.")
The new moon has been given the temporary designation P4.
But this moniker is "just a license plate to refer to it until we get a name, and we're working on that, but we don't yet have a proposal to make," said Alan Stern, principal investigator of NASA's Pluto-bound New Horizons spacecraft and a member of the P4 discovery team.
NASA Craft to Visit New Pluto Moon?
Pluto's fourth moon is located between the orbits of Nix and Hydra, which Hubble also discovered in 2005.
Astronomers first spotted P4 in Hubble pictures taken June 28 using the Wide Field Camera 3 instrument. The new moon was confirmed in Hubble pictures taken July 3 and July 18.
The team had been taking the long-exposure shots of Pluto because they were looking for theorized rings around the planet. The moon probably wasn't seen in earlier Hubble images because the exposure times were shorter.
It's possible, the scientists say, that P4 appeared as a very faint smudge in Hubble images from 2006 but was overlooked because it was obscured by scattered light reflecting from Pluto, said team member Mark Showalter, an astronomer at the SETI Institute in Mountain View, California.
After all, the new moon is only about 10 percent the brightness of Nix, he noted.
"For all these years that people have been studying Nix and Hydra with the Hubble Space Telescope, they never actually did the much longer exposures that you need to do to see something much fainter nearby," Showalter said.
Now that scientists know about P4, the New Horizons team can plan closeup observations of the tiny moon when the spacecraft reaches the Pluto system in 2015, said Stern, who's based at the Southwest Research Institute in Boulder, Colorado.
More Pluto Moons May Await Discovery
Like Pluto's other moons, P4 was likely born in the aftermath of a cataclysmic collision between the dwarf planet and another planet-size body in the early solar system, around 4.5 billion years ago.
The smashup flung molten rock into Pluto's orbit, which cooled and coalesced to form the moons.
"Almost certainly [P4] is another piece created in the giant impact that created Charon" and Pluto's other small moons, Stern said.