"It's smaller than P4," Stern told SPACE.com.
"We're finding more and more, so our concern about hazards is going up," he added, referring to the collision risk New Horizons will face when it cruises by Pluto in a few years. [The Moons of Pluto Revealed (Photos)]
The new Pluto moon has been provisionally named S/2012 (134340) 1, though it's also going by the moniker P5. It was discovered using the Wide Field Camera 3 on the Hubble Space Telescope during a series of observations in late June and early July.
P5 appears to be irregularly shaped, with a diameter between 6 and 15 miles (10 to 24 km). It zips around Pluto at an average distance of 29,000 miles (47,000 km), in an orbit thought to be coplanar with the dwarf planet's other satellites, researchers said.
Charon was first spotted in 1978, 48 years after the discovery of Pluto. Nix and Hydra were found by Hubble in 2005.
Pluto orbits 3.65 billion miles (5.87 billion km) from the sun on average, about 39 times farther away than Earth does. For more than 75 years after its discovery, the object was regarded as a full-fledged planet, but things changed in 2006.
That year, the International Astronomical Union reclassified Pluto a dwarf planet, since it shares orbital space with lots of other objects out in the Kuiper Belt, the ring of icy bodies beyond Neptune.