Finding the new moon this summer was a "fringe benefit" of a monthlong program to scan Pluto for rocky rings that could endanger NASA's New Horizons spacecraft, said planetary astronomer Mark Showalter, who's leading the Hubble scanning project. The probe is to fly by the dwarf planet in July 2015.

Even in the Hubble's best images, the new moon—some 3.7 billion miles (5.9 billion) miles from the sun—is visible only as a speck of light.

Based on its brightness, the temporarily named P5 is probably 6 to 15 miles (10 to 25 kilometers) in diameter—about two-thirds the size of P4, the Plutonian moon discovered in 2011.

The new moon adds one more level to the complexity of the Pluto system, which was already surprisingly complex for a world that's only two-thirds the width of Earth's moon, the team says.

Pluto's "Russian Doll" Moons

At about 29,000 miles (47,000 kilometers) from Pluto, the new moon orbits in a 1:3 resonance with the dwarf planet's largest moon, Charon—meaning the newfound natural satellite takes precisely three times longer than Charon to circle Pluto.