(CNN) -- While searching for rings around the dwarf planet Pluto, NASA astronomers made an unintentional discovery -- Pluto has a fourth moon.
NASA announced the discovery of the satellite, temporarily named P4, in a news release Wednesday. Astronomers using the Hubble Space Telescope were amazed they could see the moon -- believed to be a mere 8 to 21 miles in diameter -- from so far away.
"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles," said Mark Showalter of the SETI Institute in Mountain View, California. The institute led this observing program with Hubble.
NASA said the finding will add to the agenda for the New Horizons mission, which is scheduled to fly past Pluto and its moons in July 2015. "Now that we know there's another moon in the Pluto system, we can plan close-up observations of it during our flyby," said New Horizons' principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado.
The New Horizons spacecraft has already travelled more than 3 billion miles.
Astronomers believe Pluto's moons were formed by a collision between the dwarf planet and another celestial body. The impact flung material into Pluto's orbit during the early history of the solar system, NASA said.
Earth's moon is thought to have been formed by a similar collision about 4.4 billion years ago, the agency said.
The satellite is the smallest of Pluto's moons. Charon, the largest, is more than 640 miles in diameter. The new moon is located in the orbit between satellites Nix and Hydra.