Pluto may not be part of our solar system's planets anymore but that doesn't mean that it has stopped being interesting. On Wednesday, NASA astronomers discovered another hidden gem to this tiny dwarf planet: a new moon.

Astronomers using the Hubble Space Telescope had been observing Pluto to see if it has a ring and instead discovered another object orbiting the dwarf planet. The newly discovered moon is only eight to 21 miles in diameter; in comparison Pluto's biggest moon Charon is 80 times bigger.

Pluto now has four moons circling the icy orb. Charon discovered in 1978 is 650 miles in diameter. The two other moons, Nix and Hydra were only discovered in 2005. The new moon is still going by P4 until astronomers decide on a name.

The observation team led by Mark Showalter of the SETI Institute of Mountain View, California never expected to discover a new moon orbiting Pluto. The astronomers were doing a reconnaissance observation in preparation for the New Horizons mission at Pluto in 2015. The mission was designed to provide insights about distant worlds at the edge of our solar system.

"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 (5 billion km)," said Dr. Showalter

"This is a fantastic discovery," said New Horizons' principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colorado "Now that we know there's another moon in the Pluto system, we can plan close-up observations of it during our flyby."

P4 is located between the orbits of Nix and Hydra. The entire moon system of Pluto is believed to have formed by Pluto colliding with another planet-sized body.