Scientists believe Pluto's entire moon system formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup flung material that coalesced into the family of satellites (five discovered so far) observed around Pluto.
According to New Horizons Co-Investigator (and moon discoverer) Mark Showalter of the SETI Institute, "the moons form a series of neatly nested orbits, a bit like Russian dolls."
Nix and Hydra were discovered in June 2005 by a large team of astronomers – led by New Horizons science team members Hal Weaver and Alan Stern – using the Hubble Space Telescope. Nix and Hydra are roughly 5,000 times fainter than Pluto and are about two to three times farther from Pluto than its largest moon, Charon, which was discovered in 1978. Nix and Hydra are roughly 20 to 70 miles (32 to 113 kilometers) wide. They are so small and so faint that scientists combined a short exposure of Pluto and Charon and a long exposure of Nix and Hydra to create images of them all together.
Nix was named for the Greek goddess of darkness and night and mother of Charon; Hydra was named for the nine-headed serpent that Hercules fought in Greek and Roman mythology.
Astronomers using the Hubble Space Telescope spotted a fourth moon orbiting Pluto while searching for rings around the dwarf planet. Kerberos was discovered on June 28, 2011, by a large team led by Mark Showalter. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18. The moon was not seen in earlier Hubble images because the exposure times were shorter. There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured. Read more about the discovery.
The moon was temporarily designated S/2011 (134340) 1 (and sometimes called "P4") and was officially named Kerberos by the International Astronomical Union (IAU) in 2013. Located between the orbits of Nix and Hydra, Kerberos has an estimated diameter of 8 to 21 miles (13 to 34 kilometers).
The Hubble Space Telescope also figured in the discovery of Styx, Pluto's fifth moon, on June 26, 2012. Styx was uncovered by a Showalter-led team using Hubble in a searching for potential hazards New Horizons might encounter during its flyby of Pluto.
Originally designated S/2012 (134340) 1 (and sometimes "P5"), the moon was officially designated Styx by the IAU in 2013. Styx is estimated to be 6 to 15 miles (10 to 24 kilometers) across. It is in a 58,000-mile-diameter circular orbit around Pluto that is in the same plane as the other satellites in the system.