Even before the arrival of the New Horizons spacecraft, astronomers had discovered five moons orbiting Pluto. The moons may have formed from a collision of Pluto with another body billions of years ago.
Four of Pluto's moons shine brightly in this Hubble Space Telescope image from 2006.
Pluto was named for the Roman god of the underworld, so its moons are given names relating to that mythology. Styx is the river that leads to the underworld. Charon is the ferryman who carries souls along the river. Nix is Charon's mother. Hydra and Kerberos are multi-headed creatures said to guard the underworld.
This remarkable Hubble Space Telescope image taken from over 4 billion kilometers (2.5 billion miles) away shows Pluto and its largest moon. Charon is nearly half as big as Pluto. Their difference in color may be due to different surface compositions.
Astronomer James Christy discovered Charon in 1978 with an Earth-based telescope. He noticed what looked like a change in Pluto's shape that cycled every 6.4 days. He was actually seeing Charon, which is so close to Pluto that it makes a single orbit in that short time. Each also rotates on its axis every 6.4 days, so Charon always shows the same face to Pluto and never rises or sets.
This artist's concept shows Pluto and its largest moon, Charon, in the sky above one of Pluto's smaller moons. Pluto orbits at the inner edge of the Kuiper Belt, a ring of icy bodies in the outer solar system. It appears to have a rocky core surrounded by water ice, with nitrogen, methane, and carbon monoxide ices on its surface.