Pluto, the dwarf planet that was once considered the ninth planet, has a growing entourage of satellites.
On July 11, 2012, astronomers announced that a fifth moon had been discovered orbiting the dwarf planet. Researchers using NASA's Hubble Space Telescope found the moon. The discovery comes almost exactly one year after Hubble spotted Pluto's fourth moon, a tiny body currently called P4.
Charon, Pluto's largest companion
Pluto has one very large moon that is almost half the planet’s size. Discovered in 1978, it was named Charon after the demon who ferried souls to the underworld in Greek mythology. The huge size of Charon (648 miles or 1,043 km in diameter) sometimes leads scientists to refer to Pluto and Charon as a double dwarf planet or binary system. Pluto's diameter is 1,430 miles (2,302 km).
Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This means Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.
Compared with most of solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun. Also, Pluto's rotation is retrograde compared to the other worlds — it spins backwards, from east to west.
Two smaller moons
In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission — the first spacecraft to visit Pluto and the Kuiper Belt — they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon, and they are thought to be just 31 to 62 miles (50 to 100 kilometers) wide each.
Fourth and fifth moons
Scientists using Hubble discovered a fourth moon in 2011. This moon is estimated to be 8 to 21 miles (13 to 34 km) in diameter. P4's orbit is between the orbits of Nix and Hydra.
The newly discovered fifth moon is estimated to be irregular in shape and between 6 and 15 miles (10 and 25 km) across. It is in a 59,030-mile (95,000-km) diameter circular orbit around Pluto that is assumed to lie in the same plane as Pluto’s other known moons.
Pluto's entire moon system is thought to have formed by a collision between Pluto and a similar-sized body early in the history of the solar system. The collision threw out material that coalesced into the family of satellites orbiting Pluto. There may be even more moons to discover.
— Tim Sharp, Reference Editor