How Many Moons Does Pluto Have?
QUICK ANSWER
Pluto has five known moons: Charon, Styx, Nix, Kerberos, and Hydra. Charon is by far the largest, about half the size of Pluto itself, which is so large relative to Pluto that the two are sometimes called a double dwarf planet system. All five moons have names from Greek mythology of the underworld.
Pluto has five moons, but Charon is the only one that really matters in terms of size. Charon is so large compared to Pluto that the two bodies orbit a common center of mass located between them, rather than Charon simply orbiting Pluto. It's the most extreme example of a moon-planet relationship in our solar system.
What are Pluto's moons called?
Five moons, all named for figures associated with the Greek underworld. According to NASA, the five moons are Charon (the ferryman of the dead), Styx (the river of the underworld), Nix (the goddess of darkness), Kerberos (the three-headed guard dog, also spelled Cerberus), and Hydra (a multi-headed serpent). The names fit Pluto's own connection to the Roman god of the underworld. Charon was discovered in 1978, and the four smaller moons were all found between 2005 and 2012 using the Hubble Space Telescope.
How big is Charon compared to Pluto?
Surprisingly large. Charon is about 750 miles in diameter, roughly half of Pluto's 1,477-mile diameter. The mass ratio is about 1:8, meaning Charon weighs about 12 percent as much as Pluto. By comparison, our Moon is only about 1 percent of Earth's mass. Charon is so large relative to Pluto that the two bodies are sometimes called a double dwarf planet system. The relative sizes make Pluto-Charon the most balanced moon-planet pair in our solar system.
What makes Pluto and Charon a double system?
Their shared center of mass. Pluto and Charon orbit a common point in space (called a barycenter) that lies outside Pluto, between the two bodies. In typical moon-planet systems, the barycenter is inside the planet, so the moon essentially orbits the planet. With Pluto and Charon, the two bodies orbit each other around a point in empty space. Both bodies also keep the same face pointed toward each other at all times, like two dance partners staring while spinning together. This kind of mutual tidal locking is rare.
Are the smaller moons interesting?
Surprisingly so. Styx, Nix, Kerberos, and Hydra are all small (10 to 40 miles across) and irregularly shaped, suggesting they're fragments rather than original satellites. Most planetary scientists think the smaller moons formed from debris ejected when something large impacted Pluto or Charon billions of years ago. The same impact that formed Charon may have created the other moons as well. New Horizons mapped all five moons during its 2015 flyby, providing the first detailed images of the small ones.
Pluto has five moons, with Charon being so large relative to Pluto that the two are sometimes considered a double dwarf planet system. The other four moons (Styx, Nix, Kerberos, and Hydra) are small and probably fragments from an ancient impact. New Horizons captured detailed images of all five moons in 2015, finally giving us our first close-up looks at one of the more unusual moon systems in our solar system.
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