Charon and Pluto's Four Small Moons

Charon: a moon half the size of its planet

James Christy found Charon in June 1978 at the U.S. Naval Observatory in Flagstaff, Arizona. He noticed that Pluto's images looked elongated, and the bump turned out to be a moon. NASA lists Robert Harrington as a co-discoverer.

Charon and Pluto are locked in what NASA calls mutual tidal locking: the same faces always point toward each other. Pluto takes 6.4 Earth days to rotate, and Charon takes the same 6.4 days to orbit, so Charon hovers over one spot on Pluto's surface. A person standing on the other side of Pluto would never see it. The two are about 19,600 km (12,200 miles) apart.

Charon's surface is less colorful than Pluto's. NASA's page highlights one feature in particular, a reddish north polar region. New Horizons photographed Charon before its closest approach to the Pluto system on July 14, 2015, and the full set of findings is in what New Horizons found at Pluto.

A shared center of gravity

When two bodies orbit each other, they actually circle their shared center of mass, called the barycenter. If one body is much heavier, that point sits inside it. Pluto and Charon are close enough in mass that the point is somewhere else.

NASA's Pluto fact sheet, last updated on January 11, 2024, gives Pluto a mass of 1.303 × 10^22 kg and Charon a mass of 1.586 × 10^21 kg, so Charon has about one-eighth of Pluto's mass. It lists Charon's mean distance from Pluto as 19,596 km. Calculated from those values, the barycenter lies about 2,100 km (1,300 miles) from Pluto's center. Pluto's radius is 1,188 km, so the point sits roughly 940 km (580 miles) above Pluto's surface, in empty space.

In plain terms, Pluto does not sit still while Charon circles it. Both bodies move around a point between them. A Hubble news release describes the pair as a "double planet." The official classification is different: Pluto is a dwarf planet (the reasons are in why Pluto is a dwarf planet), and NASA lists Charon as one of Pluto's five moons. For the vocabulary of orbits, including how a body's path is described, see how to read an orbit.

The four small moons

Nix and Hydra were found by Hubble in 2005, Kerberos in 2011 and Styx in 2012. NASA says all of Pluto's moons are named for figures from the underworld, a pattern that began in 1930 when 11-year-old Venetia Burney suggested the name Pluto.

New Horizons showed that all four small moons are highly elongated, which NASA says is typical of small bodies in the Kuiper Belt. Their surfaces are bright, with an albedo of at least 60 percent, and their colors fit water ice. A team reporting at a 2015 planetary science meeting said Nix and Hydra appear to have formed from the merger of two or more smaller bodies. NASA says the whole moon system probably formed after a collision between Pluto and another Kuiper Belt object early in the history of the Solar System.

Chaotic spins, and how they were measured

The Moon always shows Earth the same face. Pluto's small moons do nothing of the kind. In 2015, researchers using the Hubble Space Telescope were puzzled by unpredictable changes in the sunlight reflected by the two brighter moons. A thorough analysis of all the available Hubble data showed that at least two of them, Nix and Hydra, must be tumbling chaotically. The cause is a shifting gravitational field produced by Pluto and Charon, combined with the moons' football-like shapes. Kerberos and Styx were thought likely to behave the same way, pending more study.

Then New Horizons flew through the system and photographed all four moons, which gave direct measurements. A report presented in November 2015 gave these spin periods:

• Hydra: about 0.43 days, or roughly 10 hours, the fastest of the four.
• Nix: 1.83 days.
• Styx: 3.24 days.
• Kerberos: 5.33 days.

The same report found that Styx's pole is tilted about 82 degrees and that Nix's is tilted 132 degrees, so Nix rotates backwards. Nix's rotation rate had also increased by about 10 percent since its discovery. The report describes the system as chaotic, but notes that fast spin rates help stabilize the rotation. It also says the team had no good explanation yet for why the axes are so tilted. Those measurements come from space instruments. Anyone who wants to try observing the sky at home should remember never to point binoculars or a telescope at the Sun without a proper solar filter made for that purpose.

Common mistakes and myths

• Myth: Charon is a dwarf planet. The IAU's definition says a dwarf planet is not a satellite, and Charon is a satellite of Pluto. The reasoning is in what a dwarf planet is.
• Myth: Charon orbits Pluto the way the Moon orbits Earth. Both bodies circle a point above Pluto's surface, so Pluto wobbles as well.
• Myth: Pluto has one moon, or four. NASA lists five known moons as of November 2026: Charon, Styx, Nix, Kerberos and Hydra.
• Myth: the small moons are round and spin steadily. They are elongated, and Hubble data and New Horizons images show tilted, fast and irregular rotation.
• Myth: Charon rises and sets in Pluto's sky. Because of the tidal lock, it hangs in one place, visible from one side of Pluto and never from the other.

Frequently asked questions

How many moons does Pluto have?

Five as of November 2026, according to NASA: Charon, Styx, Nix, Kerberos and Hydra. Charon is the largest by far.

How big is Charon compared with Pluto?

About half as wide. Charon is roughly 1,212 km (751 miles) across, and Pluto's radius of 1,188 km in NASA's fact sheet gives a width near 2,376 km (1,476 miles).

Why does Charon always show the same face to Pluto?

The two are mutually tidally locked. Pluto's rotation and Charon's orbit both take about 6.4 days, so each body keeps the same side toward the other.

Are Pluto and Charon a double planet?

A Hubble news release uses that phrase because the two orbit a point between them. Officially, though, Pluto is a dwarf planet and Charon is its moon.