If you visited Pluto, the first thing you would notice in the sky wouldn't be the stars. It would be Charon. Charon is so huge in Pluto's sky that it would dominate the view — bigger across than ten full moons on Earth, hanging motionless in the same spot, every hour of every day.
Charon (pronounced KAIR-on) is Pluto's largest moon and one of the strangest worlds in the solar system. The way it interacts with Pluto is unlike anything else in our cosmic neighbourhood.
Charon is Pluto's biggest moon. It is 1,212 km across — about half the size of Pluto itself. That makes Charon the largest moon in the solar system compared to the world it orbits. Pluto and Charon are tidally locked, meaning they always show each other the same face. Some scientists call them a "double dwarf planet."
Charon Is Half Pluto's Size
Most moons are tiny next to their parent worlds. Earth's Moon is about a quarter the size of Earth. Jupiter's biggest moon, Ganymede, is barely a third of Jupiter's diameter. But Charon is half the size of Pluto. No other big moon in the solar system gets close.
To picture it: Pluto is about 2,377 km across. Charon is 1,212 km across. The two of them are not really a planet-and-a-moon. They are a pair.
The Dance — Why Charon Is a "Twin"
Because Charon is so big, Pluto and Charon don't behave the way you'd expect. They don't have a strong host pulling a small follower around. Instead, both worlds swing around a shared centre point in the empty space between them. This shared point is called a barycenter.
For every other planet-moon pair in the solar system, the barycenter is inside the planet. Earth and the Moon, for example, swing around a point deep inside Earth's mantle. But for Pluto and Charon, the barycenter is in empty space, hanging between them, about 960 km above Pluto's surface.
So Pluto and Charon really orbit each other. Like two dance partners holding hands and spinning in place.
Imagine two ice skaters holding hands and spinning. Neither one is the "main" skater. They share a single point in space between them and rotate around it together. That's Pluto and Charon. They've been doing this for over four billion years.
Tidally Locked — Always Looking at Each Other
Pluto and Charon are also tidally locked to each other. That means each one always shows the same face to the other.
If you stood on the side of Pluto that faces Charon, you would see Charon hanging in the sky — never rising, never setting. Just there, motionless, always. If you walked to the other side of Pluto, you would never see Charon at all. The same is true on Charon, looking back at Pluto.
Earth's Moon is tidally locked to Earth (which is why you always see the same craters on it), but Earth is not tidally locked to the Moon — Earth keeps spinning. Pluto and Charon are both tidally locked to each other. They are the only big pair like this in the solar system.
Charon's Giant Red North Pole
When New Horizons photographed Charon in 2015, scientists saw something nobody had predicted. Charon's north pole was covered in a huge dark red patch. They named it Mordor Macula after the dark land in The Lord of the Rings.
Where did the red come from? Scientists think it is the same kind of tholin chemistry that paints Pluto's surface red. But here's the twist: the methane that creates the tholins didn't form on Charon. It escapes from Pluto's atmosphere and gets pulled across the empty space, freezing onto Charon's cold north pole. So Pluto is literally painting its own moon. More on Pluto's atmosphere.
The Canyon Bigger Than the Grand Canyon
Charon has its own dramatic landscapes. Stretching across its surface is a giant canyon called Serenity Chasma. It is over 1,000 km long and up to 9 km deep — deeper than Earth's Grand Canyon and four times longer.
Inside the canyon system there's an even stranger feature: a single mountain called Kubrick Mons sitting in the middle of a circular depression, almost like a mountain in a moat. Scientists still aren't entirely sure how it formed.
How Charon Got Its Name
In 1978, an astronomer named James Christy was studying photographs of Pluto and noticed it kept showing as a slightly oval-shaped blob. The blob bulged in different directions in different photos. Christy realised what he was seeing: a previously unknown moon orbiting Pluto.
When it came time to name it, Christy picked a name with two meanings. In Greek mythology, Charon is the ferryman who carries the souls of the dead across the river to the underworld — a perfect match for a moon orbiting Pluto, the god of the underworld. And by happy coincidence, the name also matched the first part of his wife Charlene's name. He pronounced it "SHAR-en" in her honour, though most astronomers today say "KAIR-on" with the Greek pronunciation.
In Greek myth, Charon is the ferryman.
Charon is also half of Charlene.
Charon isn't alone. Read about all five of Pluto's moons, the underworld mythology behind their names, or jump back to the main Pluto facts page.
Frequently Asked Questions
What is Charon?
Charon is Pluto's largest moon. It is 1,212 km across — about half the size of Pluto itself. That makes Charon the biggest moon in the solar system compared to the world it orbits. Pluto and Charon are tidally locked, meaning they always show each other the same face.
How big is Charon?
Charon is 1,212 km across — roughly half the diameter of Pluto. It is the largest of Pluto's five moons. Compared to its parent world, Charon is by far the biggest moon in the solar system. Earth's Moon is only about a quarter the size of Earth.
Why is Charon called Pluto's twin?
Because Charon is so big — half the size of Pluto — that Pluto and Charon really orbit each other rather than one orbiting the other. They share a centre point in the empty space between them, called a barycenter. Some scientists call them a 'double dwarf planet' for this reason.
Who discovered Charon?
Charon was discovered by American astronomer James Christy at the U.S. Naval Observatory on 22 June 1978. He spotted it by noticing that Pluto kept appearing as a slightly bulging blob in photographs — and the bulge moved over time. Christy named the moon after both the Greek ferryman of the dead and his wife Charlene.
Is Charon a moon or a dwarf planet?
Officially, Charon is a moon of Pluto. But because Pluto and Charon really orbit each other rather than Charon orbiting Pluto, many scientists argue Charon should be reclassified as a dwarf planet too, making Pluto–Charon a binary or 'double dwarf planet' system.