Pluto may have been demoted to non-planet status, but it still commands a court of five moons, as is fitting for the king of darkness; after all, Pluto is the Roman equivalent of the Greek God Hades. For decades, we knew almost nothing about these moons but in 2015, the New Horizons mission changed that.
The dwarf planet Pluto and its five known natural satellites (Charon, Styx, Nix, Kerberos, and Hydra) may be frigid, but theyre remarkably complex. Far from being boring specks, Pluto’s moons form intriguing system.
A serendipitous find
Pluto’s largest and innermost moon, Charon, was discovered on June 22, 1978, by United States Naval Observatory astronomer James Christy. Christy was conducting a routine but meticulous examination of photographic plates of Pluto, with the goal of refining Pluto’s orbit around the Sun. He observed that some of the images, which had been labeled as “defective” or of poor quality, showed Pluto as being oddly elongated.
At the time, Pluto was little more than a speck on our best imagery; a handful of pixels. It would have been very easy for Christy to dismiss the images. But he didn’t.
He noted a distinct pattern: the fuzzy blob of Pluto appeared stretched northward in some images and southward in others, with the elongation appearing to cycle over time. Suspecting the presence of an unseen companion, he dug through the observatory’s archives and found older plates from 1965 and 1970 that showed the same periodic elongation, which had also been dismissed as defects at the time. Christy and his colleague, Robert Harrington, calculated that the bulge appeared with a predictable frequency that matched Pluto’s known rotational period of 6.39 days. This confirmed that the elongation was not an artifact but a real, co-orbiting body.
They named it Charon, after the mythical ferryman of the dead, who rows souls across the rivers Acheron or Styx to the underworld. The discovery of Charon was a watershed moment in planetary science. For the first time, astronomers could apply Kepler’s laws of motion to the Pluto-Charon system, allowing for the first accurate calculation of Pluto’s mass. The results were startling: Pluto was found to be much smaller and less massive than previously believed, with only 0.2% the mass of Earth.
Ironically, the discovery of its moon sped up Pluto’s demotion from planet to dwarf planet. It also helped astronomers understand how the Pluto system began. Turns out, with violence.
More than four billion years ago, when the solar system was still a chaotic shooting gallery, a young proto-Pluto collided with another world nearly its own size. But this wasn’t a head-on demolition. Modern computer simulations suggest it was more of a glancing blow, a cosmic “kiss and capture.” The two icy bodies stuck together, settling into a stable orbit, bound by gravity. This was Charon and Pluto.
But Charon isn’t interesting just because of its relationship to Pluto. It has its own stunning features.
A remarkably rich geology
Before NASA’s New Horizons mission flew past in 2015, scientists expected Charon to be a simple, heavily cratered, and geologically dead ice ball. What they found instead was a world with a shockingly violent and complex history.
A massive system of canyons and chasms, nicknamed the “Serenity Chasma,” stretches for more than 1,000 miles (1,600 km) across Charon’s surface. Some of these canyons are four times longer and twice as deep as Earth’s Grand Canyon. It’s the ultimate scar for a satellite like Charon. But this feature didn’t form like you’d expect.
Early in its history, Charon likely had a warm, liquid water ocean beneath its icy shell. As this internal ocean froze over millions of years, the water expanded (just like an ice cube in a tray), placing immense stress on the crust until it cracked wide open
In the southern hemisphere, vast, smooth plains known as “Vulcan Planitia” have far fewer craters than other regions, indicating they are much younger. These plains are thought to be the result of massive cryovolcanism.
Yep, Charon has ice volcanoes. Instead of molten rock, a slushy mixture of water and ammonia “cryolava” erupted from Charon’s interior, flooding the landscape and burying older terrain.
But Charon’s most striking color feature is a dark reddish cap at its north pole, informally named “Mordor Macula.” This isn’t Charon’s own material. It’s a chemical stain left by Pluto. Methane gas continuously escapes from Pluto’s thin atmosphere and gets captured by Charon’s gravity. The gas freezes onto the extremely cold pole. When sunlight later hits this frozen methane, solar radiation cooks it into complex organic molecules called tholins, which have a reddish, tar-like appearance.
The other Plutonian moons
Beyond the double-world of Pluto and Charon lies a family of four smaller moons, discovered by the Hubble Space Telescope between 2005 and 2012 as scientists scouted the path for the New Horizons spacecraft. Named Styx, Nix, Kerberos, and Hydra, they are the children of that primordial giant impact, born from the same disk of debris as Charon. They are small, oblong, and bright as snow, their surfaces dominated by water ice.
And they are spinning in absolute chaos.
Remember when we said Pluto and Charon are tidally locked? That they only see one face of each other, like the Earth and the moon? Many moons in our solar system are tidally locked, but Pluto’s smaller moons are completely different. Hydra, the outermost moon, rotates 89 times for every single trip it makes around Pluto. But it gets weirder. Their spin isn’t just fast; it’s unpredictable. Their axes of rotation—the imaginary poles they spin around—are not stable. They tumble erratically through space.
If you’d be standing on the moon Nix (named after the Greek goddess of the night), the sun might rise in the east and set in the west and then rise in the north and set in the south. Or it might do an irregular motion. The length of a day would change constantly and without any discernible pattern. You’d never know where to look for sunrise. It’s a chaotic rotation.
The culprit for this is, once more, the Pluto-Charon binary.
The four little moons don’t orbit a single, simple point of gravity like Earth’s Moon does. They orbit the constantly shifting gravitational field created by Pluto and Charon as they swing around their barycenter. The gravitational pull they feel is never consistent. Because the moons themselves are not perfect spheres and look more like potatoes, this gravitational fluctuation exerts uneven torques on them. This constant, gentle nudging and twisting prevents them from ever settling down into a stable spin, at least for now.
Yet, amid this rotational chaos, lies a deep, profound orbital order. While their spins are wild, their paths around the system are a model of celestial elegance. The four moons travel in nearly perfect circles, all in the same plane, in what astronomer Mark Showalter calls “neatly nested” orbits. They are also locked in a complex, rhythmic dance called an orbital resonance. The orbital periods of Styx, Nix, and Hydra are in a precise ratio (18:22:33). This intricate gravitational relationship ensures they never get too close to one another, maintaining the system’s stability over billions of years.
Some unfinished business on the frozen edge of our solar system
The fact that we know so much about these moons to begin with is remarkable. In large part, that’s owed to the New Horizons mission.
gave us a breathtaking glimpse, but left us with a tantalizing list of new questions. For one, why are the small moons so bright? Over billions of years, the constant rain of dark, reddish dust from the Kuiper Belt should have coated them, turning them dark. Their bright, icy surfaces suggest they are being refreshed somehow, perhaps by small, frequent impacts that excavate cleaner ice from beneath, but nobody knows for sure.
Another mystery is what’s missing. The giant impact models that created this system predict there should be more debris—more tiny moons, or at least a faint set of rings. Yet New Horizons found the system to be remarkably clean. Where did all the other debris go? Did it coalesce into the moons we see, get ejected from the system, or rain down onto Pluto and Charon?
It’s extremely hard to answer these questions from Earth; this will require another visit. Scientists are already dreaming of a follow-up mission, a sophisticated orbiter tentatively named Persephone.
Unlike the fleeting flyby of New Horizons, Persephone would spend years in the system, mapping every square inch of every world, sniffing Pluto’s atmosphere directly, and using radar to peer beneath the ice shells of Pluto and Charon to hunt for the ultimate prize: a modern-day subsurface liquid water ocean. Such a mission is probably a few decades away, but it’s better to start planning early.
For now, the Pluto system stands as a testament to the beautiful complexity of our solar system. It has transformed from a simple point of light into a dynamic family of worlds we’re keen to know more of.
This article was originally published in 2013 and has been reedited to include more up-to-date information about Pluto and its moons.
