As Pluto Flyby Nears, Spare a Thought for the Dwarf Planet’s Moons

With its strangely colored surface and remarkably puffy atmosphere, Pluto promises to be among the weirder worlds we’ve ever explored. But out there, at the edge of the classical solar system, icy Pluto is not alone: It’s surrounded by at least five moons.

Recently, we learned that some of those moons tumble in orbit like drunken space potatoes instead of spinning neatly like tops. We also found out that one of them, a tiny world called Kerberos, could be curiously dark – a misfit among its brighter siblings and a wrench in the cogs of Pluto’s presumed history. Then there’s the unexpected gravitational link between three of the moons, an intricacy that hints at a more complex system than scientists could have guessed at just a few years ago.

All of this, and NASA’s New Horizons spacecraft hasn’t even gotten to Pluto yet. That’ll happen next month, when the spacecraft finally flies through this crazy system after 9.5 years on the road.

As Pluto grows larger in New Horizons’ windshield, predictions about the dwarf planet are flying fast and furious. But what about its retinue of moons? We know precious little about these moony characters, thought to have emerged from the ancient collision that produced Pluto’s mega-moon, Charon. From new worlds to bizarre shapes, here’s some of what scientists think the moons might have in store.

How Many Moons?

So far, the cast includes Hydra, Kerberos, Nix, Styx, and Charon – a moon so big it forms a binary planet with Pluto. Though Charon was discovered in 1978, scientists only spotted the four smaller moons in the last decade. Two of them, Kerberos and Styx, hadn’t even been discovered when New Horizons launched in 2006.

More moons could be lurking around Pluto. “Most people suspect we’ll find between one and three moons,” says the Southwest Research Institute’s Amanda Zangari, a post-doc on the New Horizons team.

A few months ago, Zangari put together an informal poll and studded it with specific questions about the Plutonian system – things like how many new moons the spacecraft will find, if there’s icy volcanic activity on Pluto or Charon, and the value of Pluto’s precise diameter. About 60 New Horizons team members and associated Plutophiles entered their guesses, which are likely to be their last shot at predictions “before we go and get our viewpoints completely shattered,” Zangari says.

Any additional moons are likely to be tiny — small enough to evade detection by the Hubble Space Telescope, which spotted the four smallest. So, it’ll be up to New Horizons to find them while it’s in Pluto’s neighborhood.

Recent studies suggest there aren’t many places where additional moons could survive in the already tightly packed system. But one obvious spot is outside the orbit of Hydra, currently the farthest known moon from the whirling Pluto-Charon binary. Another is just inside the orbit of Styx, the innermost of the four small moons. Ditto with the region just inside Kerberos, currently the third rock from Pluto-Charon. “Where a moon would actually work is a very specific thing,” Zangari says.

What about inside Charon’s orbit? No one thinks that’s exceptionally likely, which is great news for the spacecraft’s safety because it’s scheduled to fly through that region.

“I really, really don’t think we’re going to find anything inside of Charon,” says New Horizons team member Marc Buie, who’s also at the Southwest Research Institute. “If you wanted anything to put your hat on as being the most surprising thing New Horizons could find, that’s going to be it.”


Among the more unusual-sounding possibilities is that new moons could be sharing orbits with the ones we already know about. Put simply, orbital paths around Pluto-Charon might be crowded with multiple moons, which are called coorbitals.

“I’ve been rooting for coorbitals,” says Buie, who’s been studying the Pluto system for more than 30 years. “That would be just outstanding.”

While it might seem strange, it’s not unheard of for things in the solar system to share orbits. Perhaps the best known examples are Trojan asteroids, which trundle around the sun along the same paths as Earth, Mars, and Jupiter, among other planets. Plus, some of Saturn’s moons (Tethys and Dione) have coorbiting friends, and other Saturnian satellites even occasionally swap orbits.

“Coorbitals are common in the solar system,” Buie says. “For them to be at Pluto — that would just be fun.”

The Ferryman

New moons aside, there’s plenty to learn about the ones we’re already familiar with. Or are at least vaguely familiar with.

See that grainy bump in the image above? That’s Charon, Pluto’s binary partner in crime. Named after the mythological ferryman who carried the souls of the dead across the river Styx, Charon is about half the size of Pluto and already known to be a completely different world. It’s grayer and dimmer and doesn’t have such wild variations in surface brightness, though there is a slightly darker smudge on the side facing away from Pluto.

Truthfully, we really don’t know Charon very well at all.

“Our knowledge is so poor that who knows — maybe Charon will turn out to be more geologically interesting than Pluto,” says New Horizons team member Jeff Moore, of NASA’s Ames Research Center. “I suspect it will be the other way around, though.”

In addition to taking a close look at Charon’s surface features (and undoubtedly spotting lots of craters), New Horizons should be able to figure out what kinds of molecules are on that surface — and if some of Pluto’s escaping atmosphere ends up stuck there for a while, causing interesting chemical reactions to occur. There’s also a chance the mega-moon could have more dramatic surface topography than Pluto. In other words, ridges could be taller, canyons could be deeper, crater edges more jagged. That’s because Charon is primarily covered in water ice, which is structurally stronger than the exotic ices on Pluto’s surface. Its gravity is also weaker.

“Charon is going to be a big revelation,” Buie says. He argues that, because we know so little about Charon to start with, it’ll be the object we’ll learn the most about. “We’re going to maybe increase what we know about Pluto by a factor of four – people would argue about that, but let me just say a factor of four – and Charon, we’re going to increase our knowledge of by a factor of 40.”

If You Could Only Choose One…

But Buie says if he had to make a choice, he would try and decipher a different moon.

“If you could only get good imaging data of one satellite, which one would it be?” he asks. It’s a tough question. Some scientists might select Kerberos, because its enigmatic darkness could hold the key to understanding Pluto’s past.

Not Buie. “My answer is the weirdest one of the bunch: Nix.”

(Nix rotates chaotically as it orbits Pluto-Charon. STScI/Mark Showalter/YouTube)

Years of observations suggest Nix is oddly shaped, kind of like a stretched out egg instead of a sphere. It’s more than twice as long as it is wide, which is a peculiar shape indeed.

“I would really like to know what we can learn about where that shape came from,” Buie says. “What happens if this thing turns out to be a contact binary – where you have two circles kind of stuck together? That would really turn everybody’s thoughts on their heads.”

For years, Buie had been puzzling over the available data about Nix – which not only suggested it had a weird shape, but that it occasionally became slightly brighter than Hydra, the moon that’s next-most like it. A recent study partially solved that problem by determining that Nix and Hydra are tumbling rather than spinning. “I’ve been scratching my head trying to figure out what the heck is going on with these two guys,” Buie says. “I’m a little disappointed I didn’t come up with all those results that were in there.”

Long Shots

So. The mystery of Nix, the curiosities of Charon’s surface, and the presence of coorbiting moons are among the myriad observations that could emerge when New Horizons finally gets a good look at this rollicking place. Among the least likely observations?

Riverbeds or cities,” Moore says. “Riverbeds are more likely than cities, but riverbeds would be pretty surprising.”

Darn. I’ve been secretly hoping for alien outposts. Maybe there’s still a chance.

“Nope,” Zangari says. “Most people don’t think there are going to be aliens.”

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