Why Are Pluto’s Moons So Weird?

A view of partially-lit Nix, captured from 14,000 miles by New Horizons on July 14, 2015. (NASA/JHUAPL/SwRI)
A view of partially-lit Nix, captured from 14,000 miles by New Horizons on July 14, 2015. (NASA/JHUAPL/SwRI)

Whether you want to call it a planet, dwarf planet, Kuiper Belt Object, or some or none of the above, there’s no denying that Pluto and its family of moons are true curiosities in the Solar System. Not only does little Pluto have one moon — Charon — that’s so massive in comparison that they both actually orbit each other around a central point outside the radius of either (if you feel like adding “binary” to whatever term you prefer to use, go right ahead) but it also has four other smaller moons in orbit that kinda sorta break the rules of how moons are “supposed to” behave.

In our Solar System — and very likely in other systems as well — natural satellites are typically “tidally locked” to planets, meaning that, over time, they have settled in to a rotation that keeps the same face always aimed in toward their host planet as they orbit. Our Moon behaves this way, as do the moons of Mars and most of the moons of the gas and ice giants (with the exception of some very small and far-orbiting moons, notably Saturn’s Phoebe.) Pluto’s largest moon Charon even does this — in fact, Pluto and Charon are tidally locked to each other. If you were to stand on Pluto in sight of Charon it would always be in the same place in the sky as it went through its phases and the background stars passed behind; if you were on Charon, likewise with Pluto. (And if you stood on either in a place where the other wasn’t visible, you’d literally never see it.)

A family portrait of Pluto's moons. The 1,212-km (753 mile) Charon is seen in part along the bottom. (NASA/JHUAPL/SwRI)
A family portrait of Pluto’s moons, as imaged by New Horizons in July 2015. The 1,212-km (753 mile) Charon is seen in part along the bottom. (NASA/JHUAPL/SwRI)

But Pluto’s four wee moons — Nix, Hydra, Kerberos, and Styx, all of which were discovered between 2005 and 2012 — don’t rotate like that. Instead, they each have their own rotation rates and resonances, none of which are in face-lock with Pluto. They tumble through space more like asteroids or comets than moons, causing scientists to wonder both why and how they got that way.

Want to know more? Watch the video below from NASA’s New Horizons team featuring Discovery News host and spaceflight historian Amy Shira Teitel:

Learn more about Pluto, its moons, and the New Horizons mission here.


  1. Stormwise says:

    A good article about a really fascinating topic 🙂 What I found myself liking most about this was trying to wrap my mind around what the space around two, almost equally shallow gravity wells might look like when they lock themselves in a tight orbit with one another. Could such a thing generate a wave of sorts, that might have ejected some of the smaller satellites / debris out of the binary orbit of Pluto and Charon, while causing the remaining four smaller moons to have such unusual spins and wobbles? The problem I have with the theory that debris coalesced into the current moons (after the impact that is thought to have formed Charon), is that at least to my mind, were that the case, they should be as round as Charon.

    Also, I think the choice for Ms. Teitel as an embedded journalist for the New Horizons team was a smart one … she is clear and engaging, and I think she comes across in a way that would allow many in the general public to feel her enthusiasm about space and science. I think this is becoming more important, as the space projects increase in their scope and cost, because the budgets that sustain these projects are influenced by the priority the general public is willing to place on them. This is why I think blogs like yours are also just as important, and am grateful you take the time out of your schedule to post here – I don’t often watch YouTube; but I will generally find the time to read one of your articles, especially since I often learn something new when I do!


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