This past Monday, the planet Neptune officially got a new moon, a relatively tiny chunk of rock and ice about as wide as Manhattan is long. The object is currently dubbed S/2004 N 1, and it’s the fourteenth now known to circle that distant icy world. Mark Showalter, a researcher at the SETI Institute in Mountain View, California, found the moon in early July in archived images that the Hubble Space Telescope had snapped between 2004 and 2009. While using special software that stacks up and manipulates sequential images to reveal the motion of orbiting companions around a planet, Showalter tweaked a single line of code, switching the software’s gaze from close-in to Neptune to hundreds of thousands of miles further out. He walked away for an hour, and came back to see the software had found something curious in the old Hubble images, a small white dot that seemed to circle Neptune once every 22.5 hours. Further analyses confirmed it was a moon, one that had previously gone unseen because of its speedy orbit and small size.
Such discoveries have become old hat for Showalter, who has also discovered moons around Saturn, Uranus and Pluto. After discovering his two Plutonian moons in 2011 and 2012, Showalter held a contest to allow the public to nominate and vote on its favorite names for both new worlds. The results helped inspire the final names for the new moons, Kerberos and Styx, which were announced by the International Astronomical Union (IAU) on July 2.
Shortly after the announcement of Neptune’s newest moon, I called up Showalter to chat about the moon’s environment, its scientific value, and what he plans to name his latest discovery. An edited version of our conversation follows.
Scientific American: Was this a surprise?
Showalter: Not really, no. We went into this more focused on the ring arcs of Neptune, which are peculiar and persistent bright regions in a couple of dust rings around the planet. There were four arcs in the data from Voyager 2’s 1989 flyby, but two of those have now faded away, and we wanted to piece together what’s going on there and make sense of how the arcs are evolving. We always knew there was a possibility for more moons, things that would have been too small for Voyager 2 to see, so we had our eyes peeled the whole time. It’s definitely still a rush to find something like this, though.
Tell us more about the moon — what do we know about it?
We know its orbit pretty well. But we can only see it as an unresolved dot. We don’t even know its color, because to see these things with Hubble you have to use the whitest filters, which don’t give you info about how red, green, or blue something is. Everything else, we infer from context. We can guess how big it is based on how much light it reflects. It sits between two much larger moons, Proteus and Larissa. These and other moons near it all have similar surface reflectivity, or albedo, within 8 to 10 percent of each other. They’re about as dark as asphalt. When we make the educated guess that this moon shares that same albedo, that tells us this thing is probably on the order of 12 miles across.
What would it look like on the surface?
We don’t know for sure, but someday in the distant future, if and when we get a closer look at this thing, we’ll probably find it to be a cratered, irregularly shaped rock. One reason I’m in this field of astronomy — planetary astronomy — is that I like to visualize things, but it’s hard for me to picture a cosmological object like a quasar in my mind. It’s bright, it’s very far away, and that’s about all I can see. But when I think about the objects I study — planets, moons, asteroids, comets — they have landscapes, they can have geysers and volcanoes, they have things that are much more relatable. They’re more “Earth-like,” but also very exotic and different from what we see in our everyday lives. That combination of the familiar and the alien is something anyone who reads science fiction or watches Star Wars or Star Trek can appreciate.
I’m glad you mentioned Star Trek, since so many Trekkies unsuccessfully lobbied to name one of Pluto’s new moons “Vulcan” after Spock’s home planet. Might this new moon get the Star Trek treatment?
Let me just say first that I’m not surprised the IAU nomenclature committee rejected Vulcan despite the support of so many Star Trek fans. I was a little disappointed, but that’s a name already associated with hypothetical objects that may orbit interior to Mercury, so I knew it would be a tough sell. If they didn’t buy it, no problem. It’s still an honor just to have the opportunity to name a moon. Since Vulcan was rejected, I’ve been publicly mocked by William Shatner, and that’s an honor in its own way, too. But getting back to this new moon, the name has to somehow relate to Poseidon or Neptune, the Greek or Roman gods of the sea. At first I thought that wasn’t as interesting as naming Pluto’s moons for minions of Hades, but after a bit of reading I’ve found some great stuff, and I’ve gotten good suggestions from in and out of the research group. And we are talking about involving the public in this again, but having done it once, I know it’s a huge amount of work, whereas I could just sit down with my group in a room and decide on potential names in an hour or two.
One of my favorite possible names comes from The Odyssey, where Odysseus and his crew are on an island with a giant cyclops. That cyclops’s name is Polyphemus, and he is actually a son of Poseidon. “Polyphemus” is also good because it hasn’t yet been used for an asteroid — asteroids have already taken a lot of the great names. So that will probably be on the list. Another is a goddess, a daughter of Poseidon named Lamia. Lamia got in trouble with Zeus and was turned into a nightmarish creature that stalks and eats children. Even into the Middle Ages, people would tell their children to behave themselves, or else the Lamia will get you! So that’s another colorful one. You can probably guess that I’d prefer to name it after a hideous monster. I was a 12-year-old boy once, too, you know.
This is the sixth moon you’ve discovered, and you’re also credited with discovering rings around Jupiter and Uranus. What’s next?
Unlike those earlier moons, this new moon wouldn’t have shown up in the analyses I have done for Hubble observations of Uranus and Pluto, so it might be worth revisiting that data. There are also some very long exposures of the Saturn and Jupiter systems in the archive. Having this refined technique now, where we can take something from being undetectable in a single image to being detectable in several images combined by motion-tracking, is very powerful. One limitation of the technique is that you have to assume what you’re looking for is something in a circular, co-planar orbit, which is generally a good assumption. That’s what lets you extrapolate where an object should be in each image. Who knows, maybe we could find something in these other Hubble datasets, or for that matter even old spacecraft data! You never know what might turn up, so all of these archived observations should be reanalyzed at some point. Also, I think anyone would agree that if we sent another spacecraft out to Uranus or Neptune, there would be a huge flood of these sorts of new discoveries coming in, and many of us hope for exactly such a mission from NASA. There’s still so much we haven’t seen around these worlds because they’ve only been visited once, by Voyager 2 as it flew by [in 1989]. The big breakthroughs will always come from actually going to these places and seeing them up close.
Other than the thrill of finding and naming new objects after hideous monsters, what’s the scientific value of this?
Every one of the moons we’ve found, I think, has an interesting story to tell, and you don’t know what story the universe is trying to tell you until you find it. I think it’s possible for a truly boring moon to exist, one that tells you nothing, but so far in the history of solar system science I don’t think we’ve found one. Every moon so far has an interesting story if you look closely enough. In the case of this new moon, I keep wondering how this tiny little thing ended up wedged between two much bigger moons, Proteus and Larissa. This object has .01 percent of the mass between them. It’s minuscule, and yet somehow when they formed together it didn’t just become an extra layer of dust coating Proteus. How did it get left behind? Figuring that out will take some careful study. Neptune’s largest moon, Triton, orbits backwards and was probably captured long ago, and when that happened it must have disrupted any other moons, which means the moons we see today must have somehow re-formed afterward. Maybe this new moon can help us understand more of that early history.
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