Astronomers Spot Another Moon Around Neptune
Friday, July 19, 2013
Astronomers have detected a previously unseen moon orbiting Neptune, bringing the planet's moon count to 14. Mark Showalter, an astronomer at the SETI Institute, describes how he spotted the 12-mile moon while combing through old images taken by the Hubble Space Telescope.
IRA FLATOW, HOST:
Everybody look up to the sky this weekend and smile and wave because you're going to have your picture taken from space this weekend. Later today and tomorrow two space probes - the Cassini around Saturn and Messenger around Mercury - plan to turn their cameras back towards Earth and take your picture, a planetary portrait like that iconic Pale Blue Dot photo taken decades ago.
But earlier this week astronomers added a new dot to the solar system map. Looking at old images taken by the Hubble Space Telescope they found a previously undiscovered moon of the planet Neptune. That makes, well, the 14th moon of Neptune. Joining me now to talk about the discovery is Mark Showalter, senior scientist at the SETI Institute in Mountain View and lead researcher behind the discovery. Here in our New York studios. Welcome.
MARK SHOWALTER: Thank you, Ira. It's good to be here.
FLATOW: It's nice to have a California scientist in our New York studios every now and then.
SHOWALTER: Yes. I wish you hadn't turned up the heat quite so much.
FLATOW: Yeah. We wanted to bake you a little bit. Tell us, how did you discover this - it's just a dot in a photograph.
SHOWALTER: Well, it's actually not a dot in a photograph.
SHOWALTER: Which was part of the problem. You had to kind of combine eight to 10 Hubble images in order to see this little dot. And the problem is the dot moves a little bit. It's whipping around Neptune at about, oh, less than a day it takes to circle all the way around Neptune. And so it's not at the same place in each image. So we had to figure out how to shift the images in order to get a 30 minute exposure out of 10 three-minute exposures.
And that was how we saw it.
FLATOW: Why would you - what would drive you to go look for it to begin with?
SHOWALTER: Well, that was exactly part - that was part of the story of what we were actually trying to do. We were mainly studying the rings of Neptune and there are some little moons orbiting Neptune near these rings. And so we were just tracking - there are some arc structures, there are some very peculiar things going on.
And we realized that we had the potential to detect a moon that might have been overlooked by the Voyager spacecraft. But I had been looking much closer to the planet. I had been looking at the rings. And it was only when I pushed my search out further that I found this little guy.
FLATOW: Mm-hmm. And surprised? Were you doubting your own vision?
SHOWALTER: I certainly said wow, that looks interesting. And of course then I had to spend the next week or so looking through other images to see if it was really still there.
FLATOW: And how does this moon compare to the other 13?
SHOWALTER: It is of course very, very small.
FLATOW: Very small.
SHOWALTER: It's about 20 kilometers across. That's about 12 miles. For comparison, the next moon out is 400 kilometers across and the next moon in is about 200 kilometers across. So it's got two very big brothers on opposite sides of it and it's just trapped in the middle there.
FLATOW: Is it an icy thing? A rocky thing? What's it made out of? Do we know?
SHOWALTER: That's a good question. All we really know is that it's a dot. And so we measure how much light comes off of it and then we have to assume it's made of or is bright as the other objects in the system. Because all we really can say is how much light it reflects. And so we assume it's probably icy but it's probably covered with some dirt that makes it kind of dark. And that's how we come up with this 20 kilometer size estimate.
FLATOW: Yeah. Yeah. The question always comes to mind, could there be other ones. right? If you missed that one.
SHOWALTER: Yes. And we've of course asked that question. I think anything else that's out there is probably going to be a bit smaller than this one. I think something maybe as large as half as big as N14 would probably not have turned up in our analysis so far. So we've got more work to do.
FLATOW: Is that its name now? N14?
SHOWALTER: I'm calling it N14 just because its official name is S/2004N1 and that's just not very interesting.
FLATOW: Should we crowd source a question asking our audience what to name this moon?
SHOWALTER: Well, we just came off the completion of a campaign to name two moons of Pluto, so we're certainly talking about what the possibilities might be for how to involve the public in naming this little guy.
FLATOW: Of course everybody always says, well, let's go back to Vulcan. Remember? And the whole Vulcan thing. You were involved with that?
SHOWALTER: I was involved in that. I think this time we're going to be focusing on characters related to the seas, the god Poseidon coming from Greek and Roman mythology. So Vulcan is a little bit further afield this time, in fact, and I think it wouldn't succeed a second time if it didn't succeed a first time.
FLATOW: All right.
SHOWALTER: There are a lot of interesting names that could be used, however.
FLATOW: All right. We'll have our listeners send some email if they'd like to try to name the moon.
SHOWALTER: In fact, I just took the step the other day of buying the domain name Neptunerocks.com.
SHOWALTER: So if your listeners have any ideas, just have them send email to Mark@neptunerocks.com.
FLATOW: Still available. How do you like that?
SHOWALTER: I am - maybe if I had waited a day somebody else would've grabbed it and...
FLATOW: Well, that gets right to the question of a listener. Daniel from Ramsey, Minnesota is on. Daniel, you have a relevant question to that?
DANIEL: Well, a few years ago we had a controversy over what defines a planet coming up with Pluto. I was wondering if you ever have a similar controversy in what defines a moon.
SHOWALTER: Oh, that's a good question. Actually, a moon has a very, very general definition as just about anything that orbits something else that isn't a star. I mean, if it orbits a star then it would be something like a planet or a dwarf planet. But there are asteroids that have moons. There are planets. And of course Pluto, a dwarf planet, has moons. So anything that orbits something else is a moon.
FLATOW: But you - thanks for the call. But you might argue that the rings of Saturn and the other ones are all little moons, right? They're all...
FLATOW: ...individual pieces.
SHOWALTER: There is actually another requirement, you're right, which has to do with whether you can track it over time. And rings are more of a continuum. You can't actually say there's a particular dot and then a week later know where that dot's going to be.
SHOWALTER: So moons have to be isolated objects that you can track.
FLATOW: Even if they're the size of a grapefruit or something?
SHOWALTER: In principle if you could...
FLATOW: Could you say that? Could you say that? Yeah?
SHOWALTER: An isolated grapefruit, if you had the technology to detect it, I think we'd have to call that a moon. Maybe when that day comes people will come up with dwarf moons as the name or something.
FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR. Talking with my guest Mark Showalter, who is a senior research scientist at SETI. We've had a lot of people on SETI the last few weeks.
SHOWALTER: Oh, great.
FLATOW: It's always good to have folks on there. So I forgot what I was going to talk about. You're at SETI. You're searching for signs of intelligent life elsewhere in the universe. How do the moons of Neptune and the planetary rings in our own solar system fit into that mission? Why are you interested in that?
SHOWALTER: Yeah. That's a good question. I'm pretty sure we won't be finding intelligent life on little N14 or on the ring arcs of Neptune. But I'm part of what's called the Carl Sagan Center for the Study of Life in the Universe. We're the research branch, the pure science branch of the SETI Institute and we're a very eclectic group of people doing all kinds of research into extreme biology and astronomy and so on.
My interest is in dynamics - how systems form. The way planetary systems form and the way solar systems form are probably very similar.
FLATOW: Mm-hmm. And how are - are you specialized in finding moons? Or you're good? I mean you're looking for them on our planets?
SHOWALTER: I guess I've done it a few times.
SHOWALTER: So that makes me good at it.
FLATOW: What happens when Hubble is no longer here?
SHOWALTER: Well, eventually we will have the James Webb Telescope, which will probably be even more powerful than Hubble in terms of this kind of work. But for the time being, there's still a lot we can do with Hubble.
FLATOW: Mm-hmm. So - but the Webb is going to focus on the infrared, right?
SHOWALTER: That's right.
FLATOW: And you're looking at other visual image? Range?
SHOWALTER: We're using visual light.
FLATOW: Features, not infrared light.
SHOWALTER: Yes. But actually, infrared light has a great advantage if you're talking about the giant planets, these gas giants. They actually have absorption bands at certain wavelengths in the infrared because there's so much methane. And so you can actually pick infrared wavelengths in which the planets get very, very dark. And that makes it much easier to find moons in principle.
FLATOW: Mm-hmm. How are you going to identify what this moon is made out of? Do you care to?
SHOWALTER: I think the only thing that could really be done from Earth - I mean, obviously we'd all hope to someday see a spacecraft, something like the Cassini mission, out at Uranus and Neptune.
SHOWALTER: And that would tell us a whole lot more. The best we could do from the ground is with a lot of effort we could probably figure out what color this thing is. And that would tell you something about its composition.
FLATOW: Its color. How about its shape? Is it like, you know, they describe things as potatoes or whatever, barbells, whatever kinds of things. Have you got a shape for this one yet?
SHOWALTER: No. We don't have a shape. I suppose with enough time on a telescope you might be able to determine whether it's kind of stretched out and elongated versus more spherical. You could probably determine a little bit of information like that.
FLATOW: Would you think that this is a piece of space debris from inside the solar system that gets captured by the planet or something that's coming in from outside?
SHOWALTER: The fact that all these objects are on orbits that are very nearly circular, they're all in the same plane, they're all in the equatorial plane of Neptune, they're all going around in the same direction, that says that it's very likely they all formed out of the same cloud of debris. So this is probably not a captured object. Now, a few steps out in the Neptune system is Triton, the largest moon, which is going around backwards.
FLATOW: It's going backwards?
SHOWALTER: Retrograde option.
SHOWALTER: Yes. And so it is almost certainly a captured object.
FLATOW: And how does it get - it's in the same plane though.
SHOWALTER: Tilted a bit, but yeah, sort of in the same plane. Yes.
FLATOW: Mm-hmm. And Uranus has moons that are way out there. Doesn't it have one moon that's going in a - Uranus itself is actually...
SHOWALTER: Well, yeah. Uranus is the case where the entire planet...
SHOWALTER: ...is kind of tilted on its side about 90 degrees. But in fact, all the inner moons and the planet are rotating in the same direction and the same kinds of circular orbits. So once again it all probably formed at the same time. Something happened to tilt it.
SHOWALTER: But that probably happened later.
FLATOW: The fact that the rings on - there are rings on those planets out there...
FLATOW: ...does that say to you they all started - they had the same origin? They all formed the same way?
SHOWALTER: Well, rings are really just what happens when you condense a cloud of debris. You get moons on the outside and rings on the inside is basically what happens. There's a certain location where if you're too close to the planet, you're not going to form a moon; you're going to form a ring instead.
FLATOW: So are you looking for any more moons? Or you're taking a break?
SHOWALTER: I think we've got a technique that's working pretty well for us and we've got some older Hubble data sets from Uranus and from Pluto and it might be worth taking another look at them now too.
FLATOW: Yeah. Need any help?
FLATOW: I mean, can you crowd source this? Can you crowd source it to people out there?
SHOWALTER: That's a good question. We should think about that.
FLATOW: We've - you know, we've had - we have a neurologist crowd sourcing protein folding coming on and figuring out how proteins fold. And they got answers from our audience, so...
SHOWALTER: It's certainly something to think about, yes.
FLATOW: All right. Thank you very much for taking time to be with us. And I hope you stay cooler.
FLATOW: Mark Showalter is a senior scientist at the SETI Institute in Mountain View, California. Transcript provided by NPR, Copyright NPR.View this story on npr.org