How Many Moons Are There in the Solar System?

Transcript

ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we scan the skies for answers to the most frequent questions we get from our visitors. I'm your host, Eric, and a popular topic for our space content is the moons of our solar system. They are varied, they are fascinating, and they are numerous. Often we get asked: just how many are there? For the answer, I turned once again to Talia, one of our resident space extraordinaires for a conversation inside of our Charles Hayden Planetarium. Talia, thanks so much for inviting me into the planetarium to record this podcast.

TALIA: I know it's been a while since you've been under the dome.

ERIC: I don't think I've actually been in here since before the pandemic. Because I was home for a while - we were all home for a while, I was home for a while. I've been meaning to come in. There's multiple new shows. And so coming in for the first time before anything is open, and just being in here to record a podcast is cool.

TALIA: Well, welcome back.

ERIC: And it's also great timing, because we have big moon news. We've wanted to talk about this subject for a while. It's a great question. How many moons are in the solar system? It has so many different answers. And it's timely because we just got a bunch of new ones.

TALIA: Yeah, the number is different this morning than it was yesterday morning.

ERIC: And that's happened a bunch of times since we started working at the Museum

TALIA: Oh yeah.

ERIC: A bunch of times in the last couple of decades, a bunch of times throughout human history as we just discover more of them.

TALIA: That is true. Since the first use of the telescope to look into space by Galileo, we've just kept finding more and more moons. You know, our own moon, of course we've known about since...ever.

ERIC: Yeah, we had, we had kind of one. The total amount of known moons was one. But we didn't know what it was, so I guess you could kind of say zero known moons.

TALIA: I guess. I mean, we knew by Galileo's time, we knew it was going around the Earth. And then Galileo turned his telescope on Jupiter. And he saw these four small stars that appeared to be going around Jupiter, which he called the Medici stars after his patrons. But of course, are the things we now call the Galilean moons, the four largest moons of Jupiter, which were the first moons other than our own that we ever knew about.

ERIC: And that was kind of a big change in the way we see the solar system, the universe, where it was like those things aren't going around Earth. Those things are going around that bigger thing.

TALIA: We had confirmation that not everything was going around the Earth.

ERIC: So we had the idea that stuff could go around other stuff. And those were the first really like, there's moons. And we have a moon but there's other things that have moons. And so we went up to five.

TALIA: Yep.

ERIC: So in 1610, we had five moons in the whole solar system.

TALIA: Went from one to five. That's a big jump.

ERIC: Jupiter has a lot more than four moons.

TALIA: Yes. Currently...it's really hard to keep track. I think the current number is it might be 92. It might be 95.

ERIC: Those are the two that I saw. And it's hard to get the official...there was just an announcement in March of this year. So less than two months ago. More moons were discovered of Jupiter bringing its total up into the 90s.

TALIA: Yes, at the beginning of the year it was at 80. And then, you know, I was telling all my school kids, oh, well, Saturn has the most moons. It has 83 and Jupiter only has 80. And then suddenly Jupiter was up to 92.

ERIC: And the answer to that question changed. "Which planet has the most moons?" became Jupiter again.

TALIA: Exactly.

ERIC: And then yesterday.

TALIA: Yesterday. Look, Saturn was clearly offended at losing its moon king crown. Because it took it back with a vengeance.

ERIC: So many moons. 60-something?

TALIA: 62 new moons. Which brought Saturn's current total up to 145 moons.

ERIC: That's, like, a lot of moons.

TALIA: That is ridiculous.

ERIC: Can you talk about what goes into us bringing that number up. Moons aren't coming into existence.

TALIA: No. So what we're doing is just discovering moons that we did not know about before. So the gas giants in particular, because they have large numbers of captured moons, which are moons that formed elsewhere and only entered orbit around the planet when they got too close and the planet captured it with its gravity. So the gas giants have a lot of those. Most of these new moons, we're finding, they're captured moons. And they're small captured moons kind of far from the planet. That makes them hard to spot which is how they've gone this long undetected. You know, we're constantly getting better at finding these things. So suddenly, these little guys that have evaded detection up until this point we can find in large numbers. And it was the Canada-France-Hawaii Telescope that discovered this new bonanza of Saturnian moons.

ERIC: And the other thing is, you know, when you think about the rings of Saturn, they look gorgeous. And then when you find out they're actually chunks of ice, the size of the planetarium, the size of houses. Like, isn't each of those a moon? Like, we have to decide, like...moon is just a word. It's just like planet, you have to decide what's a moon and what's too small to be a moon and what's just like a part of the ring or a chunk. Because if you're counting all the ice chunks in the rings, then Saturn has quadrillions of moons, probably. So do we have a border right now?

TALIA: We don't have a border at the moment. In fact, there are things we call moons in the rings that are definitely called moons, they are not technically part of the rings, they are orbiting in gaps in the rings. Gaps which they cause with their gravity. But it even, like, complicates it even more, because these things are larger than ring particles, but they are in the rings. And they are part of the structure of the rings, but they are not ring particles, they're moons. As opposed to all of the other things going around Saturn, which are ring particles.

ERIC: So now that we're halfway through the episode talking about this number, we can kind of reveal that this number is meaningless, because we're always gonna find more, we're never gonna find all of them. And you can go all the way down in size to a speck of dust, and there's more moons than anyone could ever count.

TALIA: Yeah, I mean, and they also announced, when they announced all those new moons yesterday, they were like, "there's at least 30 more that we think are there that we haven't found yet".

ERIC: We're not halfway through 2023. And the numbers for Saturn and Jupiter have both changed dramatically.

TALIA: Yes.

ERIC: It's not like there's one new moon discovered, we found one new moon of Jupiter. And that brings it up from 92 to 93. Like 45, 60-something moons at the same time in the same, like, observing run or series of observations.

TALIA: We tend to find them in, like, chunks. Like, a bunch at a time. Although never 62 at once. That is ridiculous.

ERIC: Yeah. How many was it in March? It was like another 10 or 12 or something?

TALIA: Yeah. So Jupiter went up by 12. Earlier this year, which is a lot but like, you know, I remember when Saturn went up by like 20 in, like 2018, 2019, something like that. So like, that's a lot. But it's a believable number. Yesterday, you emailed me yesterday, and was like, "oh, my gosh, what is going on at Saturn?"

ERIC: I was a little bit like, "why haven't you told me about this?" Usually, you're where I get all my space information from, when you send out your newsletter, Spacing Out. And I was like, "did Talia miss this?", because I was looking up how many moons a lot of these planets have. And Saturn came up as 145. And I was like, well, that's not right. I know that that's not right. And then I looked at the date of the news article that got pulled up, and it was yesterday's date. And I was like, this just happened.

TALIA: I know you just happened to look at the exact right moment because I looked at the news that morning. And yesterday morning that hadn't been released yet. And then you emailed me in the early afternoon and I was like, "what is going on?"

ERIC: And then you emailed the rest of the planetarium crew in big huge red letters: ALERT ALERT ALERT ALERT. New number of moons! Stop saying Jupiter has the most!

TALIA: Yep.

ERIC: You just gonna keep up with the stuff when people ask about it, because it's changing so often.

TALIA: Absolutely. And we of course love our planetarium shows to be as accurate as possible, which means we have to keep tracking all of this new information that comes in.

ERIC: So Jupiter and Saturn are the leaders of the solar system in terms of moons. They're the two biggest planets. We know Earth has one. Mercury and Venus: no moons.

TALIA: No moons for them.

ERIC: Mars has two tiny little -

TALIA: Tiny little dinky things.

ERIC: It's gone back and forth a couple times the last couple years. Did they form with Mars? Are they captured? They're tiny.

TALIA: Yes, they are tiny. But there was actually a new thing that came out just a couple of weeks ago, when the Hope spacecraft flew by Deimos, the smaller outer moon. It was able to take some spectrometry readings and they suggest that Deimos doesn't look like an asteroid. Because one of the guesses was that it is a captured asteroid. Deimos doesn't look like an asteroid. It looks like Mars, which suggests that it formed potentially from Mars, potentially from a large impact or some other thing that knocked a chunk of Mars into orbit.

ERIC: I love these kinds of stories because all the moons of Jupiter, the moons of Saturn that formed along with those planets, they're very diverse and everything, but they're like, really big round worlds. And then you have Mars. Two tiny little...I mean, they're milesacross. They're the size of cities, not the size of what we think of as moons.

TALIA: They are little tiny chunks of rock. Boston-ish in size, about the size of Boston. They look like lumpy potatoes.

ERIC: I do love how Phobos and Deimos are their names, which translates to fear and panic. Which is awesome.

TALIA: Yep. It's appropriate for the planet named for the God of War.

ERIC: And definitely pause at this point, listener, and just go look up that photo of Deimos from the United Arab Emirates mission. It's so cool. And like you said, you sent it out in your newsletter and said, "it kind of looks like CGI, it doesn't look real." This is a real shot from a spacecraft orbiting Mars.

TALIA: I thought it was an artist's rendition the first time I saw it, because it's such a perfectly composed shot. And they're like, no, this is actually the shot from the spacecraft and it's like wow, alright.

ERIC: Let's wrap up the rest of the solar system, now that we've decided this number is useless and doesn't mean anything, let's get to the number. Uranus and Neptune, also pretty big. They've got moons in the dozens.

TALIA: Yep. I believe the current counts are 27 for Uranus and 14 for Neptune.

ERIC: You are correct.

TALIA: Well, it is my job to know.

ERIC: And I do love the names of those ones, because Neptune's got moons named after water deities, which makes sense. Neptune -

TALIA: The god of the sea. But I really like Uranus's names.

ERIC: The Shakespeare characters. And Pope.

TALIA: The Shakespeare characters.

ERIC: Titania, Oberon.

TALIA: Puck.

ERIC: Miranda.

TALIA: Juliet.

ERIC: I haven't read a lot of Shakespeare. But when I do I know them as moons first. And so it's a little weird to be like, "why did Shakespeare name all his characters after the moons of Uranus?" And then it's like, obviously, that's not the case.

TALIA: Other way around.

ERIC: So that's the number of moons that go around planets. It's 284.

TALIA: Yikes.

ERIC: But that's not all the moons in the solar system if you're saying that moons, as natural satellites, are anything that goes around something that's not the sun, because we've got dwarf planets,

TALIA: We have dwarf planets that have them. Pluto has five moons. And a couple of the other ones have at least one. We know Eris has one. And then there's the asteroids.

ERIC: Yeah, there's asteroids that are, like we said, the size of cities, some of the asteroids are enormous. They're a couple hundred miles across, most of them are much smaller. And some of them with our telescopes getting better, we're seeing they're actually binary systems where it's two asteroids kind of doing a dance, or there's an asteroid the size of a city with a smaller asteroid the size of a neighborhood going around it. And so far, we found about 360 of those.

TALIA: Something like that. Yeah, so that really jumps the number up a lot.

ERIC: That's more moons and go around planets. And it makes sense because even though they're small, on the scale of the solar system, the smaller something gets, the more numerous it tends to be.

TALIA: And of course, we're only talking right now about natural satellites. You know, what humans love to do: put things in orbit around other things. So we have put thousands of artificial satellites around our own planet. And over the years, we've put up to a couple dozen around, or tried to put a couple of dozen around Mars. They're not all still there. They're not all still alive.

ERIC: Yeah, there's seven orbiting now. So for all of our exploration of Mars over the last 50 years now, do we count those as moons? They're things orbiting things.

TALIA: They're things orbiting things.

ERIC: Again, this number doesn't really mean anything. We'll give you the number at the end of the episode. But it depends on your definition of so many things.

TALIA: Yes.

ERIC: And the last category is stuff that's out beyond Neptune that isn't a dwarf planet. The trans-Neptunian objects are pretty small. But we found over a hundred of those that have moons too.

TALIA: It doesn't honestly matter how small you are, you're still potentially capable of grabbing a hold of something smaller and having it orbit around you.

ERIC: That's all the moons that we know of in the solar system, all the different categories of moons. The final number I came up with is 793. But it's hard because these things change so often, and the definitions change, depending on who you ask. And there are official channels that everything goes through. But by the time one discovery gets approved, there's new ones in the news that are waiting for approval. So if you say '800', you're not that far off right now.

TALIA: Around 800 sounds...just go with the round number

ERIC: Mid-2023, 800 sounds good. I'm sure we'll have another batch that will make this episode out of date by the end of the year. But that's the number.

TALIA: That's also how astronomy works.

ERIC: Talia, thanks for telling us all about how many moons there are in the solar system.

TALIA: Thank you for asking me because I love talking about moons.

ERIC: Next time you're at the Museum of Science catch a planetarium show. You can find the schedule at mos.org which is also where you can subscribe to Talia's newsletter, Spacing Out, and keep up with the latest additions to the list of moons in our solar system. Until next time, keep asking questions.

If you liked this episode, be sure to check out:

Does the Moon Rotate?

How Big Were the Apollo Moon Landing Computers?

What's Your Favorite Unsolved Space Mystery?

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