Jupiter Wins The Moon Lottery
When Galileo first saw Jupiter through a telescope, he also discovered “stars” that would orbit around the planet in the night sky. While Galileo named them the Medicean stars—after his future patron Cosimo II de’ Medici—we know them today as Jupiter’s moons Io, Europa, Ganymede and Callisto. Since Galileo’s initial discovery, astronomers have found dozens more moons around Jupiter, and this week, researchers announced an additional 12 moons, bringing the total number up to a whopping 79.
Lead astronomer at Carnegie Institution for Science Scott Sheppard explains how he unexpectedly found these moons while searching for the elusive Planet Nine, hypothesized to lurk far beyond Pluto—and why the planet-hunting Blanco 4-meter Telescope just happened to be the perfect tool to investigate Jupiter’s skies.
Scott Sheppard is an astronomer at the Carnegie Institution of Science. He’s based in Washington, D.C.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’ll get the recipe for a 14,000-year-old pita bread, which has one very untraditional ingredient. But first, when Galileo first saw Jupiter through a telescope, he noticed stars hanging around the planet. And as he watched those stars night after night, he realized they were orbiting the planet. We now know these stars are Jupiter’s moons, Io, Europa, Ganymede, Callisto.
And since Galileo’s discovery, astronomers have found dozens of moons around Jupiter. And now this week, they’ve added 12 more, bringing Jupiter’s moons total to a whopping 79. And here to talk about the discovery is Scott Sheppard. He’s an astronomer at the Carnegie Institute for Science. And if you have questions about Jupiter’s moons, our number (844) 724-8255. You can also tweet us at @scifri. Welcome to Science Friday.
SCOTT SHEPPARD: Thanks. Thanks for having me.
IRA FLATOW: How unexpected is this finding?
SCOTT SHEPPARD: It wasn’t too unexpected, because we’re doing a survey. It’s this the deepest largest survey for outer solar system objects. So we’re trying to find things beyond Pluto, but Jupiter happened to be in our fields as well in March of 2017. And our survey can cover a big area of sky and can go deeper than other surveys have in the past. So we expected we could turn up some new moons, because we have an advantage over others that came before us.
IRA FLATOW: So you’re saying that you didn’t set out originally to point your telescope at Jupiter but it happened to be there so why not take advantage?
SCOTT SHEPPARD: Yeah. We do our survey every few months throughout the year. And we knew in March of 2017, Jupiter would be near where we are looking. So we decided to make sure Jupiter was in the center of our field so we could search for moons at the same time we’re searching for very distant things and source them.
IRA FLATOW: So how do you know they were moons and not some asteroid or another gigantic space rock? And what is a moon defined as anyhow?
SCOTT SHEPPARD: Yeah, it is actually a pretty long process. So March of 2017 is when we did the discovery observations. Then we had to re-observe the objects a few months later. And we re-observed everything that was moving at the same rate that Jupiter was moving. And that suggests it has something to do with Jupiter.
And it wasn’t until actually of May of 2018 when we figured out finally that 12 of the objects that were falling were moons of Jupiter. Several other objects we were following turned out to be comets or main-belt asteroids. So it did take a year to determine how many moons we had found.
IRA FLATOW: And so why does Jupiter have so many moons? 79, right, in the last count?
SCOTT SHEPPARD: Yes. So that’s 79. And there’s two reasons Jupiter has this may moons. One is it’s the biggest planet in the solar system. So it has a very big sphere of influence where if something gets too close to it, it can control that and create a moon out of it.
A second reason it has a lot of these moons are finding– the moons that we found are outer moons. So they’re further away from Jupiter than the Galilean moons are. And we find these in groups a lot of the times. And what we think is there were originally just a few large or parent bodies that were hundreds of kilometers in size and they’ve been collisionally broken apart over time. So we’re finding a lot of fragments from collisions that have happened in the Jupiter area.
IRA FLATOW: Now this may sound like a little weird question, because you know most of us are used to looking at one moon in the night sky around our planet. Are all the moons orbiting the same way?
SCOTT SHEPPARD: Yeah. Not all of Jupiter’s moons orbit the same way. The Galilean moons, which we believe formed with Jupiter go around Jupiter. They orbit Jupiter in the same direction that Jupiter spins. And then we have two different outer kind of groups. We have the prograde group, which does the same as the Gallileans. They go around the same direction Jupiter spins, and those are about 10 million kilometers from Jupiter.
And then beyond those, we have what we call the retrograde group, and they go in the opposite direction that Jupiter spins. And so we have these two outer groups that go in different directions. And one of the new objects we found actually doesn’t obey those two laws. It’s actually a prograde object that’s in the retrograde group. So it’s going down the highway in a wrong direction, which suggests it probably has collided head on with some of these retrograde objects in the past.
IRA FLATOW: Does it have a weird shape that would tell you that? We don’t know the exact shape, but it’s the smallest moon we found around Jupiter. It’s about one kilometer in size. So it’s probably very irregular in shape. It was probably actually bigger in the past. It was probably 10 to 100 kilometers in size, but it’s been ground down to mostly dust through collisions over the age of solar systems. So what we see today is probably the remnant of a once bigger moon out there.
We have a tweet coming in from Heather Hill who says, do any of Jupiter’s moons seem to have an atmosphere?
SCOTT SHEPPARD: The Galilean moons can hold on to some very tenuous atmosphere, but nothing thick. The only moon that really has an atmosphere in our solar system is Titan, and that’s around Saturn.
IRA FLATOW: Our number 844-724-8255 is our number if you’d like to join us. Let’s go to the phones. Let’s go to Matthew in Pittsburgh. Hi, Matthew.
MATTHEW: Hi. How’s it going?
IRA FLATOW: Hi there. Go ahead.
MATTHEW: So I was wondering why the four Galilean moons look so different from each other, and whether these new moons are also that diverse.
IRA FLATOW: Good question.
SCOTT SHEPPARD: Yeah, it’s a good question. So the Galilean moons look different because they’ve had quite different histories. Io or Eo, it’s been called either way, is the closest one to Jupiter, and it’s being tidally flexed. It’s very close to Jupiter, so it gets pulled by Jupiter, but it also gets pulled by the further out Galilean satellites as well. So it’s kind of like if you take a rubber band and stretch it back and forth, it gets really hot. So Io gets very hot inside because it’s getting stretched back and forth.
And that’s why it actually has all these volcanoes we find on it. It’s the most active volcano object in our solar system. And so that completely changes its surface composition to sulphur because of all these volcanoes, whereas the first one out of the Galileans, Callisto, is very far out there, and it’s mostly because it’s so cold. And it’s for these two different reasons– one’s much further away, it’s much colder, the other one is getting tidally stretched– that you see quite differences on the surfaces the Galilean moons.
IRA FLATOW: If Jupiter has so many moons and you have 79, and then the next planet next to it is Saturn, it has all these rings, which are actually like thousands, gazillions of tiny little moons running around, right, why has one, you know, got rings and why has one got little moons around it?
SCOTT SHEPPARD: Yeah, so Jupiter actually has rings as well, but it’s very tenuous and they’re very close to Jupiter. And Jupiter’s rings actually come from– it has four very small inner moons that are closer to Jupiter than the Galilean moons are. And these get bombarded by micrometeorites. It’s kind of like if you go out at night and you see a meteorite in Earth’s atmosphere. Those things are raining down on these small moons all the time next to Jupiter. And when they hit the surfaces of these very small moons– these are only a few tens to 100 kilometers in size– they release particles off the surface of the moons, and those particles escape and they start orbiting around Jupiter. So Jupiter has rings from that process as well.
Saturn’s rings are probably quite different. We believe– we didn’t know if they were very old or fairly new, and when I say new I’m talking tens to 100 millions of years old– but it looks like from Cassini data that came in over the last few months Saturn’s rings are newer than we thought. And so they could be a broken-up moon, some kind of recent broken-up moon. But that’s still to be determined exactly where Saturn’s rings came from.
IRA FLATOW: A lot of tweets are coming in. Let me get one from Joshua, who says if Earth had more than one moon, how would it affect tides?
SCOTT SHEPPARD: Yeah, so Earth is special because we have one moon, and it kind of stabilizes Earth’s rotation axis. So it’s kind of important for life on Earth, because if our rotation axis was able to move around, that wouldn’t be a good thing. So if we had another moon, at some times those two moons could align with each other, be on the same side of the Earth, and tides would even be greater. And then sometimes those two moons would be on the opposite side of Earth and they could cancel each other out. So yeah, it would be a quite different tide system here on Earth if we had more than one significant moon.
IRA FLATOW: Do you think, then, that one moon is significant for the development of life on Earth?
SCOTT SHEPPARD: A lot of people think a moon is very helpful for our planet because it stabilizes the rotation axis of a planet, and so currently, the Earth’s rotation axis is 23.5 degrees inclined, and it more or less stays near that number. And so the seasons on Earth are very constant, very similar every year to year. But if you let that rotation axis vary, the seasons could get much harsher at other times if the rotation axis changes. And so that would be pretty bad for life here on Earth.
IRA FLATOW: Andrea tweets, are they newly discovered or newly formed?
SCOTT SHEPPARD: The moons around Jupiter are newly discovered. We believe they’ve been there since Jupiter formed, about 4.5 billion years ago. And the ones that we discovered, these outer moons, we don’t believe they formed with Jupiter. We actually think they were captured by Jupiter during its formation process. Jupiter cannot currently capture moons very efficiently because you need to somehow slow an object down to capture it. You need to dissipate energy from its orbit, and that currently doesn’t exist. But when Jupiter was forming, there was a lot of gas and dust in the system, and that could slow an object down and allow it to be captured.
IRA FLATOW: Alan tweets, why are planets and moons always circular?
SCOTT SHEPPARD: So the biggest ones are circular. If you’ve ever seen an astronaut in space and you see them throw water out in a zero gravity environment, that water ball will go to a sphere because that’s the least amount of energy for an object to have. And so if you have a very big object where gravity dominates, it’s got to turn into a sphere because that’s the least amount of energy for the object to have.
But these moons that we found are small. They’re only a few kilometers in size, and they’re likely not spherical. They’re probably very elongated, irregular shaped bodies because their surface tension, the material strength of the body, is probably stronger than gravity is, and so they can keep these elongated shapes over gravity.
IRA FLATOW: So they’re too small to be like Europa, which might have oceans or things on it.
SCOTT SHEPPARD: Yeah, so all the Galileans are very spherical because they’re very big objects. Ganymede, the largest moon in our solar system, is bigger than the planet Mercury. So that just shows you how big some of these Galileans are.
IRA FLATOW: I want to get one quick question in before we have to go about your search for Planet Nine. How’s it going?
SCOTT SHEPPARD: Yeah, so our main goal is to find a dwarf planet and planets beyond Pluto. We call them way, way out there objects. And we’ve covered about 20% of the sky we wanted to cover. So we’re still doing that survey. We found several new dwarf planet type objects that we’re tracking. And hopefully, the more we find these small objects, they can lead us to a bigger object due to its signature on the orbits of the smaller objects.
IRA FLATOW: So you’re very hopeful you’re going to still find it.
SCOTT SHEPPARD: We think it’s more likely than not that this planet beyond Pluto exists. But it’s not a guaranteed thing. And that’s kind of what in the next few years we hope to determine.
IRA FLATOW: Can you train your telescope on another planet looking for more moons, or would you suspect there are more there around Jupiter, we just haven’t seen them. If there’s 79, why not 179?
SCOTT SHEPPARD: Yeah, so we’ve actually checked in the last year or two, we’ve checked all the planets, and Jupiter was the one where we haven’t gone to extreme faint magnitudes, extreme depths for the very small ones yet just because its sphere of influence is so big, it’s hard to cover the whole area around Jupiter. But we’re using the largest wide-field camera on a large class telescope in the world that only was put on a telescope in the last few years. So we have a big advantage over people in the past, and that’s why Jupiter was so successful.
IRA FLATOW: Scott Sheppard, thank you for this fascinating– for taking time to talk with us today.
SCOTT SHEPPARD: Thanks for having me.
IRA FLATOW: You’re welcome. Scott Sheppard, an astronomer at the Carnegie Institution for Science. We’re going to take a break, and when we come back, Emperor Charlemagne– how much do we really know about what’s lurking in our DNA? Is it possible we could be related to Charlemagne? Maybe Cleopatra. Carl Zimmer is here with his new book to talk about it. We’ll be right back after this break.
Lucy Huang is a freelance radio producer and was Science Friday’s summer 2018 radio intern. When she’s not covering science stories, she’s busy procrasti-baking.