How We Found Jupiter’s 79 (At Least) Moons

By Korey Haynes | December 27, 2018 4:18 pm
a tiny moon in front of jupiter

The moon Io is tiny compared to mighty Jupiter, but still among the easiest of Jupiter’s many moons to spot. (Credit: Cassini Imaging Team/SSI/JPL/ESA/NASA)

Jupiter is king of the planets. It’s huge, it’s bright in our night skies, and even four of its comparatively tiny moons are bright enough to see with the most basic of telescopes. We’ve sent nine probes either into orbit or on a close flyby of the planet. And yet, as recently as this past year, we discovered not one, but twelve new moons around Jupiter, bringing the total to 79. How haven’t we exhausted this particular moon mine yet?

The Easy Targets First

The answer is that most of Jupiter’s moons aren’t the grand companion that our own moon is to Earth, at nearly a quarter as wide as its host planet. The four moons first spotted by Galileo in 1610 — Io, Europa, Ganymede and Callisto — are big enough compared to our moon, but absolutely puny when compared to Jupiter, the planet they circle. And those are the easy targets. It makes discovering new moons against its bulk difficult.

It took the advent of photography before astronomers discovered any more moons around Jupiter, and the work over the next century or so was painstaking. By the time Voyager cruised by in 1979, the giant was up to 13 moons. Voyager added three to the count: Metis, Adrastea, and Thebe.

All three of these plus Amalthea (discovered in 1892 by famed astronomer E.E. Barnard) and the original Galilean moons comprise Jupiter’s regular moon group. This means they’re more or less spherical, orbit in the same direction that Jupiter spins and do so on well-behaved, near-circular orbits that don’t tip much out of the plane of Jupiter’s equator. In other words, what you probably imagine a moon to be.

The rest are the irregular moons, and these make up the vast majority of Jupiter’s satellites. These tend more toward potato shapes, and their orbits are often eccentric, tilted, or even retrograde, meaning they fly backwards to Jupiter’s spin. Most are probably captured asteroids or the results of long-ago collisions of larger bodies — perhaps past moons of Jupiter. They’re tiny and tend to orbit farther out from Jupiter than the regular moons. This makes them much harder to spot.

Astronomers found a few of these irregular moons. But after Voyager, the discoveries stopped for about two decades.

A Population Explosion

diagram showing prograde moons in blue, retrograde moons in red

One of Jupiter’s newest moons orbits prograde (normally), but since it’s among the retrograde (backwards) moon group, it’s probably marked for a deadly collision before too long. (Credit: Roberto Molar-Candanosa/Carnegie Institution for Science)

And then Scott Sheppard appeared on the scene. The Carnegie Institution for Science astronomer’s teams are responsible for 60 of the 79 known jovian moons — all irregular, but still an impressive feat. Sheppard’s team has been discovering moons around Jupiter since 2000. Just this past year, they added a round dozen to the list. The new moons add to our understanding of Jupiter’s neighborhood and help astronomers understand how the planet formed and its surroundings evolved over time.

It is true, though, that some of Jupiter’s moons have been “discovered” more than once. These glorified space boulders are sometimes spotted in images but their orbits are poorly understood. So when astronomers look for them again in a few months or years, sometimes they turn up missing and have to be found again.

These irregular moons are quite tiny — only a few miles to tens of miles across. They bear little resemblance to the complex worlds of Europa and Ganymede, or even our own moon. Instead, they are mostly misshapen hunks of rock, orbiting far out from Jupiter’s bulk. So the telescopes that find them have to be sensitive, and either look at a large swath of space or get very, very lucky.

Spotting Tiny Specks

The probes we sent to Jupiter, while far closer than Earth-bound telescopes, are mostly busy looking at the planet. They, too, would have to get quite lucky to catch one of these tiny irregular moons by accident while trying to image the planet. And frankly? The possibility of finding one more tiny space rock doesn’t tempt scientists who want to understand the deep mysteries of Jupiter’s storms or interior. They’re not wasting precious mission time looking very hard.

Sheppard’s most recent successes actually came while he was looking much farther out, trying to find a possible Planet Nine far past the giant planet’s orbit. But since Jupiter happened to be in the same region of the sky, Sheppard and his team checked to see if they could find any photo-bombing moons in their images. They got lucky, though hard work went into the find as well.

Astronomers have gotten much better at wide-field surveys where they scan large chunks of the sky at once. Our telescopes, of course, have also gotten better. But mostly, you have to be willing to spend a long time looking for very dim objects to discover any new satellites around Jupiter.

Most individual irregular moons aren’t considered groundbreaking discoveries on their own (though a few weird exceptions exist). But taken as a whole? Jupiter’s rowdy brood of moons, regular and irregular, tell a long and interesting story about what life is like around the solar system’s largest planet.

CATEGORIZED UNDER: Space & Physics, Top Posts
MORE ABOUT: solar system
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  • Erik Bosma

    Rocks, like rings, shouldn’t count as moons.

    • OWilson

      There’s only one Sun, one Earth, and one Moon.

      The rest are stars, planets and satellites.

      • jonathanpulliam

        This is oilson physiclet:

        “Get me a Coke please!”

    • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

      Dwarf moons? Nah – moonlets.

  • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

    If you are attempting a gravitational slingshot about Jupiter ~in the plane of the ecliptic, you might exercise an unusual interest in orbital debris.

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  • Mike Richardson

    It’s probably best to think of a gas giant’s family of satellites as one that includes permanent members as well as foster children and occasional new adoptions. To explain this analogy, think of Jupiter’s Galilean moons and other regular large moons as permanent members born into the family. Then you have the permanent or semi-permanent gravitational captures, the adopted children. Finally, some of the moonlets are temporary captures, loose aggregations of rubble that later disintegrate, or satellites in unstable orbits that eventually collide with other moons or the gas giant itself. Like human families, these families of worlds show a large degree of diversity.

    • jonathanpulliam

      Poor analogy from a poor observer.

      • OWilson

        It takes a solar system to raise a moonlet!

        But they are not all born equal! :)

        • Never_Simone

          You got your butt handed to you, Ow. Suck it up and move along.

          • Mike Richardson

            Happens pretty often around here. As I noted on this thread, he just keeps doing the same thing and expecting a different result. I think Einstein had something about that, in fact.

      • Mike Richardson

        Poor attempt at trolling, but par for the course, eh, oven boy? They say you’ll spend the New Year doing what you did at the beginning. Looks like a sad 2019 for you.

        • OWilson

          Trolling usually involves name calling from the usual trolls :)

          • Mike Richardson

            Your lack of irony awareness never ceases to amuse me. If anyone would be an expert on trolling, it would be you — though you seem to have missed Johnny trolling you below, “oilson.” 😁

          • OWilson

            Just an attempt at a pun, Mikey. He’s just an amateur :)

            Far more innocuous than your usual favorites for me: “alcoholic”, “closet fascist”, “mental incompetent”, “senile”, “racist”, “plagiarist” and the many others on the list I have somewhere :)

            Now that’s industrial strength trolling!

            Lol

          • Mike Richardson

            It’s a shame you’ve provided so much evidence, at various times, for every one of those characterizations in quotes. And actually quite sad if you do in fact keep a list of every observation other folks make about your behavior you don’t like. Your chronicle of victimhood must rival the Encyclopedia Britannica by now, but is probably a few volumes short of any listing of insults you’ve leveled at others over the years. You really are quite the hypocrite.

            As for fantasizing about your wife, sorry, but the operative word is “sympathize.” I know you don’t like providing the context of these remarks, so I will. Some time ago, you mentioned a wife in the Dominican Republic. In a short space of time, in another thread, you referred to having a girlfriend. Since those are not interchangeable terms, I concluded you must either have an open marital relationship, or are cheating on your spouse, deceiving or “duping” your wife. Now, you could always clarify your current marital /relationship status, or allow others to assume the worst based on your own statements. I’m not bothered either way.

            Finally, I think a climate change denying, Trump admiring, individual who brags about the benefits of not getting regular medical checkups should probably not be so quick to call anyone else’s opinions deluded. Kinda the pot calling the kettle black, ya know? 😏

          • OWilson

            Lets see :)

            “Climate change denying” – wrong, the climate has, is and always will change. I regularly post links to NOAA’s satellite record, showing the change over the last 40 years.

            “Trump admiring” – partially correct, given my relief that those who wanted “Slick Willie” (as those who knew him best in Arkansas nicknamed him) and his missus back in the White House were defeated.

            “Bragging about the lack of medical check ups” -wrong, I was relating a very personal experience with the drug dispensing industry and advocating a healthy balanced diet and lifestyle as just one method of combating the very serious obesity and opioid dependency crisis in your country. I am a senior citizen, healthy, at my optimum weight, despite a family genetic dispostion to obesity, and have taken no prescription or other drugs since 1982.

            And you? :)

            “Closet fascist” – wrong! I escorted students to Auschwitz on four occasions to see the obcenity they wrought on civilization.

            “Alcoholic” – wrong! an occasional Lite beer in my tropical paradise.

            “Racist” – wrong! My “partner” is considered black as are the locals I live happily among.

            As to whether she is “duped”, she would have to answer that question herself! :)

            But I appreciate your long standing interest in my life, and thank you, on behalf of the readers for dropping the atrocious name calling, at least in your response.

            This is after all, a science blog, where that sort of stuff is totally inappropriate, although your “oven boy” ad hominem to another poster above here, indicates that sometimes you just can’t help yourself!

          • Mike Richardson

            Wow, looks like I struck a nerve or two. 😉

            Good.

            Now, your “climate has, and always will change,” statement is a standard rhetorical dodge of climate change deniers. What you deny is that humanity has had any significant impact on the climate, despite the findings of the vast majority of researchers in the field, and the totality of NOAA’s data (not just your cherry-picked month-to-month readings provided out of context).

            To your second attempt at a refutation, you have indeed described yourself as an “unabashed Trump supporter,” despite the extreme narcissism, immaturity, and ignorance he displays on a regular basis.

            Regarding your statements regarding avoiding doctors, they’re part of the record on Disqus, and you’ve basically said nothing contradicting what I noted.

            As far as “closet fascist,” I often think with your support of the most extreme far right politicians today, that you have come out of the closet. Maybe you’ve visited Auschwitz, but you’ve also mischaracterized the regime responsible for those horrors as “socialist” and refuse to acknowledge that the far right is every bit as terrible as the most extreme elements of the far left (which you also tend to generalize as representing all “leftists”). And if you truly were upset over what you saw at Auschwitz, you would have been more upset at JP’s comments that his opponents could be “burned in ovens to provide useful calories,” than my calling him out for it. “Oven boy” is a rather mild epithet for someone who brings up a final solution like that.

            I stand by my assessment of your prejudice, and indeed racism demonstrated against certain groups. Having a partner of another race or ethnicity might be step up from “I have black friends,” but I’ve seen plenty of folks where I live engage in sexual relations with people they later use racial slurs to describe. You, for example, repeat the racially charged nickname of “Pocahontas” for Elizabeth Warren, and have taken many opportunities to express particular disdain for Native Americans. I also recall your frequent use of the phrases “gang-banging ghettos,” “Democrat plantations,” and other such descriptions for minority groups. Whatever your personal relationships may involve, you’ve shown a great degree of prejudice in your posts, which is all part of the record.

            Perhaps, though, you aren’t an alcoholic, though you have spent more words describing your fondness for Presidente Lite than any family member or friend. I just go by the frequency of your mentions of something as an indicator of your fondness for it.

            You should apologize for diverting this conversation away from the topic of planets and moons, but you are sadly mistaken if you think anyone will share your delusion of “following you down your trolling swamp black hole, and taking the discussion waaaay off topic, yet again!” Need I remind you, I made the initial on-topic comment, and you felt the need to chime in to my response to JP, taking us down this path. You wanted this, Einstein, and you got it. So while I would enjoy further discussing the variety of solar system satellites, we instead had to talk about your favorite topic, apparently — you. Unfortunately, I don’t think you did yourself any favors here, as usual. 😉

          • Mike Richardson

            Wow, looks like I struck a nerve or two. ;). Good.
            Now, your “climate has, and always will change,” statement is a standard rhetorical dodge of climate change deniers. What you deny is that humanity has had any significant impact on the climate, despite the findings of the vast majority of researchers in the field, and the totality of NOAA’s data (not just your cherry-picked month-to-month readings provided out of context).

            To your second attempt at a refutation, you have indeed described yourself as an “unabashed Trump supporter,” despite the extreme narcissism, immaturity, and ignorance he displays on a regular basis.
            Regarding your statements regarding avoiding doctors, they’re part of the record on Disqus, and you’ve basically said nothing contradicting what I noted.
            As far as “closet fascist,” I often think with your support of the most extreme far right politicians today, that you have come out of the closet. Maybe you’ve visited Auschwitz, but you’ve also mischaracterized the regime responsible for those horrors as “socialist” and refuse to acknowledge that the far right is every bit as terrible as the most extreme elements of the far left (which you also tend to generalize as representing all “leftists”). And if you truly were upset over what you saw at Auschwitz, you would have been more upset at JP’s comments that his opponents could be “burned in ovens to provide useful calories,” than my calling him out for it. “Oven boy” is a rather mild epithet for someone who brings up a final solution like that.
            I stand by my assessment of your prejudice, and indeed racism demonstrated against certain groups. Having a partner of another race or ethnicity might be step up from “I have black friends,” but I’ve seen plenty of folks where I live engage in sexual relations with people they later use racial slurs to describe. You, for example, repeat the racially charged nickname of “Pocahontas” for Elizabeth Warren, and have taken many opportunities to express particular disdain for Native Americans. I also recall your frequent use of the phrases “gang-banging ghettos,” “Democrat plantations,” and other such descriptions for minority groups. Whatever your personal relationships may involve, you’ve shown a great degree of prejudice in your posts, which is all part of the record.
            Perhaps, though, you aren’t an alcoholic, though you have spent more words describing your fondness for Presidente Lite than any family member or friend. I just go by the frequency of your mentions of something as an indicator of your fondness for it.

            You should indeed apologize for diverting this conversation away from the topic of planets and moons, but you are sadly mistaken if you think anyone will share your delusion of “following you down your trolling swamp black hole, and taking the discussion waaaay off topic, yet again!” Need I remind you, I made the initial on-topic comment, and you felt the need to chime in to my response to JP, taking us down this path. You wanted this, Einstein, and you got it. So while I would enjoy further discussing the variety of solar system satellites, we instead had to talk about your favorite topic, apparently — you. Unfortunately, I don’t think you did yourself any favors here, as usual. 😉

          • Mike Richardson

            Wow, looks like I struck a nerve or two. 😉 Good.

            Now, your “climate has, and always will change,” statement is a standard rhetorical dodge of climate change deniers. What you deny is that humanity has had any significant impact on the climate, despite the findings of the vast majority of researchers in the field, and the totality of NOAA’s data (not just your cherry-picked month-to-month readings provided out of context).

            To your second attempt at a refutation, you have indeed described yourself as an “unabashed Trump supporter,” despite the extreme narcissism, immaturity, and ignorance he displays on a regular basis.

            Regarding your statements abot avoiding doctors, they’re part of the record on Disqus, and you’ve basically said nothing contradicting what I noted.

            As far as “closet fascist,” I often think with your support of the most extreme far right politicians today, that you have come out of the closet. Maybe you’ve visited Auschwitz, but you’ve also mischaracterized the regime responsible for those horrors as “socialist” and refuse to acknowledge that the far right is every bit as terrible as the most extreme elements of the far left (which you also tend to generalize as representing all “leftists”).

            And if you truly were upset over what you saw at Auschwitz, you would have been more upset at JP’s comments that his opponents could be “burned in ovens to provide useful calories,” than my calling him out for it. “Oven boy” is a rather mild epithet for someone who brings up a final solution like that.

            I stand by my assessment of your prejudice, and indeed racism demonstrated against certain groups. Having a partner of another race or ethnicity might be step up from “I have black friends,” but I’ve seen plenty of folks where I live engage in sexual relations with people they later use racial slurs to describe. You, for example, repeat the racially charged nickname of “Pocahontas” for Elizabeth Warren, and have taken many opportunities to express particular disdain for Native Americans.

            I also recall your frequent use of the phrases “gang-banging ghettos,” “Democrat plantations,” and other such descriptions for minority groups. Whatever your personal relationships may involve, you’ve shown a great degree of prejudice in your posts, which is all part of the record.

            Perhaps, though, you aren’t an alcoholic, though you have spent more words describing your fondness for Presidente Lite than any family member or friend. I just go by the frequency of your mentions of something as an indicator of your fondness for it.

            You should apologize for diverting this conversation away from the topic of planets and moons, but you are sadly mistaken if you think anyone will share your delusion of “following you down your trolling swamp black hole, and taking the discussion waaaay off topic, yet again!” Need I remind you, I made the initial on-topic comment, and you felt the need to chime in to my response to JP, taking us down this path. You wanted this, Einstein, and you got it.

            So while I would enjoy further discussing the variety of solar system satellites, we instead had to talk about your favorite topic, apparently — you. Unfortunately, I don’t think you did yourself any favors here, as usual. 😉

          • OWilson

            Have a nice day, Mikey! :)

          • Mike Richardson

            Once again, your weak efforts at projection and attempting to defend your trolling and prejudiced views are an epic fail, here for everyone to see. I’m sure those arguments sounded much better in your head. Another occasion where you would have been better served by not commenting at all, but I’m pleased to see you never learn and just keep repeating the same mistakes. Adios for now. 😁

          • OWilson

            Just keep it clean, Mikey, ya hear? :)

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