The outer Solar System is far enough from the Sun that water doesn’t get blasted away into deep space. In this icy realm, there are many worlds with vast quantities of water ice. Today let’s look at the icy outer moons and dwarf planets.
Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020) (The National Academies)
Interiors of icy bodies in the solar system (as of 2010) (The Planetary Society)
Hill Sphere (OSIRIS-REx)
Oort Cloud (NASA)
Scattered Disk Objects (Swinburne University)
Centaurs (Swinburne University)
Kuiper Belt (NASA)
Why icy worlds? (Northwest Earth and Space Sciences Pipeline)
Voyager Mission Overview (NASA)
What are the Galilean Moons? (Universe Today)
Controversy over the discovery of Haumea (Wikipedia)
The Dwarf Planet Eris (Universe Today)
Ep. 613: Pluto’s Demotion: 15 Years Later (Astronomy Cast)
Ep. 1: Pluto’s Planetary Identity Crisis (Astronomy Cast)
Hypothetical Planet X (NASA)
What Is A Dwarf Planet? (Universe Today)
Hydrostatic Equilibrium (Swinburne University)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast Episode 619: Icy Moons and Dwarf Planets. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey Pamela, how ya doing?
Dr. Gay: I am doing well. How are you doing?
Fraser: Doing great. We mentioned this in the preshow but the new decadal survey, the 2020 decadal survey, is out. You can freely download it and read all of the cool new science missions that are being suggested and backed by the National Academy of Science. It’s a pretty exciting time to see what the future holds for space and astronomy and various missions and so on.
Dr. Gay: It’s gonna be a lot to go through. I know that they are looking for a next set of great observatories. And I am looking forward to seeing what we as a community are able to come up with and implement. And I just hope that this brings that next new level of –
Dr. Gay: – innovation.
Fraser: So, cool. All right. So, the outer solar system is far enough from the sun that water doesn’t get blasted away into deep space. In this icy realm there are many worlds with vast quantities of water or ice. Today let’s look at the icy outer moons and dwarf planets. All right. Let’s talk about the outer solar system. The icy objects. I’m trying to think. Are they the same? Are they different? What would be your standard – what’s your baseline here for an icy world? Beit a moon or a dwarf planet out there in the Kuiper Belt.
Dr. Gay: Looking out at our outer solar system, we have lots of objects that have subsurface oceans that have densities that are closer to the density of water than the density of granite. And in looking at their densities, we start to see this continuum from rocky stuff to icy stuff. And often because things are so far away, we’re not really sure where things are on that spectrum. But we know from their thin atmospheres of methane, ethane, ammonia, things like that, that these aren’t objects like our world.
Dr. Gay: They’re something more like Pluto. And while Pluto is kind of our OG icy planet.
Fraser: Right. So, then, if you took a bandsaw and –
Dr. Gay: Yes.
Fraser: – sliced Pluto or one of these other objects in half, what would you see as you took the two hemispheres apart?
Dr. Gay: There’d be a rocky core. Around it there would be either a continuous ocean or pockets of ocean depending on how the world is structured and how it’s giving up its heat. And then above that you have layers of ice that can be kilometers to tens of kilometers thick and have their own mountain ranges from where the icy plates have come together, where craters have formed, and a variety of other, well, cryogeologic features.
Fraser: And as you mentioned, in some cases, even a thin atmosphere.
Dr. Gay: Yes. And it’s because they’re so cold and because that they’re so tiny that we have such thin little atmospheres.
Fraser: So, then, let’s sort of – you know – as we’re gonna be talking about these objects in today’s episode, what would you like to add to the bucket? Where are we gonna start? Are we gonna include Enceladus? Or Europa? Where do you wanna start?
Dr. Gay: Let’s first start by looking at the where these suckers are because it’s easy to forget how big our suns – what we call the Hill sphere – that region of space where something is going to find itself bound to our sun instead of bound to some other object. And our Oort Cloud can go out as far as 200,000 astronomical units.
Fraser: Like two lightyears.
Dr. Gay: Yeah.
Fraser: It’s far. Yeah.
Dr. Gay: Yeah. And that keeps it a good portion of the way to the next nearest star. So, when you start looking at this massively expansive area you find that there’s that spherical Oort Cloud where things have been flung. There is a disky portion of the Oort Cloud that’s lined up with the rest of the solar system. Then there’s this area called the scattered disc which is where we think a lot of the comets come from. And these are objects that have been flung outward by the planets and are a compliment to the centaurs which have been flung inward by the planets.
Dr. Gay: So, these things are getting flung out. Things are getting flung in. This elliptical portion. And then of course there’s the Kuiper Belt region which is closer in to Neptune. And when you start looking at all these different sections, they don’t have a whole lot of mass that we’ve discovered so far. Although there could be some giant planets as we’ve talked about hiding out there. But it’s in massive volume that these regions are taking up.
Fraser: And yeah. I mean when you think about just the scale of the inner solar system compared to that, it’s a fraction.
Dr. Gay: Yeah.
Fraser: Most of the solar system is what you just described.
Dr. Gay: Yeah.
Fraser: Times orders of magnitude. It’s so big. And some of these objects have been captured by planets. Some are still orbiting roughly in the Kuiper Belt. And as you said others have been kicked into really weird orbits.
Dr. Gay: And some of them just formed around the planets. And this is where you see that science works the same in a whole lot of different environments. So, it looks like most of the moons – not all but most of the moons that are of a size around Jupiter and Saturn – they formed there. But then when we go and look at Triton, it’s orbiting backwards. It’s bigger than Pluto. It appears to be a captured Kuiper Belt object.
Fraser: Yeah. Yeah. It’s hard to understand how it might’ve formed in place in the wrong direction.
Dr. Gay: Right. Right.
Dr. Gay: And the meltiness of its surface seems to indicate that it had to dump energy at some point and that led to the meltiness.
Dr. Gay: And you can explain that from an object – again – captured into an orbit that was highly elliptical. And the constant changing of the tidal forces as it went in closer and then zoomed back out would lead to it dumping the energy into these thermal effects. The orbit would circularize and Triton would refreeze into the solid object we see today.
Fraser: So, you mentioned Triton. I’d like to just do a tour of some of these objects which you have been cramming into your brain. So, I’m gonna let you decide what you’d like to talk about.
Dr. Gay: So, we’re fairly familiar with these moons that count as icy objects. So, there’s Triton. Proteus out at Neptune. Going out to Uranus, we have all of the Shakespearean moons. Some around the Ariel, Umbriel, Titania, and Oberon. We don’t know a lot about them because we basically have data from the Voyager Missions. We’d like to have more data. We don’t. So, I can tell you that they’re icy, they’re cold, and they orbit Uranus. And that’s about all I can tell you.
Fraser: Right. They exist.
Dr. Gay: They exist. But then coming into Saturn – where we had Cassini for so long – this is where we start to understand some of the really cool physics that’s involved when we look at Enceladus. Enceladus is a icy world that is experiencing tidal forces from Saturn. And part of the effect this has had is pressure built up inside. And it had to break through somewhere.
And when it broke through, it broke through in the slightly weaker areas near one of the poles forming the first stripe. And it turns that the pressurized water that came out through its surface would then go up and then as the world rotated beneath, it would build up on the surface where it built up over time. And there’s pressure still on the inside leading to the next stripe.
Dr. Gay: And this happened to get us to this whole series of parallel tiger stripes on Enceladus. And it’s really amazing to look at.
Fraser: That’s interesting. I didn’t realize that the stripes were formed in series as the moon’s orientation changed to Saturn. And I’m sort of imagining Hawaii’s hotspot that forms the Hawaiian Islands –
Dr. Gay: Yes.
Fraser: – moving. It’s sort of a similar situation.
Dr. Gay: This is more like if you constantly are throwing snowballs onto weak ice and you can only throw them for 30 feet, you’re gonna end up with a buildup of snowballs 30 feet away that are eventually gonna crack that ice. Here, that 30 feet has to do with material came out through the crack, the world is rotating beneath, the material builds up on that weak surface until it cracks at that new place. Now you have two cracks that are geysering up material. Planet rotates under the material, it builds up, it gets another crack. And it was only once it hit four cracks that enough pressure was released that it’s not creating additional cracks.
Fraser: Hmm. Oh. Okay. So, it has stopped.
Dr. Gay: Yeah.
Fraser: All right.
Dr. Gay: Yeah.
Fraser: We’re gonna move on in your tour of interesting icy objects in a second. But it’s time for a break.
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Fraser: And we’re back. All right. So, what else has been fascinating you in this region? Like I said, you’re in charge here. I’m just gonna let you call the shots.
Dr. Gay: All right. So, from Saturn, let’s cruise on over to Jupiter where the Galilean moons were the first ones that really caught our attention as being worlds like no other. Here we’re looking at Io which is not an icy moon. It’s a Galilean moon. I’m going to just lay it right there. Today is not about Io. It’s more about Europa. Another one of these classic worlds that we see Cryovolcanism. We see geysers. It’s about Ganymede that appears to be a cratered icy shell.
And they also have an ocean underneath, but we don’t know as much about it. It’s about Callisto which, again, we just don’t know as much about, but we see that cratering. And looking at these different worlds we see through all their variations in colors, all their variations in structures, hints of what we might see if we’re someday able to get out to these significantly further away dwarf planets that folks like Mike Brown have been turning up. Not at a happy 5AU like Jupiter rounding a lot, but more like 40 AU. 90 AU.
Fraser: Right. Yeah.
Dr. Gay: Which makes observing a lot more difficult.
Fraser: Yeah. I mean, Galileo was able to see the Galilean Moons in his first very bad telescope.
Dr. Gay: Yes.
Fraser: And yet, as you say, Mike Brown with some of the biggest telescopes in the world can just barely detect similar objects that just happen to be 20 times farther away.
Dr. Gay: And part of what we’re dealing with is the difficulty in catching things moving. If you decide to take your Galileo lookalike scope – probably made of plastic – and go outside and look up at Jupiter, over the course of the night you can see its biggest moon zipping back and forth as they go round and round. And they appear to go back and forth because of their motion in front and behind Jupiter. Now that’s a single night you can see this happening. As you look at things that are further and further away it gets harder and harder to see their change in position over a single night. With Pluto you can just barely do it with a reasonable sized telescope.
Now with these more distant objects one of the reasons that there’s this massive resurgence in discovery of objects in the early 2000’s is because prior to that they’d been looking for things that were moving at basically 2 arcseconds a night-ish. Ish. And then they found Sedna, which was going at 1.75. And it was realized, “Oh, no. We need to start looking for things that are moving slower and slower.”
And within one week we saw – due to some nasty politics – the discovery announcements of Haumea Mea, of Eris, of all of these little, tiny objects. And part of the reason for this was Mike Brown and his team had been sitting on the discovery of a lot of these objects while they got additional data, firmed up the orbital parameters, firmed up discoveries of moons, and they had submitted a abstract to a conference where they were planning to announce an object they had at that point codenamed Santa. Because it was discovered in December. And this is one of the fabulous things about how Mike comes up with names for his objects.
Dr. Gay: So, he had it codenamed Santa. He had another one that was codenamed the Easter Bunny because it was found around Easter. And it will never be known exactly what happened. But a Spanish team looked through all of his observing logs, accessed the abstract, and within a couple of weeks of doing both of those things, announced Haumea’s discovery. They had their own data.
Dr. Gay: But –
Fraser: But it’s entirely possible that they – by watching the activity of the finest planet-hunter, finest on Earth –
Dr. Gay: Yeah.
Fraser: – they got a hint of what he might’ve discovered.
Dr. Gay: Yeah. And part of the reason that it’s so challenging is they’d had the data that they claimed was the discovery data for quite some time and hadn’t found anything. And it was right after the abstract went in and the abstract was public and the log files Mike found out were public. He didn’t realize all of these things were public. That suddenly these things are getting announced.
And when Haumea was announced by the Spanish team he didn’t immediately know that this was based on all of these attempts to look at his log files and everything else. But he very quickly went public with everything he had, submitting discoveries of moons, submitting Eris. And when it was found out that it was the Spanish team that had accessed his logs, this led to, basically, filing of official academic grievances that –
Dr. Gay: And all of this really overshadowed just how un-amazingly cool Haumea Mea – there’s a bunch of these objects that have names you say twice. Haumea is not one of them.
Fraser: Yeah. Yeah, you could say Haumea, Haumea. All right.
Dr. Gay: So, Haumea, one of the reasons he was sitting on it, is in discovering it, they had discovered it has coloration that makes it appear really rocky. It has moons that are super icy. And there’s an icy debris field nearby. And so, the thinking goes that this may be an object that went through one heck of a nasty collision, got spun up, so it assumed this flattened shape that it has. And the moons are essentially knocked-off chunks that used to be part of the object. It’s just a super cool object that looks like it had so many things in our solar system. A super violent past.
And he was doing all of this research and wanted to tell this full story all at once. And then got beat to the discovery announcement, which just hurts the soul because at the time this was the 10th planet. Now it was shortly after this, like 24 hours after this, that he’s like, “All right,” drops down the observations and says, “I have an object more massive than Pluto.” And again –
Fraser: Right. But do you know what?
Dr. Gay: – overshadowed.
Fraser: That’s – wait, wait, wait. That’s a perfect moment –
Dr. Gay: All right.
Fraser: – to go for another break.
Dr. Gay: All right.
Fraser: And then we’ll find out, what was that object?
Dr. Gay: All right.
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Fraser: And we’re back. What was that object?
Dr. Gay: All right. So, there is an object codenamed Xena, with a moon –
Dr. Gay: – that is codenamed Gabrielle, and it is of arguable radius. But it is thought – from our best data now – to be smaller in radius than Pluto, bigger than Pluto in mass. This means it’s a denser object, a rockier object, than Pluto. NASA originally called this object – which at the time of the announcement was not named officially – NASA called it the 10th planet in the solar system.
Dr. Gay: And with the discovery of Haumea, which was big, with the –
Fraser: Haumea Mea.
Dr. Gay: I’m determined to screw that one up.
Fraser: Yeah, you are. It’s funny.
Dr. Gay: The discovery of Haumea, with the discovery of this object that would go on to be called Eris, with this discovery of an object that would be called Eris, it set the IU into an interesting squabble. Where you had the naming committee for naming Kuiper Belt objects being like, “We shall name things after creator deities. Because that’s the rule.” So, for instance, when you look at Gonggong, and Makemake – and these are names that are supposed to be duplicated. And this is why I screw up Haumea. When you look at these –
Dr. Gay: – objects, they have names that are the creator deities of various peoples around the world. And Mike has worked to get the Hawaiian Island deities to get the native peoples, the Indigenous peoples of California’s deities, to reach out to all of these places that the astronomy has touched and use the names that are important to those peoples. But if an object is a planet instead of a Kuiper Belt object, you aren’t restricted to being a creator deity in how you name it.
Fraser: Right. Clever.
Dr. Gay: So, is it a dwarf planet? That term didn’t exist at the time. There were planets. There were Kuiper Belt objects. And so, the question was, which committee was in charge of the name?
Dr. Gay: And from arguing over which committee gets responsibilities, things trickled through to, “Let’s just define planet once and for all,” unfortunately.
Dr. Gay: Because the definition they came up with, hopefully, is not the definition for once and for all. But it’s that definition and the discovery of first Haumea and then of Eris, that has led to Mike Brown being forever known as the Pluto Killer.
Fraser: Right. And obviously that was our first episode. We revisited this idea a few weeks back, that this was the chain of events that led to Pluto’s loss of planethood. At this point though, how many of these things are there out there? If you include all of the moons that qualify. If you include all of the –
Dr. Gay: Oh, man.
Fraser: – dwarf planets, all of the larger objects in the Kuiper Belt, anything that’s reached hydrostatic equilibrium, how many objects are we looking at out there in the solar system?
Dr. Gay: We’re looking at tons. I don’t know the exact number at the moment, but the thing to remember is there are two moons bigger than Mercury.
Dr. Gay: And so, we are going essentially from things the size of the asteroid Vesta, kind of is like – well, not quite a circle. So, we’re gonna go a snirt bigger than Vesta and work our way all the way out through to Ganymede, which is bigger than Mercury. We’re gonna go past the size of Mercury and hopefully find things in the future with the sought-for planet 10 that is hopefully out there that we’re waiting for LSST for. So –
Dr. Gay: – tons of objects, this is part of why the IAU was like, “We don’t wanna have that many planets.” No. It’s a lame reason. But that was part of the reason.
Fraser: Yeah. Well, it’s funny. This whole issue has – it caused obviously such a PR crisis.
Dr. Gay: Yeah.
Fraser: Everyone was really mad that Pluto is not considered a planet anymore. But even we’ve taken to just calling these things worlds. Like, I no longer –
Dr. Gay: Yeah.
Fraser: – distinguish between planet, moon – I mean, I do. But in my mind they’re all worlds that – if none of the old words are gonna work, let’s just use another word. And –
Dr. Gay: Yeah.
Fraser: – there was an interesting proposal. We actually have an article in Universe Today about this that various people are coming together and saying, “Let’s just lose the concept of moon versus planet versus dwarf planet.” There’s so many –
Dr. Gay: As a –
Fraser: – of them and there’s –
Dr. Gay: – geological meaning.
Fraser: Exactly. Yeah.
Dr. Gay: I think it’s useful for dynamics. Like –
Fraser: This or with that.
Dr. Gay: – the fact – yeah – the fact that Eris has its moon Dysnomia. The fact that we have moons around Haumea and Makemake. And Pluto has so many. And Gonggong has the one. All right. Orcus. Salesia. Quaoar’s Weywot. I just love the name Weywot.
Fraser: Quaoar. Weywot. Yep.
Dr. Gay: Yeah. All these objects that are order of 500 to 1,500 kilometers across, all these objects that have moons, that the moon simply means they’re big enough to hold onto something. And so –
Dr. Gay: – dynamically, that word has meaning. And it’s cool to think that things so much smaller than the Earth’s moon have their own moons. And then asteroids have moons. Moon is a dynamical –
Dr. Gay: – nomenclature.
Fraser: And so –
Dr. Gay: I’m down with keeping moons.
Fraser: – in this paper they –
Dr. Gay: Yeah.
Fraser: – propose that anything above 500 kilometers qualifies as a planet. As a world. Whatever. Like, it’s reached hydrostatic equilibrium.
Dr. Gay: Yeah.
Fraser: Capable of having moons go around it. And that by that measurement, if you count 500 kilometers and above, there are more than 100 in the solar system. So, someone’s gonna have to come up with a brand-new version of my very excellent mother just served me and then –
Dr. Gay: No.
Fraser: – move on with the whole other paragraph to get through –
Dr. Gay: That’s a novel there.
Dr. Gay: Not literally a novel, but –
Dr. Gay: Now, in that number, they’re counting all of the asteroids and rocky things that side as well, aren’t they?
Fraser: Mm-hmm. Yeah. Yeah. Ceres.
Dr. Gay: Yeah.
Fraser: Ceres. As you said, Vesta’s a little too small. The moon Io. Ganymede. Etc.
Dr. Gay: Yeah. Yeah. It’s getting to the point that while there are tons of icy objects that are no-named and defined, once you start including all of the rocks, once you start looking at the theoretical models for all the things that we haven’t discovered yet that are predicted to be out there based on the amount of the sky that we have accurately observed, the parts of the sky that are just too messy to accurately observe yet, there’s gonna be a whole lot more found.
Fraser: Mm-hmm. Yep. Yeah. Yeah. I mean, here we are less than a year until Vera Rubin comes online, and it could double the number of objects that are out there.
Dr. Gay: Yeah. And what gets me is we have now found enough of these outer solar system objects that appear to be scattered disc objects, by and large were Kuiper Belt objects. We’ve gotten to the point that some of them are sufficiently boring that no one is particularly invested in making sure they have a name. It’s like there’s a whole lot of asteroids out there that no one has bothered to name because there’s just a lot of asteroids. Well, I was going through the list of all of the largest of the icy objects a month or two back and I was just like, “What did 2002 MS4 do so wrong –
Fraser: To not get a cool name.
Dr. Gay: – that no one has” – yeah. It has no name. It is unnamed.
Dr. Gay: It was discovered in 2002.
Dr. Gay: And so, I actually reached out on Twitter to Mike Brown and his response was basically, “Yeah. It’s too boring.” It’s just like I love that we’ve found enough of these that some of them are interesting and some of them we admit are boring at this point.
Fraser: That’s awesome. All right. Well, I think we’ve reached the end of this week’s show. What a wonderful solar system we happen to find ourselves in. Thank you Pamela.
Dr. Gay: Thank you. It’s my pleasure.
Fraser: All right. Do you have some names for us this week?
Dr. Gay: I do. As always, we are brought to you by you. This is a team effort. We have Beth in the background producing us today. Nancy, as always, keeping us in order. We have Rich doing the audio editing. Allie doing the video editing. So many people out there are moding. All of this takes your help. And I’m so glad that we’re able to pay our staff who do the thankless job of making Fraser and I look and sound good.
And those of you who are supporting us on Patreon are the ones we count on every month to allow us to pay them every other week. So, this week I wanna thank Moose and Deer, Gfour184, Schercm, Arthur Latz-Hall, Cernanski, Brent Kreinop, Steven Coffey, The Mysterious Mark, Gabriel Gauffin, Dustin A Ruoff, Andrew Stephenson, Kseniya Panfilenko, Planetar, Rachel Fry, Joe Wilkinson, Sean Freeman with Blixa the Cat, Antony Burgess, Elad Avron, Glenn McDavid, Dean, The Air Major, John Oiseth, Roland Warmerdam, Bart Flaherty, Claudia Mastroianni, Tim Gerrish, Articfox, Stephen Veit, and Birko Roland.
Thank you all so much. And I looked at the numbers over the weekend and roughly 1 in 50 to 60 people that are out there subscribing are actually part of our Patreon community. So –
Fraser: Thank you.
Dr. Gay: – if you’re one of those 49 not donating, you get a whole lot more cool content if you’re part of the Patreon community. So, consider joining at even just $1.00 a month and we’ll send you a lot more stuff.
Fraser: Awesome. All right. Thanks everyone.
Dr. Gay: Thank you.
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