This week, we return to our starting point, where Astronomy Cast began: Pluto. 11 years on, we have a whole new appreciate for the dwarf planet Pluto. We’ve visited it, probed it and taken pictures. It’s time for an update.
We usually record Astronomy Cast every Friday at 1:00 pm PDT / 4:00 pm EDT/ 20:00 PM UTC. You can watch us live on here on, or the AstronomyCast YouTube page.
If you would like to support Astronomy Cast, please visit our page at Patreon here – We greatly appreciate your support!
If you would like to join the Weekly Space Hangout Crew, visit their site here and sign up. They’re a great team who can help you join our online discussions!

Download the show [MP3] | Jump to Shownotes | Jump to Transcript

This episode is sponsored by:Barkbox and Hello Fresh

Show Notes

Pluto info site at NASA
Education info for Pluto
New Horizons flyover of Pluto
Tombaugh Regio
Pluto’s Heart and other features
Moons of Pluto
Djanggawul Fossae


Transcription services provided by: GMR Transcription

Female Announcer: Today’s show is brought to you by Hello Fresh. Visit and use the Promo Code: Astronomy30 to save $30.00 off your first week of deliveries.
Fraser: Astronomy Cast, Episode 456: Pluto Revisited. Welcome to Astronomy Cast, a weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. Hey, Pamela, how’s it going?
Pamela: It’s going well. How’s it going with you, Fraser?
Fraser: Good. This is the first time we’ve seen each other virtually since seeing each other in reality in St. Louis.
Pamela: And that was kind of awesome with the exception of I did not see Corona. But you got to see it and everyone else got to see it, so I took one for the team.
Fraser: So, for those of you who have forgotten, we had a big live event in St. Louis. 135 of our closest friends joined us. Live concert. We got a chance to go down and watch the eclipse live in Carbondale. All of us saw it except for Pamela, who there was a big cloud obscuring it and she –
Pamela: Little cloud.
Fraser: Yeah. Took that bullet, so the rest of us could see it. So, thanks, Pamela. And so, I think now we’re hoping to maybe do some other kind of event, maybe, once a year, so stay tuned as we figure out what comes in the future. This week, we return to our starting point where Astronomy Cast began: Pluto. 11 years on, we have a whole new appreciation for the dwarf planet. We visited it, probed it, taken its pictures, it’s time for an update. Let’s just roll back the old Astronomy Cast memory, Pamela. 11 years ago, our first episode of Astronomy Cast – what did we talk about?
Pamela: We talked about how just a few days previously – well, several days previously, at the International Astronomical Union Meeting in Poland, there had been a vote that redefined what a planet is such that you can’t have planets outside of our solar system, so it really does need to be reworked. And in the process of redefining “planet”, well, Pluto lost its planethood.
Fraser: And of course, everybody dealt with it well and they’ve long forgotten about it now, 11 years later.
Pamela: Nope. This is one of those things where my husband was able to go out and he bought me a cute little solar system ruler for reasons I have no idea. He thought it was cute. And it had Pluto on it. I mean, you can still go out and get things that list all the planets, and Pluto is listed among the planets. And all of the other wannabe planets are left off because there is planet bias. I, for one, celebrate Ceres, the first extra planet that has since been forgotten, that it used to be a planet. So, there’s other worlds.
Fraser: Yeah, I still get yelled at whenever I mention that it’s a dwarf planet and not a planet anymore. And of course, my argument to people is we’ve got Eris, which is roughly the same size as Pluto, and it goes around the sun, so does it get to be a planet? If not –
Pamela: And Ceres. We have Ceres.
Fraser: Sure. But if not, why not, right? So, that means that you have 10 planets. You’re never going back to 9. Now you’ve got 10; you have Eris. Well, what about, as you say, Ceres? It’s roughly in the same scale, too, so it’s 11 planets. What about Haumea, and Makemake, and all these other ones? 10, 12, how many planets do you want? Are you okay with 13, 15? So, you can’t have 9.
Pamela: Well, you can just not with Pluto, and this is where we’re at is – the analogy that I use is if you were some very, very organizationally motivated aliens who were in the process of gathering up materials to redo your own solar system. And you came to our solar system with giant Tupperware containers, you would start by throwing all the gassy worlds into one container by kind, so Jupiter, Saturn, Uranus, Neptune, all in a bin. There you go.
Then you’d probably go and grab all the rocky worlds and throw them in a bin. And here things will probably get a little bit messy, because you either just grab the things going around the sun, so you have Mercury, Venus, Earth, Mars, but do you grab the moon? Do you grab some of the moons? Here it gets a little bit messy, but because you have moons in the mix, the mess probably gets settled out where Mercury, Venus, Earth, Mars get thrown in a bin, and then you grab all of the other spherical balls of worlds whether they be moons or asteroids or Kuiper Belt objects, and you throw all of them in a bin.
Then you stick all the stuff that’s gonna melt in a different bin, and you put it off to the side, so that when it makes a mess, it doesn’t make a mess where you worry about it. And then you throw all the asteroids and everything else in another bin. So, in that schema of cleaning the solar system, Pluto is in a bin with a bunch of moons and a bunch of non-planets.
Fraser: I’m fine with it not being a planet, but I’m also fine with anything – like Alan Stern made this proposal that anything spherical is a planet. I’m fine with that, too. I don’t care, I guess, is the thing. I don’t really mind because I really feel like the solar system has changed so much, that we’ve discovered so many new things, we’re discovering all these new objects that it is just influx all the time. The change is our only companion. Don’t panic. Everything’s gonna change again. Just embrace it.
Pamela: This week, Astronomy Cast has a new sponsor, and that new sponsor is Hello Fresh. My husband and I, we’ve used subscription boxes off and on across the years all those different times when it just got a little bit too busy around the house to make it to the Farmers’ Market, to make it to the grocery store, to make it to all those places you go to make sure you have high-quality meat and high-quality vegetables. With Hello Fresh, they actually just deliver everything to our door in not a whole lot of packaging, just enough to make sure everything is kept fresh, everything is kept separate, and everything is recyclable.
Now, I don’t cook a lot, and most of what I cook is stuff I learned from relatives, and so following recipes for me is always a questionable experience. But they made everything simple. I took on a recipe called Pork Tenderloin a la Orange. I don’t think I’ve eaten this before, but I created something I liked. It had kale, and pecans, and wild rice. It’s a summer salad with cranberries, and it was just tasty and good, and it only took me about 30 minutes, about the length of a podcast to pull everything together and, well, get everything on plates and served out, so I could get back to work.
So, if your schedule is as busy as mine, go ahead and give it a try. You can get $30.00 off your first week of deliveries if you go to and use the promo code: Astronomy30. It comes to your door, it’s packaged well, everything is fresh, and it’s easy to make. Even for the person who really doesn’t know how to cook. So, enjoy.
Fraser: But Pluto is amazing, and we have seen some wonderful things at Pluto thanks to the New Horizon spacecraft, so let’s less talk about how sad everybody is that it’s been deplaneted, and let’s talk about how wonderful a world it is and what we discovered.
Pamela: And it’s a world that is finally getting names so that we can talk about all these amazing things we’ve found using actual names. So, first of all, Pluto has heart. And this is one of those things that I think as New Horizons got closer and closer to this little icy world, we were all captivated by because it became visible fairly early on. We have named officially with the International Astronomical Union this heart-shaped, pale, nitrogen-ice plain, Sputnik Planitia, and this is one of the more awesome features because it was so unexpected. We were really expecting that Pluto would be this beat-up, icy, cratered thing, and instead we found this beautiful, smooth heart just staring at us from the equator.
Fraser: I put the image up for the people who are watching the live show. I really love the shape of this. This was unexpected to see these kinds of structures on the surface of Pluto. They were expected ices, but they weren’t expecting the way – so, when we’re looking at the heart and the surrounding area, what are we looking at?
Pamela: We’re still trying to figure that out.
Fraser: I thought we knew some stuff.
Pamela: Well, we know some stuff. The amount of stuff that we’ve learned since our first show in 2006 – heck, we’ve changed the number of moons that we know that Pluto has. But when it comes to looking at these surface features, what we know right now, or at least what we think we know right now, is that Sputnik Planitia is an icy nitrogen sea that has near it floating mountains of regular water ice, and around it there is this weird dark material that when you look at it just right, it looks like a whale. And this whale of dark material we think is probably some sort of organic material that may have gotten left behind when there was some sort of amazing collision between Charon and Pluto at some point in the past.
Fraser: That’s kind of amazing that they might have had interacted with one another – crashed into each other?
Pamela: It’s one of these really hard to model situations. So, when we look at Pluto, and Charon, and all the rest of the moons – and we do need to note when we first did this show, Pluto was kinda hanging out by itself with Charon, which did you realize that Charon was discovered in our lifetime because I did not –
Fraser: 1979.
Pamela: It was found in ’78, but given acknowledgement in ’79, and then finally names in ’85. But then its other moons were discovered since we started this show, so Cerberus was discovered in 2011, Styx was discovered in 2012, Nix and Hydra – well, okay. They were discovered in 2005, but we’re still learning about this system.
And so, all these moons were found and they’re all in residence, so when you look at how long it takes them to go around Pluto, you have between Charon, and Styx, and Nix, and Cerberus, and Hydra this amazing set of two to four to five to six residences of their period once you start taking into account some of the procession that’s in this system. So, everything is locked in place. It means it’s been there for long enough that all of these residences could settle in, and when you run the models to try and figure out what kind of energies could be involved, one of the first models that was run assumed that Charon must’ve formed much like Earth’s moon where many billion years ago, something came along, hit Pluto hard, kasplash, now you have the Charon system and Pluto together.
But the energies don’t work out. The orbits are too close, the mass is too similar, everything is too settled in for that to have been the case. Another model which seems to be working better – and we’re still figuring this stuff out – another model that seems to be working better has Charon and Pluto forming near one another, and then colliding into one another, sharing some material back and forth, splashing things up, partially melting, forming organics on their surfaces creating this dark whale on Pluto and the dark mortar, which is not the official name, on Charon. And it was a collision that partially melted things that may have slowed them down and locked them together gravitationally.
Fraser: And so, I think the news that you’re sort of meandering around is that the International Astronomical Union – what, yesterday? – officially accepted the 14 surface feature names on Pluto. And a lot of these names it had been sort of in use already, but got official acceptance.
Pamela: And many of the other names that we’re using still aren’t official, so the whale shaped dark feature on Pluto is – well, Cthulhu is part of its name, and that is not yet official.
Fraser: Should be.
Pamela: It should be and it fits the naming schema. What’s kind of awesome is the naming schema they have landed on celebrates both explorers where you have various features that are named after the first people to defeat Everest, and you have Tombaugh Regio for the discovery of Pluto, Sputnik Planitia to celebrate the first spacecraft to orbit the earth, but then you also have features that celebrate the first nations of Australia with – I’m gonna mispronounce this – Djanggawul Fossae, which is one of the regional gods for the aborigines.
So, we have all of these different things that are getting recognized and celebrated, and by including the underworld, there is the potential that maybe, maybe – I don’t know what kind of odds to place on this – maybe Cthulhu can actually become part of an actual map of Pluto someday.
Fraser: That would be awesome. So, yeah. You can see this announcement out there on the interwebs and both – do a search for the informal names because there’s some great names in there, but as you said, you’ve got Hillary Montes, you’ve got Tenzing Montes – those are the people that first went up Everest. You’ve got Virgil, Elliot, Burney, Tartarus Dorsa. So, there’s a bunch of really interesting names, and of course, the big wide area is the Sputnik Planitia.
And the fact that these features are named – but I think one of the things that was just so amazing was that how Pluto looked kind of like Earth in that you’ve got different kinds of features made of different kinds of substances. But here, we’ve got oceans made of water and mountains made of dirt, of rock, and on Pluto, you’ve got frozen oceans made of hydrocarbons and mountains made of ice.
Pamela: And we can’t totally explain all of this yet, which this is why we do science. We do science because we don’t know all the answers and we want to. And we just need the funding to send all the spacecraft out to see all the different things. And with Pluto, going into this, we thought this was a dead, boring world with a little bit of atmosphere, and we expected a whole lot of craters, and we expected it to be solid all the way through, and we now think we were completely wrong. We’re finding this nitrogen-rich atmosphere that the sun shines blue through. We’re finding that there’s probably a liquid water or other liquid ocean that may be as much 100 kilometers deep. Our ideas were wrong, and this excites us to no end because being wrong for lack of data means, well, we just guessed wrong – and it’s okay to guess wrong now and then.
And the actual universe just keeps being far more exciting that anything our silly human brains could imagine without enough data. About a year ago, I was driving home and saw this large, fluffy, fluffy puppy on the side of the road playing in a ditch. And I couldn’t let that stand, so I pulled over, and I caught him, and I brought him home, and I cleaned him up, and when no one claimed him, I named him Eddie. And a few days later, I saw an ad on Facebook come up for Bark Box. And I haven’t had a young dog in a long time, and I was like, “Okay. I’ve gotta give this a try because this dog needs toys, this dog need treats, this dog has all the energy in the world,” and he still has all the energy in the world a year later.
And we have been getting monthly Bark Box for an entire year, and our house is filled with happy, little, tiny toys which haven’t fallen apart, which haven’t been destroyed, and I don’t think it’s just because my dog is gentle because my dog has destroyed other things. I think it is because Bark Box toys stand up to your typical Australian Shepherd. And the treats – we have had all manner of treats. We have had Booberry treats from Halloween, we have had banana treats in a Safari-themed box, and they’re all-natural, and they’re made in the U.S.A. or Canada. And my dog has loved everything he’s gotten, and it’s gotten to the point that he now keeps his eye out for the small, rectangular cardboard box that he knows I’m gonna let him open.
And the joy that Eddie and I have each month, you could have this, too, because Bark Box – they’re one of our sponsors for Astronomy Cast and you can get a free month of Bark Box with your subscription to either a 6-month or 12-month plan. Just go to and, well, make your dog happy.
Fraser: Was it earlier this year? Actually, Alan Stern has started to shake the trees to get another mission to Pluto. Maybe this time orbit her or land her, which would just be amazing to be able to get back to Pluto and this time land, study the surface from close – can you imagine a Rover zipping around the surface of Pluto and some of these features?
Pamela: I don’t think you zip on ice. That just seems like a really terrible thing to try.
Fraser: Crunching carefully and delicately up these ice mountains.
Pamela: Yeah, I want to send something that is essentially covered in suction cups. I don’t know how else to send something to an icy world that has mountains the same size as the American Rockies on a world that is so much tinier. Just think of the scale heights on this. And it’s smooth ice. This is fresh ice. We see what appears to be convective cells from the ice continuing to be on the move, continuing to change, and I’m just a little bit excited about how awesome this little world is. And I have to admit, my favorite part of all of the discoveries that New Horizons made is actually utterly ridiculous. And my favorite discovery is the surface of Pluto is younger than the amount of time that we have had honeybees evolved on the planet Earth.
Fraser: That’s awesome. I mean, the thing that’s quite funny is the data from New Horizons only recently finally arrived back on Earth. It took the better part of two years to slowly make its way back at this really slow bit rate communication, and so now – I know that you were at the last planetary meeting in Texas, and they had made a bunch of announcements, and so they’re still kind of trickling out the announcements of the things that they’re finding on Pluto. So, what are some of the other things that they’ve discovered on Pluto recently?
Pamela: Oh, man. Where to start? Well, first of all, the atmosphere that it has a variety of different layers to it, and it actually is set up in such a way – we refer to this scientifically as inversion layers – such that the atmosphere away from the surface is warmer than the surface, and that’s just kinda cool, and weird, and awesome. There are areas that are ancient that are covered in craters, so the question then becomes how is it that you end up with this region over here that clearly isn’t getting resurfaced while you have these areas over here that clearly have been completely flooded in and have different materials, and trying to decouple all these processes is something we’re still trying to figure out how to do.
And the different ways that they’re approaching this is they’re trying to build computer models where either it gets hit – collision – whether it be some other object or Charon where the north/south poles are, change such that the axis of rotation today is radically different compared to the surface from what it was in the past. And we’re at this stage of throwing stuff into computer models and seeing which model comes out matching reality the best.
We haven’t even had enough time for a dissertation student to complete a dissertation working on this data, so I think it’s gonna be two or three sets of Division of Planetary Science, and Lunar and Planetary Science, and European Planetary Science meetings where everyone gets together, compares their models, competes for publications, moves on, reiterates – it’s gonna be a few cycles of this before we start to say with certainty that whale of a formation, whether it be called Cthulhu Regio or not, is covered in organics that are fresh versus are old. It’s gonna take us a while to figure these things out.
Fraser: And you mentioned the moons that we already knew about thanks to Hubble and previous observations. And one of the big surprises is that there weren’t any other moons discovered, like, they’ve all been found, and that was unexpected.
Pamela: That was completely unexpected. It was a serious concern from 2012 forward that as they got closer and closer to Pluto, they might begin to resolve some sort of a dust-ring, some sort of a debris-ring. The worry was that with five moons, why wouldn’t there be more? Why couldn’t there be more?
Fraser: Little ones.
Pamela: Spacecraft destroying ones.
Fraser: Yeah, they were pretty concerned just about the path that they had to take through the system. They were ready to make changes to New Horizons’ trajectory to avoid moons if it started to discover them as it got closer.
Pamela: And it just worked out that we were able to find all of them with Hubble and other ground-based data well ahead of time. And we don’t have good data on all of them yet – Little Styx was way too far away from the spacecraft and way too tiny to get good data. But we’re now starting to realize, well, Hydra is a lumpy puzzle-piece, and amusingly enough, Cerberus looks like a dog bone and Nix is your standard potato. Styx, we don’t know. It’s a blob a few pixels across, but hey, it’s more than just a dot of light now.
So, we’re starting to put together our solar system. We’re starting to realize based on how shiny these things are that they’re probably waterized in a large part, and well, the more you know, the better the models, the better you understand, and the more excited we are to science. So, science.
Fraser: Well, the thinking I know right now is that the age of these moons are all the same. That they’ve done a bunch of crater counting and determined that all of the craters were all sort of – that allows you to figure out the age of the object and that these moons are roughly all formed together in some just gigantic collision that happened a long time ago.
Pamela: And getting everything to be in these nice, near residences that they’re currently in with their exactly 18 to 22 to 33 which rounds down to 3 to 4 to 5 to 6 when you get all of the moons engaged to get everything in residence takes time. Things may not start there and they get gravitationally put there, so this is kinda cool. Now, another interesting thing that we’re looking at is unlike some of Jupiter’s moons that are in residences and tidally locked, this system isn’t tidally locked, so we were able to get different views, different sides, catch the rotations, get hints at the 3-dimensional shape of these little moons. It’s only hints because, again, only a few pixels across, and our spacecraft, New Horizons, flew through way too quickly, but it’s data we didn’t have.
Fraser: Were there questions that you think the scientists wanted to get to the bottom of that they weren’t able to find them? I guess with the lack of moons, but were there other things that people were hoping to see or hoping to find that they didn’t find?
Pamela: I think the entire “Oh, wow, Pluto’s smooth. Huh. That’s new,” that was pretty much the initial set of happy expletives that came out of most of our mouths because when you are expecting something to be completely cratered, and you have your crater counters on board to spit out quick estimates of ages of the surfaces, and you initially don’t see any craters. It kinda completely changes your plan off attack, so it’s no longer “We didn’t get the data we are hoping for to study this thing that we’re planning to study. It’s the that thing we were planning to study, yeah, that wasn’t actually a thing.
Fraser: Right. Like, as you said, that gigantic area is the largest glacier in the solar system. So, nobody was anticipating to find the biggest glacier in the solar system on Pluto. Once they did, then they had to get to work to figure this out.
Pamela: And suddenly, we’re not talking about hydrodynamics instead of solid-body physics, and everything gets much more different. And what was, I think, also remarkable to a lot of us was just how different Pluto and Charon are because Charon is this completely beat-up world. It has these amazing cracks and valley that kinda make Mars look smooth, and that’s hard to do.
And the discoloration, the difference in color between these two worlds – it’s truly remarkable where they both do have these dark, organicy, we think, looking patches, but then all of Charon is pretty dark. And it’s just wrapped in all different directions with these massive features that just wrap around his fault lines and show us how the surfaces cracked and collapsed over the ages.
Fraser: And now, of course, New Horizons is still going. It’s gonna be reaching its next target in 2019, I think.
Pamela: And they’re thinking that one of their future targets, if they’re fully approved even beyond that one, they will get closer to it then to anything else they’ve been, and will get even higher resolution images, which is awesome.
Fraser: So, the object is called –
Pamela: They all have license plate numbers.
Fraser: Yeah, yeah. 2014 MU69 is the next object, and it’s gonna get three times closer to this than it did to Pluto, and image it. Because now, I mean, it’s completed its main part of its mission. You can get really close, you could take some risks with this [inaudible] [00:31:07].
Pamela: Exactly. So, is MU69 a different number for the same thing you just said? It says New Year’s Day 2019, New Horizons is visiting MU69, but many things have multiple license plates.
Fraser: [inaudible] 2014 MU – 2014 is when it was discovered.
Pamela: Okay, yes.
Fraser: Yeah, sorry. That’s its license plate.
Pamela: And so, it’s going to be able to see features that are simply tens of meters across and that’s kind of awesome.
Fraser: So, I guess we’ve got an update on what’s happening with Pluto. Now we’re gonna wait for some more science to come out, some more planetary meetings from more discoveries to be made in all of this New Horizons science. And then we need to have that next mission, we need to have that lander, that Rover, or the ice skater. Cross-country skiing probe.
Pamela: I want suction cups. I wanna be able to climb up those mountains in low gravity without accidentally bouncing into outer space.
Fraser: Alright, well, we’ll pass along your suggestions to Dr. Stern. I’m sure he’s – he’ll – he can’t wait to incorporate suction cup mountain climbers.
Pamela: Yeah, no one is building that. I want it, no one is building that. So, yeah. It’s amazing how science has changed in the years we’ve been doing this, and here’s to hoping that in another 10 years, we’re coming back and there’s another spacecraft that is nearing Pluto once again.
Fraser: Awesome. I can’t wait. Thanks, Pamela.
Pamela: My pleasure.
Male announcer: Thank you for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain and Dr. Pamela Gay. You can find show notes and transcripts for every episode on You can email us at, tweet us @AstronomyCast, like us on Facebook, or circle us on Google+. We record our show live on YouTube every Friday at 1:30 p.m. Pacific, 4:30 p.m. Eastern, or 20:30 GMT. If you miss the live event, you can always catch up over at or on our YouTube page.
To subscribe to the show, on your pod-catching software, add or subscribe directly from iTunes. If you would like to listen to the full, unedited episode including the live viewer’s questions and answers, you can subscribe to Astronomy Our music is provided by Travis Searle, and the show was edited by Chad Weber. This episode of Astronomy Cast was made possible thanks to donations by people like you. Please give by going to
Duration: 34 minutes

Download the show [MP3] | Jump to Shownotes | Jump to Transcript


Submit a Comment

Your email address will not be published.