Ep. 615: Planet 9

After Pluto lost its planethood, we were down to 8 planets. But there’s growing evidence of another world (or worlds) out beyond the orbit of Pluto. Is Planet 9 out there and how will we find it? Could there even be a Planet 10?

Show Notes | Transcript

Show Notes

International Observe the Moon Night (NASA)

CosmoQuest Hangout-a-thon

What Is the Solar Cycle? (NASA)

Aurora (Swinburne University)

TWITTER: Fraser’s aurora picture

Aurora – 30 Minute Forecast (NOAA)

Space Weather App

AMAZON: Gravity chair

AMAZON: Camping poncho

Ep. 410: Planet 9 Facts and Fiction (Astronomy Cast)

Ep. 613: Pluto’s Demotion: 15 Years Later (Astronomy Cast)

Mike Brown (Caltech)

Konstantin Batygin

Scott Sheppard (Carnegie Science)

Chad Trujillo

Kuiper Belt (NASA)

Subaru Telescope

The orbit of Planet Nine (Brown & Batygin)

Vera Rubin Observatory

Dark Energy Survey

Giant Comet Found in Outer Solar System by Dark Energy Survey (NOIRLab)

Meet the 42: ESO images some of the biggest asteroids in our Solar System (ESO)

Dog-bone asteroid Kleopatra makes a fetching portrait (EarthSky)

htVery Large Telescope (ESO)


Evidence Indicates There’s Another Planet the Size of Mars in Our Solar System (Interesting Engineering)

Lucy (SwRI)

Saturn Has a Fuzzy Core, Too (Sky & Telescope)


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Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy Cast Episode 615, The Search for Planet Nine. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand 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. Hi, Pamela, how are you doing?

Dr. Gay:                      I am doing well. There is so much going on right now. The day after we record this, Saturday, it’s International Observe the Moon Night. Next weekend is Hangout-a-thon. Fall is here. Halloween. For those of you watching the video, Felix the Dragon is hanging out behind me celebrating October. Life is good. Life. Is. Good.

Fraser:                         So, I want to shamelessly self-promote the sun today, which is, of course, we’re entering – we’ve left the solar minimum. We are now driving towards the solar maximum. And that means auroras.

Dr. Gay:                      Oh.

Fraser:                         And so, we had an incredible aurora display that hit a big chunk of the Earth in the last couple of days from when we’re recording this. I was really lucky. I had surprisingly clear skies and could see auroras off on the horizon. Beautiful display I was able to catch on my phone, of all things. But people in New York State, people in sort of some of the middle latitudes were able to see it, as well.

                                    So, now is the time if you are interested, if you’ve never seen an aurora and you live, I would say, above the 35th parallel, then you have a good-ish chance a couple of times a year of being able to see an aurora. But you’ve gotta be prepared. So, get yourself acquainted with NOAA’s aurora alert. Find an app that you can put on your phone that’ll tell you when there’s aurora activity in your area.

                                    Have a plan for a place you can go where you have dark skies and a view towards the correct pole. In our case, we wanna be able to have a nice, dark view to the north. If you live in the Southern Hemisphere, you wanna a nice, dark view to the south. Plan this all out and then when the aurora activity is peaking – and you’re not gonna feel like doing this; you’re gonna feel like it’s cold and you don’t wanna go out – go out. If it’s clear skies, go see it. And –

Dr. Gay:                      This is the one time you don’t want a hammock. And any of you who’ve been watching for a while know I am a strong proponent of hammocks. But there are these really nice infinity chairs, zero-gravity chairs, whatever you wanna call them, they’re ones that let you lean back but not all the way. We will link to them in the show notes.

                                    Get yourself something that is extraordinarily comfortable and then get insulation for beneath you and above you. Prepare to mummify yourself. There are these amazing camping wraps that are like sleeping bags you can walk in. This is what you want. This time of year and March are the two times of year that you wanna go out and see this stuff. It’s the best alignments.

Fraser:                         Yeah. And you don’t – with a meteor shower, say, the Perseid meteor shower, you know that you’re gonna be seeing meteors every minute or so. And so, you don’t have to wait very long. But with auroras, you could go out and see nothing. And then you can go out again and see nothing. And then you can go out again and see just a faint hint on the horizon. But if you wait two hours, three hours, you could get one of these fantastic aurora displays where the auroras are just raining down on top of you, the sky is dancing from horizon to horizon. And it’s an absolutely magical experience that you’ll never forget.

                                    But you won’t see it unless you get out in it. And so, it takes some preparation and it takes some patience, and it takes a certain amount of just being willing to endure while you wait for the chance. But, I gotta say, when they happen, they’re like nothing else you’ve ever seen. So, if that’s on your bucket list, then now is the time to start preparing an action plan to take advantage of it.

                                    All right. Let’s get into the show. So, after Pluto lost its planethood, we were down to eight planets. But there’s growing evidence of another world or worlds out beyond the orbit of Pluto. Is Planet Nine out there? And how will we find it? Could there even be a Planet Ten?

                                    I guess we all know this. We talked about planet Pluto and its loss of planethood just a couple of episodes ago, a refresh. So, now, Planet Nine. And we have done an episode on Planet Nine. But give sort of the quick version of what Planet Nine is.

Dr. Gay:                      Planet Nine is a world that has been most publicized by the team of Michael Brown and Konstantin Batygin who looked at the orbits of a suite of Kuiper Belt objects that had super weird orbits. And they ran a bunch of models to see if there was a way to explain these orbits and realized that one of the best ways to explain them was with a – initially they calculated it out to be a 10 Earth-mass either Earth-like world or Neptune-like world that is hundreds of astronomical units, probably 200 to 300 astronomical units or times the distance of the Earth to the Sun away. And is just out there influencing all these smaller bodies to enter these weirdo orbits.

                                    Now, they weren’t the only ones that came up with this idea. And people often forget that even earlier, Scott Sheppard and Chad Trujillo had also come up with the idea of looking at a different set of screwed up orbits, the Kuiper Belt objects. They also postulated there might be something else out there, similar mass. They were using different objects so they got slightly different orbits. There’s some controversy there that I suspect we’re gonna get to, but –

Fraser:                         A fight. Yeah, I was gonna mention that. That, of course, their discover was met with incredible lack of skepticism, right? Their suggestion that these worlds were out there. Everyone just fell in line and got to work searching for them. No. No.

Dr. Gay:                      Well, they did get to work searching for them, I will give them that. This is one of those things where I think pretty much everyone is, like, “Eh? I wouldn’t bet money on them being right, but I would bet telescope time.” Because if this is right, it is so, so cool. And currently, there are teams using the Subaru telescope in Hawaii to frantically – well, frantically is the wrong word; they’d have more telescope time if it was frantic. To desperately –

Fraser:                         Desperately scurrying around, twirling dials, switching knobs.

Dr. Gay:                      Well, there’s a whole lot of not enough telescope time but a lot of emotional energy in what telescope time there is to going out and searching the regions of the sky that haven’t been eliminated yet as the possible home for a massive object.

Fraser:                         So, then, I’m just sort of ­– sort of think about what that evidence looks like and how sure are they that this object is out there?

Dr. Gay:                      So, when they first did – when Michael Brown and Konstantin Batygin first did their calculations, they came up with a 1 in 10,000 chance that what they were seeing were completely random alignments. But over the years, as more data has been acquired and as more effort has been taken to remove from the calculations any worlds that are being perturbed by Neptune, it has dropped to more like a 1 in 250 chance that this is just random.

                                    And 1 in 250 isn’t that unusual. But we want it to be there, not gonna lie. We want the world to be there.

Fraser:                         Yeah. And it does feel inevitable seeing an object like Pluto, Eris, Sedna, there’s a lot of these really big objects out there in the outer solar system. It seems inevitable that something bigger is lurking out there, either something that formed in place or something that was kicked out from the inner solar system through all the various interactions. We know that Jupiter hurled a lot of material out into the solar system. And so, it’s not surprising that something is out there.

                                    One in 250? I like those odds. And so, they updated – I know that Brown and Batygin updated their research fairly recently, just a couple of months ago, with a firmer estimate of not only what the odds are but even where to look.

Dr. Gay:                      Yes. And it turns out the object is probably both closer, brighter, smaller than originally thought. And if it’s out there, it was earlier on calculated that it was probably somewhere in the vicinity of the Orion constellation, but in this massive swath of sky. And the possible of area of sky has now been narrowed down.

                                    Unfortunately, this is an area of the sky that’s extraordinarily rich in background objects and background nebulosity and all sorts of things that makes it super hard to find a not-entirely-bright, very slow-moving object. And one of the things that Mike Brown has said that I deeply appreciate is we’ve probably already imaged this object if it’s out there. We just don’t know it because of the rate at which it’s moving. It’s just not going, “Hey, I’m a moving object.” It’s just hanging out there pretending to be a star.

Fraser:                         So, you described a bunch of objects of varying sizes and of varying distances. It seemed pretty vague. So, let’s talk about the hunt. You mentioned that it’s almost certain that there’s a photograph out there right now that contains an image of Planet Nine in it, and yet nobody has realized that that’s what they’ve got their hands on. Why is that? Why wouldn’t they know that they’ve gotten an image of Planet Nine?

Dr. Gay:                      Well, one night’s observations may not be enough, especially if you don’t have perfect seeing, to notice this object moving across the sky. So, it may end up that if you have several hours of observations that you happen to combine, that it might look a little bit elongated, maybe; probably not. And it’s that slow motion and the fact that researchers generally will only get a few nights consecutively to go searching that causes these issues. You have to have nights that are much more spread out to successfully go looking for these kinds of things.

Fraser:                         Right, right. So, the discovery of asteroids is all about time-domain astronomy. That you have to take pictures night after night after night and then flick between the images that you’re looking at to look for the object that is moving, bit by bit, through the field of view. And in many cases, yeah, an astronomer is absolutely taking an image of some field, some long-duration image, so they can look at a galaxy or study a supernova or whatever. But they’re not coming back a month later and doing it again and then comparing the two. Their telescope time is done and the telescope’s been gone.

                                    And so, they’re not set up to search for these kinds of objects. And you do need a very powerful telescope, like the All-Sky scanning telescopes like the – a lot of these automated telescopes are just not gonna be able to pick up an object this faint.

                                    So, let’s talk about the wonderful observatory that will probably be able to find it.

Dr. Gay:                      So, unfortunately, COVID has delayed this telescope, like so many things. The Large Synoptic Survey Telescope, which has been renamed the Vera Rubin Observatory, and it’s now doing a – LSST is the name of the survey they’re doing and I know longer know what the acronym stands for. But I know the telescope is awesome and nearing completion. We just need to be able to finish it. And once it’s open, once it has first light, once it’s able to hit its full cadence, it’s going to be imaging the entire visible sky every few nights.

                                    And while it’s a Southern Hemisphere telescope, it’s not so far south that it can’t see the parts of the sky that we’re interested in. And that means that it’s going to have the kind of cadence where it’s going to be able to find every fairly large Pluto and bigger object, and Neptune and another 10 AU or so out objects, and bigger objects to greater and greater distances, and shinier objects to greater and greater distances. And we’re finally gonna know what’s out there and be able to either find this object or prove which of the alternative hypotheses turned out to be the real reason that there isn’t actually another planet or two out there.

Fraser:                         We had a bit of a sneak preview of this. There’s a recent mission, or recent observation, called the Dark Energy Survey Camera. And they were looking for evidence of dark energy and trying to sense dark energy’s presence on the universe. They’re looking for supernova. Things change. And while they were at it, it also happens to be a terrific asteroid, Kuiper Belt, object hunter. And so, over six years of observations, they were able to observe a thousand Kuiper Belt objects, half of which were brand new, never seen before.

                                    And so, just as these missions come together and as they do longer observations, more and more of these objects turn up. And when you do turn one of the most powerful telescopes in the world on these targets, then you get a tremendous amount of detail. This is sort of a rabbit hole, totally unrelated, but did you see this incredible set of images of the 42 asteroids from the European Southern Observatory?

Dr. Gay:                      Yeah. It was just like they used some of the best telescopes in the world to look at space rocks and got some kind of amazing results. And the one that kind of made me sad is there’s a dog-bone shaped asteroid that looks –

Fraser:                         Kleopatra?

Dr. Gay:                      Yeah. It looks super like a dog bone in the Arecibo radar data. And it looks much less impressive directly imaged. And I was, like, “I want my dog bone asteroid.” So, it’s like the face on Mars. Sometimes too much resolution does you in with pareidolia.

Fraser:                         And so, if someone is able to find the location of Planet Nine, then the European Southern Observatory’s very large telescope will turn on it and deliver the best possible images that we have. Until James Webb flies in a couple of months.

                                    So, let’s talk about, then, Planet Ten.

Dr. Gay:                      Yeah. So, work that Kat Volk and others did, she was doing a lot of this as an early career researcher, looked at the distribution of Kuiper Belt objects – the fact that there’s a cliff in their distribution out somewhere around 60 AU, roughly – and did some modeling and realized there could be Mars-sized objects out there influencing these orbits, creating resonances. And what’s amazing, to me, at least, is with our current definition of planet, if you stick a Mars-sized object out into the Kuiper Belt, there’s a good chance it won’t be able to clear its orbit and won’t be technically a planet.

                                    And so, this gets into all sorts of neat semantic arguments of what do you do when you find a new Mars hanging out, unable to clear its orbit? And hopefully a discovery like that will be made and will reopen these debates.

Fraser:                         All right. Well, let’s have a mini-version of that debate in a second. But it’s time for another break.

Fraser:                         And we’re back. So, I guess what you’re saying here is if we find an object that is the size of Mars – so, bigger than Pluto – and yet it hasn’t cleared out its orbit, yet it’s spherical, orbits the sun, is it a planet?

Dr. Gay:                      Technically, no.

Fraser:                         Technically, no. Even if it’s the size of Neptune and still hasn’t cleared out its orbit? It’s a big –

Dr. Gay:                      Well, that’s where it gets squirrely.

Fraser:                         It’s a big space out there, yeah.

Dr. Gay:                      So, Neptune would probably count as a planet because it’s substantially larger than everything around it. But something the size of Mars isn’t orders of magnitude bigger than Pluto. And it’s that difference where Neptune hasn’t cleared its orbit. Its orbit contains Pluto, for crying out loud.

Fraser:                         Sure. But it dominates its orbit.

Dr. Gay:                      It dominates its orbit. So, Neptune counts as a planet. Jupiter shares its orbit with the Trojans that Lucy’s hopefully going to launch and go explore. And so, we have these exceptions where it’s, like, well, that counts as sufficiently large enough. And a Mars-sized object probably wouldn’t and would reopen this debate.

Fraser:                         Yeah. People would definitely – if there’s an object bigger than Mars, people will definitely call it a planet.

Dr. Gay:                      That’s my hope. I just want a geology-based definition, something that says if it’s capable of having differentiated crust, volcanism, cryovolcanism, active tectonics, that is a planet. Or a moon. And we call them both worlds. I want the word ‘world’ to come back.

Fraser:                         Sure. I just do not care. No part of me cares.

Dr. Gay:                      All right.

Fraser:                         Yeah. I have zero depths to give on this one. We’re never going back. I think that’s the thing that’s really important to understand. Yeah, we had nine planets because that’s what you grew up with. But not if you lived in the 1900’s. Then, you grew up with Ceres and Vesta and you had 12 planets. And then they went back down to eight. And then you got Pluto and then you had nine. And then Eris is the same size as Pluto, maybe bigger. Does that get to be a planet? What about Haumea? Makemake? It goes on and on and on.

Dr. Gay:                      I want Ceres back.

Fraser:                         Ceres could come back. And so, you’re never gonna get nine planets. And if you want Pluto to be a planet, no problem. You just then, also, have to memorize Ceres, Eris, Haumea, Makemake, Sedna, it goes on and on and on.

                                    So, then, you probably have to memorize 20 objects and have them.

Dr. Gay:                      That’s okay.

Fraser:                         My very excellent mummer, mother, just served –

Dr. Gay:                      You need a paragraph.

Fraser:                         Exactly. You’re gonna need a giant paragraph to remember all of your planets. Is that what you want? Is that what you want for your children?

Dr. Gay:                      I come from a nation with 50 states and 50 state capitols that we had to memorize.

Fraser:                         Right. So, you can handle – you have 50 slots in your brain for information.

Dr. Gay:                      Yeah.

Fraser:                         Okay. But, yeah. It doesn’t matter. It’s all just – each one is its own special world. And I hundred percent agree with you. Let’s shift to world. Because then Enceladus gets to be a world, Titan gets to be a world. Yeah. You’re on to something.

Dr. Gay:                      It’s an amazing set of geology out there. And I didn’t know I liked rocks until we started sending space probes out to find salty, briny volcanism on Ceres. It’s kind of awesome and we need to acknowledge that small things have dynamics, too.

Fraser:                         So, then, what does the future hold for this? I mean, assuming the – when Vera Rubin comes online, it’s not assuming; there’s no launch that needs to happen. It’s on a mountain, it’s on a nice, stable mountain in Chile –

Dr. Gay:                      And they can repair it as needed.

Fraser:                         They can repair it, yeah, yeah. So, we are just – and, it’s not long. Like a year, we’re about a year away from first light from Vera Rubin. So, –

Dr. Gay:                      Yeah. Once they can get everyone back on the mountain. So, there’s still the getting everyone back on the mountain.

Fraser:                         Right.

Dr. Gay:                      But yeah. It’s close.

Fraser:                         So, then, it’s gonna find Planet Nine. It’s gonna maybe find Planet Ten. But is there a lot of these things out there? What do you think are the limits of what Vera Rubin could find?

Dr. Gay:                      My opinion on that keeps shifting as we keep realizing how many worlds the known parts of the solar system have been clobbered. It’s been realized that both Jupiter and Saturn have slightly fluffy cores thanks to measurements of their mass distribution by orbiting spacecraft. And the way you get fluffy cores is you hit them really hard. We are pretty sure that Earth got its moon from a collision with a Mars-sized object, or possibly two collisions. Venus appears to have been inundated with these series of collisions to get it to its state of rotation and mass distribution.

                                    So, if we had all of these collisions going on, it means there was a whole lot of stuff going on and not all of these collisions had to lead to either the impactor being destroyed or the impactor being conglomerated into the surviving world. There are notions that maybe Venus experienced a series of glancing blows that just slowed that sucker’s rotation down.

                                    All of these collisions make me think there’s room for five, ten, we can’t imagine exactly smaller, rockier worlds. And possibly one or two Neptune-sized things out there. We know Neptune-sized objects are basically the primary planet found out there. And the dearth of them in our solar system is something confusing. So, maybe we’ll find them.

                                    When we look at forming solar systems with ALMA, they’re huge. We don’t know how big our planets could be out in the edges where we can’t see them yet.

Fraser:                         And so, I wonder could there be something Jupiter-sized out there? Could the solar system have captured an object from another star system? Or maybe a rogue planet or something like that? It’s hard to understand how big the Oort Cloud really is. It extends out almost two light years from the Earth. There could be a big object out in the Oort Cloud that we would never know about and never detect.

Dr. Gay:                      This is where missions like WISE are so important. One of the things I heard at a conference that just sort of hit me as, okay, you have changed my world view of our solar system is because it’s so hard to tell the distance to brown dwarfs, we could’ve detected that our solar system’s actually in a binary system with a brown dwarf and we just couldn’t measure the motions correctly. And if there’s a Jupiter at a reasonable distance, it probably would have been found by WISE if it was in the areas of the sky that WISE observed. There’s always the chance that something’s in a crazy orbit that, because it’s moving so slow, it was in a patch of the sky we’ve missed so far.

                                    But the universe likes to say, “I will take your ‘that’s impossible’ and prove you wrong.” And I’m no longer willing to say that’s impossible. I’m pretty sure it is a very low probability that we’re actually in an unknown binary system. I think it’s probably a very low probability that there is a Jupiter-mass object out there that WISE didn’t find. But Neptune and Mars-sized things? Sure, have at it. Those can be out there hiding.

Fraser:                         Yeah. So, just to put a pin in it, right, that idea of the WISE telescope, its job is to search for the kinds of cool, brown dwarf, large Jupiter objects in our nearby vicinity. That question was asked and answered thanks to WISE. And so, now we know there’s probably not some brown dwarf, giant rogue planet. But there could be a vast collection of objects that were too small, too cool for WISE. End school.

                                    All right. We’ve reached the end of this week’s episode. Thank you, Pamela. That was super fun. And I think, I hope, we do find many more objects. More is better.

Dr. Gay:                      And, hopefully, while we’re recording.

Fraser:                         Sure, yeah. Now, do you have any names this week?

Dr. Gay:                      I do. As always, I have to say thank you to all the people out there that make this show possible thanks to their sustained support through Patreon. Patreon really is the best way that you can help us out. And if you make an annual donation, paying for the entire year during this month of October, it will count towards our Hangout-a-thon goal.

                                    All right. So, this week I wanna thank Omar del Rivero, Cooper, Alex Raine, Kenneth Ryan, Dean McDaniel, Jeremy Kerwin, Don Mundis, EranSegev, Steven Shewalter, Karthik Venkatraman, Daniel Loosli, Shannon Humber, Frode Tennebø, Janelle Duncan, Bongman McBluntsmoke, J. Alex Anderson, Mark H. Widick, Marek Vydareny, Jim McGihon, John, Matt Rucker, Michelle Cullen, Abraham Cottrill, Anitusaur, Philip Grand, Mark Steven Rasnake, and Father Prax. Thank you all so much for being part of what allows us to put this show out.

Fraser:                         Thanks, everyone. And we’ll see you next week.

Dr. Gay:                      Bye-bye.

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