Podcaster: Fraser Cain & Dr. Pamela Gay
Title: Astronomy Cast: Ep. 747: Rogue Planets
Organization: Astronomy Cast
Link: http://www.astronomycast.com
Description: Streamed live on Mar 10, 2025.
Most planets orbit stars. That’s the rule, right? Well, maybe not. In fact the vast majority of planets could be floating freely through the Milky Way. Today we’re gonna talk about rogue planets. Sometimes planets just go rogue. Let’s learn about planets living free from stars.
Bio: Fraser Cain is the publisher of Universe Today and Dr. Pamela Gay is a Senior Scientist at Planetary Science Institute and a Director of CosmoQuest. They team up to do Astronomy Cast, a weekly facts-based journey through the cosmos
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Transcript:
[Intro]
It’s the 365 Days of Astronomy podcast, coming in 3, 2, 1.
[Fraser Cain]
AstronomyCast, Episode 747, Rogue Planets. Welcome to AstronomyCast, our 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, I’m the 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 are you doing?
[Dr. Pamela Gay]
I am 90% better than I was when my bronchitis was at its worst, but I’m still going to cough, and I’m still very grateful for Rich, and any of you watching this live, it’s definitely a no-makeup kind of experience.
[Fraser Cain]
For both of us, yeah.
[Dr. Pamela Gay]
You had one hell of a week last week.
[Fraser Cain]
I really did. Yeah, so for people who know, we didn’t release an episode last week, and that was because I was too busy to record AstronomyCast, which is weird, because it doesn’t take much time. But yeah, my website went offline hard last week, and backups were failing, and we had to rebuild the website on a new server, and I decided to sort of live a dream that I’ve been planning for the last six months or so, and that is to remove all of the advertisements from the website and go 100% supported only by the patrons.
And because the ad industry is just in freefall, search engine traffic is being no longer delivered by Google because you get the AI slop at the top of the search results, and so it was destroying my business. And at the same time, we’ve gotten more and more patrons coming on board, and so I was able to make… I said, that’s it, I’m done.
So no ads on the website. I want something that’s just clean, that a teacher can feel great about showing off in their classroom, and we are 80% able to afford this with our current run rate of writers. And my hope is that over the next couple of months, as I sort of tell the story of what we’re doing, we’ll be able to fill that gap, and then we will be a completely independent space news reporting agency that isn’t concerned about advertisements or stats or Google or any of that.
And so if that’s exciting to you, if you want to support us, go to patreon.com.universe today, and we’re going to have more information about what actually happened over on my YouTube channel shortly, and I think hopefully give people a fairly dramatic understanding of what’s happening to the media industry from someone who is inside of it. And I’m the canary in the coal mine. I am the person who runs very lean, and yet it was unsustainable for me.
So hopefully we’ll get to this place that is sustainable forever, and I can employ people and keep going and report and don’t care about what happens outside of just making sure that we tell good space news stories. So I’m excited, I’m freaked out, but I’m excited.
[Dr. Pamela Gay]
And I’ve enjoyed watching just where you’ve gone. The images are big, the site is bold, and what you’re doing is amazing.
[Fraser Cain]
It’s clean. It’s really clean. There’s no ads, there’s no JavaScript, there’s no tracking, there’s no Google Analytics, there’s no sponsors, there’s nothing.
It’s just pure pictures and text, and it is one-tenth, each article is one-tenth the file size that it was before, because I’ve been able to remove all that and just focus on just the simplest HTML implementation. So it’s pretty cool. Well, most planets orbit stars.
That’s the rule, right? Well, maybe not. In fact, the vast majority of planets could be floating freely through the Milky Way.
Today, we’re going to talk about rogue planets. All right, so rogue planets. Yeah.
So I guess, what is a rogue planet?
[Dr. Pamela Gay]
So scientifically they get called, and I always call them rogue planets, so I have to look at this. They’re technically planetary mass objects, independent planetary mass objects, or free-floating planets.
[Fraser Cain]
Right. Free-floating planets is the term that I’m most familiar with. But I agree.
I like rogue planets. Although, that opens up the mistake of rouge planets, which I get in the comments where people are talking about that. And so, just to be clear, we’re talking about rogue planets, not rouge planets, which are not a thing.
[Dr. Pamela Gay]
They’re something I paint, but that doesn’t mean they’re real.
[Fraser Cain]
Yeah, maybe. Okay, so free-floating planets, rogue planets, and so what are they?
[Dr. Pamela Gay]
These are objects that are less than 13 times the mass of Jupiter. They’ve been found as small as the planet Earth, and they exist out between the stars. We have found them in two different scenarios.
They, of course, exist just hanging out in the galaxy. They were first discovered with microlensing. There was a group of them back in 2011 of 474 microlensing events.
Ten of them were deemed to be planetary mass objects.
[Fraser Cain]
And so, for people who don’t understand what a microlensing event is, what’s going on? How are astronomers making this observation?
[Dr. Pamela Gay]
So, teams look at dense populations of background stars. In this case, they were looking at the Magellanic Clouds, and as a object orbits around the Milky Way and passes in front of one of these distant stars, its gravity will cause light beams that were otherwise not directed at the planet Earth to get bent into our field of view. This is a gravitational lensing event.
And because it’s just a star or a planet that’s doing this gravitational lensing, it’s called a microgravitational lens or a microlensing event. So it’s just something small using its gravity to magnify light.
[Fraser Cain]
And so, like an astronomer will be watching a field of stars, and then one of those stars will brighten.
[Dr. Pamela Gay]
In a very characteristic way, that is a sharp brightening and a sharp decrease. And the amount of time and the amount that gets magnified is related to its mass and its orbital parameters.
[Fraser Cain]
Right. That’s incredible. So they’re not seeing the planet, they’re seeing this very well understood brightness curve of the star that is telling you that a, you know, the equivalent of like a giant glass lens floated in front of the star and focused its light at us for a second before drifting on past.
[Dr. Pamela Gay]
And we’ve seen examples of star does the microlensing, and then there’s a secondary, much smaller, different amount of time microlensing event that we attribute to planets.
[Fraser Cain]
Right. You’re getting the star and then the planet.
[Dr. Pamela Gay]
Yeah. And then they’ve been finding, like I said, that first 2011, it was 10 out of 474 events were driven by planets. And a number of these have been found since then.
This is how they figured out one of these objects is Earth-masked. We’re not going to be able to see a random Earth-masked object flying through the galaxy unless it’s very nearby. But it’s, it’s gravity can do things that we can see using background light.
[Fraser Cain]
So we, I did a story on rogue planets and how they did a survey for rogue planets using the NeoWise telescope. They did a survey for just looking for, for planets in the solar system and they couldn’t find anything Jupiter mass and so on out there in the outer solar system. So it leaves the possibility of, of Neptune sized objects, you know, that’s the planet nine possibility.
But they, they said like if there was a rogue planet, like we wouldn’t see it within about like a thousand astronomical units. So really until we just don’t have the capability right now to see objects planet sized floating relatively nearby the solar system, we need that, that microlensing strategy. And, and yet there is another way that these rogue planets have been found.
All right. So what’s the other way that we’ve found rogue planets?
[Dr. Pamela Gay]
So looking at star farming regions, particularly Orion in the infrared, where these are small hot objects, planets give off the bulk of their light in the infrared. We have found, I wasn’t part of this. You weren’t part of this, but astronomers have found all these different objects, particularly binary objects that they have called jumbos, which is Jupiter mass binary objects.
And what’s weird about these is they have velocities that are consistent with the velocities of the stars in the open clusters and star farming regions that they’re looking at. So here are these objects that aren’t orbiting stars that are clearly planets in star forming regions with star like velocities.
[Fraser Cain]
And those were first found by Hubble and then confirmed with these recent observations with web. And I mean, they found hundreds, they found hundreds in the, in the Saturn range, the Jupiter range. But as you said, all the way down to, um, I think it was like a sub Saturn into sort of the, the big Neptune range while those earth ones, those come from the gravitational lensing events.
But you know, there’s gotta be more of them further down the size regime because that’s sort of like how the, you know, the commonality of, of different mass objects work. So chances are we’ll find plenty more if we get more powerful telescopes. So you’ve got, and then you said these jumbos, right?
Like I forget the number, like 19% of something of these objects are orbiting one another. So you’ve got two Jupiters in orbit around each other, free floating in the Orion nebula, nowhere near a star.
[Dr. Pamela Gay]
Yeah. The initial survey of these found 9% of, of the free floating objects were actually two objects.
[Dr. Pamela Gay]
Yeah.
[Dr. Pamela Gay]
And, and so that also broke our minds. The, the original expectation was that these rogue planets were things that got yeeted out of solar systems via some sort of a three body interaction via some sort of, of violent incident that just was like, and now you fly away. Now the thing about these kinds of violent interactions is you then, as we’re used to seeing with stars, end up with something with a high velocity.
When we see high velocity stars, we know it’s something that got yeeted through a multi-star interaction. So when we start seeing binary systems, it’s like, how, how does that even happen? How did those not get torn apart?
[Fraser Cain]
Right. Like you can understand one getting kicked out, a single planet getting kicked out because it came too close. Like chances are tons of planets were shed out of the solar system in the early history and that that would contribute to, to the rogue planets out there.
But to get two in perfect gravitational balance as a binary object out there, that is a puzzling mystery.
[Dr. Pamela Gay]
Yeah. And, and then the majority of these other ones that we were finding, again, they had velocities consistent with the stellar velocities in these clusters. These were not high velocity planets.
So suddenly we had a really cool dynamical mystery and, and this let the theorists do cool stuff.
[Fraser Cain]
I mean, do we, do we have a comprehensive theory on, we do. Okay. All right.
I didn’t think we had, but I’m, I’m ready to hear it then.
[Dr. Pamela Gay]
It came out either last week or two weeks ago. It’s a brand new paper and, and it’s a, it came from the Chinese Academy using JWST data. And what they did was they modeled the interactions of circumstellar disks of systems that were flying near one another, just like galaxies might fly near one another.
And just like galaxies will form bridges of material as they sweep past each other in galaxy clusters, they found that circumstellar disks sweeping past each other at like three to four kilometers per second in these star forming regions will create bridges of material. And the dynamics of these bridges of material will cause two knots to form that will develop into two binary planets.
[Fraser Cain]
Wow. So, so you’ve, hold on, so you’ve got two star systems that move past each other. You get this bridge of material going between them and then that sort of turbulently makes two binary planets to be able to be extracted from this system and then they float off in their own direction.
[Dr. Pamela Gay]
And because the kinematics of it is you have two solar systems going past each other three to four kilometers per second, they both transfer momentum to the planetary system that forms these jumbos and they end up with velocities consistent with the motion of the stars in the cluster instead of being high velocity planets. So they were able to explain the binary formation. They were able to explain the velocities.
And of course they can also form singular planets through this mechanism and still get the right velocities. So it looks like there’s two different methods for these forming the, the two solar systems pass in the night and violence within one solar system.
[Fraser Cain]
And there was like a third, another theory that was like the alternative, which was that they just formed in place. That if you have enough material that for, that collapses down and you get enough failed brown dwarf, yeah, you get a failed brown dwarf, which is already a failed star. And, but the problem is how do you get enough of the heavier material to form something like that?
Like, and, and for it to be able to remain. So there’s one theory that I, that I’d reported on that I really liked and you know, I don’t think anybody’s really put a lot of credence to it, which is that they were actually bigger before maybe even stars and that the combined radiation of the Orion nebula sandblasted them down to jumbos.
[Dr. Pamela Gay]
And one of the arguments I saw in a different paper was you can also end up with the shock waves of supernovae blasting into each other. And in this shock wave of heavier mass material, higher atomic number material, you can end up with planets forming at these junctions of shock waves as well.
[Fraser Cain]
Yeah. But I think, you know, just seeing like, it shows you, it’s like this new discovery is made. You’re finding all of these planets.
Suddenly you’re finding that they come in binary pairs and then the theorists get to work and they try all the different ideas and then we will get to the scientific consensus. And so it, I think for a lot of people who are, you know, we’re here thinking about dark matter, dark energy, a hundred years, 50 years, 20 years after these, these observations were made, you’re not there for that back and forth as astronomers work their way through the, the, the troubleshooting tree. Right.
[Dr. Pamela Gay]
Yeah.
[Fraser Cain]
But, but, but with this one, you get to watch it in real time. You probably remember when these jumbles were announced just like two years ago. And yet here we are now they’re working their way through the possibilities.
They’re making more observations, they’re rejecting ones and doing the simulations and we will get to the scientific consensus of, of how they form. So then what would one of these rogue planets be like if you could fly your spaceship to one of these systems? What would you see?
[Dr. Pamela Gay]
Algae system. Yeah. Yeah.
So it in part depends entirely on the mass and the age. These are things that start out with the heat of formation, the heat of gas that gets compressed. And initially you can end up with a system that is warm enough through some combination of contraction and decay of radioactive materials inside of it, that it might even be warm enough to have very lazy life.
Now you’re not going to end up with anything that requires sunlight clearly, but the kind of life like we see down in the Mariana Trench could potentially exist if it evolved fast enough. And that’s the problem is this stuff isn’t getting regularly heated by a star. And in a star forming region like Orion, you do have all of these O and B stars that are massively heating things up, that they might flow in and out of the range of getting blasted by these OB stars, but you can’t count on it.
And once several million years have passed, you no longer have a dense open cluster. You have something that’s opening up more and more. Billions of years later, it’s no longer even a cluster.
And now you have a cold, probably dead world as those radiation decays run out and you’re left with a chunk of daughter atoms and coldness. But what about moons?
[Fraser Cain]
Do you think these rogue planets could have moons?
[Dr. Pamela Gay]
They could, depending on their formation mechanism. It looks like the jumbos could have moons. It looks like the ones that formed in shocked systems could have moons.
It’s unclear about the yeeted ones. That depends on the dynamics. But it’s possible.
And those moons are going to cool off even faster because small things cool faster as we all learn from making pies of various shapes.
[Fraser Cain]
So then there was, you know, we’ve talked about the Heyshen worlds and so one really interesting possibility is that if you do have like a world with a hydrogen atmosphere that is a moon of a gas giant, for example, then that delivers enough energy. Like if you think about Io, right? Imagine if you surrounded Io with water and an atmosphere, it would be warm.
It would be dark, but it would be warm. And so you could find this sweet spot where you’ve got this planet with enough internal heat that is able to keep its ocean warm. It’s got a thick hydrogen atmosphere that’s keeping its water liquid at the surface.
And you would have a world that is not receiving sunlight, but has a liquid surface. And so there’s some really interesting possibilities for these planets. Obviously, you know, not having sunlight is probably a deal killer in a lot of situations.
But, but still, I think there’s, you know, we are, you know, our imaginations start to open up as we think about this. Do we have a sense then of like how many are out there?
[Dr. Pamela Gay]
We do. I do want to add one thing to what you just said. You have to have multiple moons or the orbits will circulate over time.
You have to create an elliptical orbit to get those tidal forces going. Now, given the number of these that are potentially out there, I see space for that to exist depending on what paper you read. And I love the chaos of how many they do or don’t think are out there.
So for context, there are 100 to 400 billion stars in the Milky Way, depending on what paper you read. And there are anywhere from four times on the conservative end to a hundred thousand times on the, oh my God, planets and more planets than stars. The typical number I found was billions to trillions of rogue planets in the Milky Way.
[Fraser Cain]
So more than stars.
[Dr. Pamela Gay]
Yeah.
[Fraser Cain]
Yeah. Yeah. Yeah.
Yeah. And, and hold on. I want to just quickly personally was like, could that explain dark matter?
And the answer is no. Because when you consider, say the solar system, the sun is 99.89 or 99.8% the mass of the solar system. And so you could have thousands of other earths inside the solar system and it still wouldn’t be of much mass compared, you know, half more than half of that mass is Jupiter.
So you could add a lot more mass to the solar system or a lot more planets and it wouldn’t contribute to the mass of the solar system. It wouldn’t account for dark matter. So but I love this idea that when we look out into space and we think about the kind of interstellar gulfs that we would need to cross to go to the nearest star system, there could be hundreds or maybe thousands of rogue planets in the gaps in between us and that nearest star that there are, um, you know, if we can get the timing right, that there could be gas stations out there filling the void that we just weren’t even aware of 10 years ago.
[Dr. Pamela Gay]
You still have to steer for them. I mean, the thing is that, yeah, they can be out there, but it’s just like every time we send a probe out through the asteroid belt, we have to make sure we point carefully if we want to visit just one asteroid.
[Fraser Cain]
Yeah. You don’t have to avoid the asteroid, the asteroid belt, you have to aim for them.
[Dr. Pamela Gay]
Exactly. And, and so it’s, it’s going to be something similar to that. If we’re able to ever do a really solid census of like what’s out there between us and Alpha Centauri and other nearby-ish stars, sure, there could be some out there, but we’re going to have to aim for them.
And doing that census is so hard, but this is why we need to just keep looking up, keep doing projects like OGLE that are looking for all these microlensing events, keep building infrared telescopes, and hopefully we’ll turn up more and more as our telescopes get larger and larger over time.
[Fraser Cain]
And so can you foresee this time when we have, I mean, I wonder, like we probably won’t be able to directly observe them until our telescopes are monstrous.
[Dr. Pamela Gay]
Yeah. Yeah.
[Fraser Cain]
But, but we will, with this gravitational microlensing, we’ll build a better survey of what’s out there and around us.
[Dr. Pamela Gay]
Exactly. And, and knowing how many to look for starts justifying the funding. And that’s one of the frustrations is all of this, like so much.
Science advances along three different axes, human creativity, technology, and funding that goes into funding the humans and the technology. Right. And right now we’re looking at massive budget cuts to the National Science Foundation in the United States, potentially a 50% cut coming to NASA’s Science Mission Directorate funding.
And that slows down the advancement of science and ultimately reduces how many people choose to go into science and are able to stay in the field of science. So yeah, totally possible. Need more money.
[Fraser Cain]
In the US.
[Dr. Pamela Gay]
And, and sadly we are one of the major by dollars places that money is getting spent just because we’re big.
[Fraser Cain]
That’s, that’s interesting. You know, I think I’ll do some research into what is the state of space research around the world and how that compares.
[Dr. Pamela Gay]
Because like China- China’s defeating us right now.
[Fraser Cain]
Yeah. Like China’s just releasing an enormous amount of science.
[Dr. Pamela Gay]
Like now when I look through archive- China, China, China, the Chinese Academy of Science is doing amazing work.
[Fraser Cain]
Yeah. There’s a lot of Chinese based, you know, research and, but, but here in Canada and other places it’d be interesting to sort of sense how that, how that is starting to change. Because if the US is literally taking its eye off the ball in terms of space science, yeah, you’re going to see places like China and stuff fill that, fill that gap.
So it’d be interesting to sort of see if I can catch that transition in live. But anyway, we’re off topic now. Very cool.
One of my favorite topics, Pamela, rogue planets. Thank you so much.
[Dr. Pamela Gay]
I love this. And we were able to do this thanks to our patrons. This week I would like to thank the following humans.
Okay. This week I would like to thank Sergio Sanisivero, Bill Smith, Brett Mormon, Jarvis Earl, Slug, G. Caleb Sexton, Andrew Moore, Evil Melky, Breznik, Andrew Allen, Cody Ross or Rose rather, Brian Cook, Robbie the dog with the dot, Kate Sindretto, Helga Bjorkog, Stephen Veidt, Christian Magerholt, Andrew Palestra, Gerald Schweitzer, Zero Chill, Les Howard, Gordon Duis, Kim Baron, Katie Bairn, Masa Herleyu, Alex Cohen, Matt Rucker, Antosaur, Stephen Coffey, Michael Regan, Diane Philippon, Philip Walker, Sean Matz, Cooper, Sam Brooks and his mom, Jeff Wilson, Matthias Hayden, Kami Rassian, Scone, Glenn McDavid, Tim Garish, Robert Cordova, David Bogarty, John Thays, Christian Goulding, Frank Stewart, Time Lord Iroh, Jim of Everett.
Thank you all so very much and we will be here next week.
[Fraser Cain]
Thanks, everyone. See you then. Bye bye.
You’re listening to the 365 Days of Astronomy podcast.
365 Days of Astronomy
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