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Most of the exoplanets we’ve found are around stars, where they belong. But a few have been found free-floating in interstellar space. The evidence is growing that there are a lot of them out there, maybe even more than planets with stars. How do they form and how can we learn more about them?
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Show Notes
This episode discussed the recently discovered population of planets wandering interstellar space without a star. Here are the key points:
- Rogue Planets: These are planets that are not bound to any star and travel freely through interstellar space.
- Discovery:
- Originally found in the Orion Nebula in the early 2000s using the Hubble Space Telescope.
- Later identified through microlensing events, where the planet briefly magnifies the light of a background star as it passes in front of it.
- Formation: There are two main theories for how rogue planets form:
- Ejection: A star system interaction (like a close encounter with another star) can gravitationally fling planets out of their original system.
- Independent Formation: A collapsing cloud of gas and dust might directly form a free-floating planet without ever igniting a star at its core.
- Binary Rogue Planets: The recent discovery of binary rogue planets suggests they might have been ejected together from their parent system during a close encounter.
- Frequency:* Estimates range from 2 to 10,000 rogue planets per star in the Milky Way.
- Future telescopes like Roman Space Telescope might provide more accurate numbers.
- Habitability: The potential for life on rogue planets is渺茫 (miǎo máng) (渺茫 means渺茫 (miǎo máng) unlikely or渺茫 (miǎo máng) doubtful), but there might be some possibilities:
- Young rogue planets might have residual heat that could support life on moons tidally heated by the planet.
- Life could potentially exist in subsurface oceans on moons, similar to Europa and Enceladus.
- Challenges of Studying Rogue Planets:
- They are faint and difficult to observe directly.
- Microlensing events are rare and offer only a brief glimpse.
- Future Observations:
- James Webb Space Telescope (JWST) might be able to find young, hot rogue planets directly.
- Nancy Grace Roman Space Telescope could identify thousands of rogue planets through microlensing surveys.
- Vera Rubin Observatory might capture additional microlensing events for confirmation.
- Impact on Interstellar Travel: Rogue planets could potentially serve as:
- Refueling stops for future interstellar travel.
- Gravitational slingshot points for spacecraft acceleration.
Overall, rogue planets are a recently discovered population with many unknowns. Future telescopes and missions will hopefully provide more information about their abundance, characteristics, and potential for harboring life.
Transcript
Fraser Cain:
Join Patreon for an ad-free experience at patreon.com/astronomycast. Astronomy Cast Episode 721 Rogue Planets. Welcome to Astronomy Cast our weekly facts-based journey through the cosmos. We will help you understand not only what we know, but how we know what we know. I’m Fraser Cain. I’m the publisher of The Universe Today and 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 L. Gay:
I am doing well, although one of my lights took a mighty leap right before we went live. So, if you’re watching this on YouTube, thank you for being here and I apologize for my lighting. But you, how are you after your grand adventures?
Fraser Cain:
Oh, it’s great. We had an amazing time in Japan. It’s surprisingly affordable. The travel was effortless. The level of English there is incredible. I can’t think of anything that went wrong. I didn’t buy my rail pass in advance. So, we had to pay cash for all of the rails that we did. We spent a lot of time in front of machines punching in destinations and putting in exact change to be able to go where we wanted to go and it would have been a lot easier.
I think we saved money, but it was a pain. Apart from that, the food was amazing. The people were great. The accommodation was spotless. I can’t recommend traveling and visiting Japan now more highly. So, yeah, no, I can’t wait to go back actually. I’m sort of like already planning my next adventure back to Japan.
Dr. Pamela L. Gay:
All right. I need to go check airfares as soon as we’re off the stream.
Fraser Cain:
Okay. Most of the exoplanets that we found are around stars where they belong. However, a few have been found free-floating in interstellar space. The evidence is growing that there are a lot of them out there, maybe even more than planets with stars. How do they form? Also, how can we learn more about them? So, free-floating planets, rogue planets, what are these things?
Dr. Pamela L. Gay:
They are worlds that by one manner or another managed to not have a star. What’s cool is these isolated planetary-mass objects may sometimes, sometimes in just the right circumstances, form completely on their own without a star, which starts to confuse what a brown dwarf is even more so. Or they get ejected from their solar system by one of several different flinging, yeeting mechanisms.
Fraser Cain:
Okay, well, before we get into how they exist, let’s talk just about how we found them. So, how did we first discover the presence of these free-floating exoplanets?
Dr. Pamela L. Gay:
It turns out that if you look closely, you can see them. They were originally found in the Orion Star-forming region back in 2000. If you look very closely, crawling across this rich star-forming nebula you will start to see [inaudible] [00:04:11] of solar systems forming. You will see baby stars of every possible mass. As you go, you will actually start to see some things of non-stellar mass. This is how the very first ones were discovered by two separate teams that published not knowing anything about the other team back in 2000.
Fraser Cain:
In 2000.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
So, we are 24 years ago and these are observations made with the Hubble Space Telescope.
Dr. Pamela L. Gay:
The thing is that wasn’t the only way that they were found. They were also found in the 20-teens. I don’t know what we’re calling 2011.
Fraser Cain:
2010s. Yeah.
Dr. Pamela L. Gay:
2010s. I like 20-teens. It sounds good.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
There was also a group, a Japanese group from Osaka University that was trying to understand how much stuff is out there masquerading as dark matter. They were using microlensing to measure the presence of all of these otherwise non-luminous sources of mass. While they were doing their survey, they came across objects, just a handful of them, that so briefly magnified the light of a star, that the only way to explain the magnification was a planet-masked object causing the microlensing event to take place.
Fraser Cain:
This is similar to this process of gravitational microlensing where they’re finding exoplanets around a star. You get one star passing in front of another star. You get this distortion of the light and then further distortions due to the exoplanets that are following the star as they’re passing through. You can actually measure the size and the mass of these exoplanets. However, in the case of the rogue planets, the free-floating planets, you’re just watching a star, no star, and then suddenly it gets a distortion and then the distortion goes away.
Dr. Pamela L. Gay:
Very brief.
Fraser Cain:
Yet all the information that you need is in that distortion telling you the mass of the planet as it’s passing in front.
Dr. Pamela L. Gay:
So, this started with the Osaka University team led by Takashi Asami. A Polish team picked it up as well. Since then we’ve just seen team after team, we’ve seen things coming out of Ogle have gone looking, but with microlensing, the problem is you see the planet once and that’s it. That’s the only chance you’ve got.
Fraser Cain:
Yes.
Dr. Pamela L. Gay:
There’s no confirmation taking place.
Fraser Cain:
Right. There’s no follow on observations. Yeah. It’s over. Yeah. It’s kind of gone, but sort of back to that original Hubble discovery, Webb turned its eye on the same regions in the Orion Nebula. What did it find?
It found over 500 free-floating worlds of which roughly 9% are binary free-floating worlds.
Fraser Cain:
Okay.
Dr. Pamela L. Gay:
It is not something anyone predicted or expected. A recent publication may help us understand how the heck we get there. I have to give a shout-out to, and I apologize if I mispronounce your names, McCullen and Pearson for their pair of papers that are working very, very, very, very slowly through peer review. The discoverers of these binary objects put out a pair of papers in archive. They’re working their way through the peer review system. They are not peer-reviewed yet, but the paper explaining how the non-peer-reviewed results came to be is through peer review. So, we have a bit of a point of confusion here.
Fraser Cain:
All right. So, free-floating binary exoplanets. How?
Dr. Pamela L. Gay:
So, the thought is we have to take a step back first and look at one of the mechanisms that allows these free-floating planets to exist in the first place. So, if you have a tiny solar system or a big solar system, it really doesn’t matter, and it happily has planets going around and around its star and another star comes sweeping past. That star that goes sweeping past can gravitationally disrupt your happy little solar system and cause planets to fly away. Now the catch is if you have a star with two planets that are on the same side of the star, and when they’re on the same side of the star, so they’re gravitationally seeing each other, another star comes through. It’s possible that there’s two planets that are presumably going in the same direction and that passing star’s gravity can cause those two planets to hold on to each other and lose hold of both stars.
Fraser Cain:
Right.
Dr. Pamela L. Gay:
So, it’s a side-swipe that gravitationally disrupts the system and sends those two planets flying.
Fraser Cain:
Right, and that’s trying to explain. I mean, like if all we saw in the Orion Nebula were these single planets, then these side swipes would make a lot of sense. Not just like a side swipe, but also planet, you know, there’s a lot of mayhem that goes on inside a planetary system.
Dr. Pamela L. Gay:
Right.
Fraser Cain:
We know that we had this migration process. So, you know, who knows how many planets were kicked out of the solar system early on? Okay, fine. However, to see 9% of your exoplanets in binary pairs means that something happened to them together. That side swipe seems like a bit of a long shot that you’re gonna get 9% of the time two planets are gently plucked away from their star and sent hurtling off into interstellar space. That’s pretty out there.
Dr. Pamela L. Gay:
It’s the only published explanation we have so far.
Fraser Cain:
What’s the other mechanism, right? The other idea is that they just formed in place, that no star was ever involved.
Dr. Pamela L. Gay:
Right, exactly. So, yes, and this gets back to what is the core set of ways that you can get these things. So, rogue planets, in general, are thought to have either formed that way, to have been in a solar system, and been yeeted out through three-body interactions within the solar system. So, this is where interacting with some other world plus the star caused them to get ousted. Or sideswipes occur.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
So, either external force, internal multi-object dynamics causing yeeting, or they form externally. So, it’s also possible that you have these things forming independently in binary systems. What gets me is we only see big things right now. We only have the capacity to see these gas giant-sized objects. We’re not yet seeing free-floating rocky objects.
Fraser Cain:
Right.
Dr. Pamela L. Gay:
So, we don’t fully understand what is the frequency that these things occur to different-sized objects? What is the frequency at which different-sized objects can be free-floating? There’s so many things that we don’t know. What we do know is there’s this cool paper that just came out that is saying that you can, according to the numerical models, get two things gently removed if the side swipe is just right.
Fraser Cain:
Right, right. Also, there have been papers saying you can get a world that you can get if you have less gas… I mean, think about like a brown dwarf, right? You have a brown dwarf.
Dr. Pamela L. Gay:
Yes.
Fraser Cain:
You have less material. It doesn’t have enough material to form a star, but there is enough that comes together that you get this failed star. It might very well be that if you just have a smaller amount of material, you get maybe a Jupiter. Even less material, maybe you get a Saturn. However, you’re not gonna get the same mixture of heavier elements that you might get in a more traditional star system where you’ve got this larger gravity [inaudible] [00:13:00] is pulling material in. You have these rocky areas that are forming certain kinds of planets.
Dr. Pamela L. Gay:
Or, I think another way to look at it is in a solar system like ours, you have a massive quantity of mass that has sufficient amounts of the stuff and things that become rocky worlds that you can have rocky worlds.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
However, a single isolated cloud, even if it has the same ratio of materials because the majority of the material is hydrogen and helium, you aren’t going to get a rock out of a tiny cloud that is majority hydrogen and helium. You’re gonna get a gas giant that has heavier elements in the same frequency that they occur in our solar system just mixed around differently.
Fraser Cain:
Yeah. So, now we know these things exist. Do we have a sense of how many there are out there?
Dr. Pamela L. Gay:
I love the dispersion in the papers I’ve found so far. The general suggestion is there’s probably at least two per star. However, there could be tens of thousands.
Fraser Cain:
Yes. I know exactly what you’re talking about.
Dr. Pamela L. Gay:
How do you get it?
Fraser Cain:
Right. So, let’s imagine, right? Say there’s 100 billion stars in the Milky Way.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
So, there could be 200 billion rogue planets or maybe 10 trillion rogue planets moving around inside the solar system.
Dr. Pamela L. Gay:
Right.
Fraser Cain:
I’ve got to just stop right now and people are going to go, wait a minute, could that explain dark matter? The answer is no.
Dr. Pamela L. Gay:
No.
Fraser Cain:
Moving on. Yeah, it’s not enough. Their planets are in the solar system, rogue planets, or are 1% of the mass of the solar system.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
It is the sun that is the vast majority of the mass in the solar system. So, no, rogue planets. If they were dark matter, then they would be shining and we would see them and we would detect it. Also, they’re not 10 times the matter. So, sorry. But okay. So, let’s say that there is, like even double is exciting.
Dr. Pamela L. Gay:
Right.
Fraser Cain:
A thousand times more is incredibly exciting but what does that mean then for how far away these things are in the Milky Way?
Dr. Pamela L. Gay:
So, I think this means that we need to start looking more and more for newly discovered comets and asteroids that are on highly elliptical orbits. Are they alien in nature and I don’t mean little green men. I mean, did they come from other stars?
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
I think if these things do exist in those tens of thousands kinds of numbers, we should be seeing a lot more interloping rocks coming at us from the great beyond. That’s cool. That’s a way to get rocky samples from a solar nebula that wasn’t our solar nebula. I like that idea.
Fraser Cain:
Mm-hmm.
Dr. Pamela L. Gay:
It also raises the probability of an interloping planet coming flying through our solar system. I want to see that story very much, please, and thank you.
Fraser Cain:
Yeah. But stay far away. Like, don’t get too close.
Dr. Pamela L. Gay:
Yes. Yes. It also means that when we see these weird stars suddenly dim by X amount, we have new things to potentially blame. It means that there’s a whole lot of junk out there that we need to figure out how to incorporate into our models and that’s cool too.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
It’s mostly just a whole lot of, this is interesting data. If it’s only two per star, our models are fine. If it’s 10,000 per star, why haven’t they started interloping more? Did we just not notice?
Fraser Cain:
So, let’s talk about the potential for habitability. Could there be life on these planets?
Dr. Pamela L. Gay:
It depends on how old they are. So, when they first form, they’re going to be super-hot, just everything’s super-hot when it first forms. Over time, that internal heat is going to get radiated away. Over time, any radioactive materials in the mix are going to do the radioactive half-life thing and stop being radioactive and start being completely different child particles. Over time, all of these things are gonna just cool off to little frozen nuggets floating through the galaxy and beyond. It’s hard to have life without energy.
Fraser Cain:
Mm-hmm. But when we think about places like Jupiter with its icy moons and, you know, the heat for IO, like IO is the most molten place in the solar system and it is not coming from the sun.
Dr. Pamela L. Gay:
Yeah. That’s tidally heated.
Fraser Cain:
Right. So, could you get this situation where you’ve got a giant exoplanet and moons that are tidally interacting and are being kept warm in the same way that Europa, Enceladus, and so on. Yeah.
Dr. Pamela L. Gay:
Yes. So, then it starts to be, now we’re looking at the case of life with no light. There’s no reason to think that can’t happen because we have it at deep-sea hydrothermal vents here on the planet Earth. Yes. If we manage to form rogue planets that take their moons with them or form with moons and those moons have the needed constituent materials. So, here we’re looking at these minuscule clouds somehow having enough rock and water to form icy moons. Or to have stolen them when they got yeeted from where they came.
Fraser Cain:
Right.
Dr. Pamela L. Gay:
Yes, there is the potential. Now comes the question of is this the kind of environment, and we don’t know yet, capable of sustaining life?
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
It’s possible.
Fraser Cain:
You’ve also got the decay of radioactive elements, which are a big part of the Earth’s internal heat. So, you kind of add those together and there’s a chance.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
However, good luck learning anything about them because they’re far and there’s no star to eliminate them. So, really, my next question is how can we learn more? What can we do to find these things with more precision and learn more about them?
Dr. Pamela L. Gay:
The frustration is like, we know that the area of the Orion and the Trapezium star forming raised shins clusters are super, super dense. In that super, super dense environment, we’re going to find the rare objects. However, once they leave, we’re now looking for cooling objects, and JWST is going to be part of our ability to do that. But we’re just gonna have to hope that some of these objects are interloping in the fields we’re already looking at.
Fraser Cain:
Right.
Dr. Pamela L. Gay:
Directed searches for them. JWST isn’t a survey scope. I think our best hope is maybe we’re looking at other background objects within the plane of the Milky Way for some reason and there happens to be one in the field. That’s what we’re going to be looking for is looking at other objects within the Milky Way. What’s going on?
Fraser Cain:
There is a telescope that’s coming that’s gonna be doing exactly this. This is the Roman Space Telescope, Nancy Grace Roman. Its larger goal is to be due to this gigantic survey of the universe to try and map out the concentrations of dark matter, and dark energy. Also, understand the expansion rate of the universe, but it has this incredibly wide field of view. It’s looking for objects, and change in brightness across this field of view.
It’s theorized that it will find thousands, maybe tens of thousands of rogue planets through those microlensing events that you mentioned earlier on. So, we are gonna have a telescope that is going to find a lot of them and give us a number of how many there are out there. Give us a sense of their mass, you know, how massive are they? But it’s still one and done. They pass through in front of the star and then they’re gone and they’re no more observation allowed.
Dr. Pamela L. Gay:
However, Roman is working in the infrared. So, it also has the potential to do direct imaging of young ones. We’re going to have Roman at the same time that we have Vera Rubin. And with the LSST doing its survey thing across the sky, in the best of all possible worlds, we happen to catch some of these microlensing events with both scopes at the same time and get that confirmation.
Fraser Cain:
Yeah.
Dr. Pamela L. Gay:
More I think Roman, I’m really hoping Roman does some direct imaging. Then Rubin is going to be catching everything. It’s just that simple. Rubin is going to be catching everything.
Fraser Cain:
Yeah. There was a mission out a few years ago called WISE and it was doing an infrared survey of the area around the solar system.
Dr. Pamela L. Gay:
Mm-hmm.
Fraser Cain:
In fact, we know that there are no large planets in the outer solar system like no Jupiter-sized world out to about 10,000 astronomical units because of the survey that was done by the WISE telescope. It also found brown dwarfs, and things like that. So, we know that if planet nine exists out there, it has to be roughly like between Earth and say Neptune size.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
It can’t be bigger, because that’s been ruled out. However, there could very well be one of these rogue worlds. If they really are that dense, as we’re saying, then they’re going to be within tens of thousands of AUs of the sun. Like they’re out there around us, all around us. How, how does this sort of change your thoughts about exploring the Milky Way? Say we get to some far future where we’re able to fly to another star system. Are these the destinations, not the stars?
Dr. Pamela L. Gay:
So, it depends on what you’re going for at the end of the day. There is the possibility that you get on one of these that has a particularly high velocity so that you can cover a larger swath of the galaxy in a given number of generations. However, if you’re looking at a world that is either just a frozen gas giant or a frozen gas giant with perhaps some warmed-up moons around it that require massive amounts of drilling equipment to get down to any liquid water, that’s not the most energy-efficient destination you can go toward. I think that the idea of flying out to your [inaudible] [00:24:48] as starting points is going to, for the time being, remain our ultimate short-term destination. I do however really like the idea of taking the idea of Jules Verne’s off on a comet and turning it into off on a planet and setting up a future where this is how we explore the galaxy.
Fraser Cain:
Yeah. I love the idea that these rogue planets are all around us. They’re passing relatively close to the solar system. What would have been a thousand-year journey suddenly becomes a hundred-year journey that these could potentially be refueling stops.
Dr. Pamela L. Gay:
Yeah.
Fraser Cain:
Or even if we map them out well enough that they are gravitational slingshot points that you can use to accelerate your spacecraft to its final destination but it changes. When I started to learn more about these rogue planets, it started to feel like that revelation that you realize that you’re more bacteria than you are a person. That the galaxy has more rogue planets than regular planets. We just don’t see them because we only can see what’s around the stars.
Dr. Pamela L. Gay:
Again, maybe it might be two per star.
Fraser Cain:
Sure. Yeah. Merely two per star or it might be more. It might be less. Who knows? Like we’ve got, we’ve got a way for Roman to do a survey to really tell us the answer, but, but it still sets my imagination on fire. I just think it’s such a great idea and I would love to know more. Hopefully, in future missions, and telescopes we’ll know more. We’ll eventually get to a point where we’re –
Dr. Pamela L. Gay:
I want an interloper.
Fraser Cain:
Yeah, we’re analyzing the atmosphere, the weather.
Dr. Pamela L. Gay:
Imagine if a Mercury just suddenly flew through, like a rogue Mercury coming for a visit from another star.
Fraser Cain:
Yes.
Dr. Pamela L. Gay:
I’d be down for that.
Fraser Cain:
Yeah, that would be amazing. Awesome. All right, thanks, Pamela.