Moons orbit planets, planets orbit stars, stars orbit within galaxies. It’s orbits all the way down. But occasionally objects can receive a powerful kick that sends them off on a journey, never to return.
Superman (DC Comics)
Iron Man (Tony Stark) (Marvel)
Conic Sections (Math is Fun)
Parabola (Math is Fun)
Hyperbola (Math is Fun)
Ellipse (Math is Fun)
Ballistic Trajectory (Universe Today)
FAQ – Earth (Planetary Science Institute)
Escape Velocity (Let’s Talk Science)
The Three-Body Problem (Scientific American)
PODCAST: Ep 102: Gravity (Astronomy Cast)
In Depth | Oort Cloud (NASA)
Overview | Comets (NASA)
Types of Comet (CometWatch)
In Depth | Oumuamua (NASA)
Ukrainian Astronomers Discover ‘Exocomets’ around Another Star (Scientific American)
Slingshot Star? (Science Magazine)
JOURNAL: A search for runaway stars in 12 Galactic supernova remnants (Astronomical Notes)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast episode 660, Runaways. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain. I’m the publisher of Universe Today. I’ve been a space and astronomy journalist for over 20 years. With me is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela. How you doing?
Pamela: I am doing well. If I don’t sound the way I normally sound or look the way I normally look, I still haven’t recovered where all my stuff is after I was in Vegas for Annie Wilson’s wedding [inaudible] [00:01:29] one of our community members and one of my good friends. Unfortunately, during the travels, several people got COVID, so there was a come home, dump all belongings, get the phone call that people had tested positive, quarantine, and then chaos ensued.
Fraser: But you didn’t.
Pamela: I did not get sick. I had a random sore throat, which I’m gonna blame on things I was allergic to. I escaped.
Pamela: So, folks, wear your masks. I wore my K95 like my life depended on it because it probably did. Yeah, so I managed to stay safe. I just haven’t found my microphone or my webcam. But congratulations to the beautiful couple. It was a fabulous ceremony. And everyone is now home.
Fraser: Good. Well, hopefully by next week, you will have started to put your life back together.
Pamela: That’s the goal.
Fraser: Moons orbit planets. Planets orbit stars. Stars orbit within galaxies. It’s orbits all the way down. But occasionally, objects can receive a powerful kick that sends them on a journey never to return. So, let’s talk about orbits first. Let’s say that you are on the surface of Earth and Superman is going to punch you really hard. Describe the —
Pamela: That would be so bad.
Fraser: Yes, it would. Now, describe the trajectories, here, that we could be looking at if Superman gives you a punch.
Pamela: The pause here is the structural integrity of the human body is such that I’m not sure there’d so much be a trajectory –
Fraser: He’s punching another Superman.
Pamela: He’s punching Iron Man with the suits. Let’s go with –
Fraser: Yeah, he’s punching the Hulk.
Pamela: Let’s go with Iron Man because I have faith in that steel suit.
Fraser: Okay, sure. Superman’s punching Iron Man.
Pamela: So, Iron man is going to travel on what’s called a comic section. So, imagine, if you will, a round cone, like good old-fashioned ice cream cone, and you can cut a plane through that and in a bunch of different ways. If you cut it just right, you’ll get a perfect circle. And depending on how you do it, you may end up with a not entirely closed surface. So, you can end up with what are called a parabola, a hyperbola, or an ellipse. Now, if –
Fraser: Or a circle, which is, I guess, a type of ellipse.
Pamela: Is a type of ellipse. So, ellipses are defined as a nice closed shape that has two foci that, if you tie a string to both of them and you move that string around the foci, you’ll end up with the ellipse. And depending on how far apart the foci are, you get a flatter and flatter ellipse. And if those two foci are in the exact same point, you get a circle.
Fraser: Isn’t it foci? Maybe it’s foci, hmm. Right. So, maybe it’s Canadian just versus American English. So, Superman gives him a punch. Not very hard. Just a light tap. He goes up. He comes back down.
Pamela: So, that’s a ballistic trajectory. Now –
Fraser: And it is a parabola?
Pamela: Well, you don’t really have enough information to tell because he collided with the surface of the object. Now, it’s entirely possible to have a perfectly healthy elliptical orbit that just happens to intersect the surface of a planet, which would be unfortunate. Except, in this case, Iron Man gets to keep breathing air that doesn’t have to come from his suit. So, that’s ballistic.
Fraser: Right. I guess you chose Iran Man for a reason here. And we know that to go into orbit around the Earth is 7.6 kilometers per second, 20,000 kilometers per hour. If Superman punched –
Pamela: Mostly sideways.
Fraser: Yes. If Superman punched Iron Man that fast, he still wouldn’t be in orbit.
Pamela: Yeah, it all depends on the exact angle of the punch. So, if Superman punches him straight up so that there is a line through the center of mass of the Earth that extends straight through the surface of the planet, through Superman’s fist, and sends Iron Man straight up, that kind of a punch is going to lead to a very skinny ellipse, so skinny, in fact, that it is a straight line and, again, a ballistic trajectory. And Iron Man is going to hit the surface of the planet on his way back down –
Fraser: Right. He’s gonna go up.
Pamela: – hitting Super Man if Super Man doesn’t move.
Fraser: He’s gonna come back down. Even though he’s punched him into orbital velocity speed, he’s not gonna be able to go into orbital velocity. Even if he punches him at an angle, he’s just always gonna be taking these parabolic ballistic trajectories and coming back down to the surface. So, how can Superman make sure that Iron Man doesn’t return to hitting the Earth again?
Pamela: He has to hit him sideways. So, if he hits him so that you have the surface of the planet, you have Iron Man on the pedestal off the surface of the planet, and –
Fraser: No, he’s on the surface. He’s just standing on the surface. Just comes up and punches him.
Pamela: Okay, so if he just plain punches him, he’s going to have to hit him hard enough that he goes into an elliptical orbit that has him going off the surface, coming back, grazing the surface of the planet – may there not be mountains on the other side – and then coming back, and he hits Super Man again. So, no –
Fraser: But if he punches him a million kilometers an hour, he’s just gonna escape velocity. He’s not coming back. That’s where I’m going with this.
Pamela: Okay, yes. He can escape if he hits him hard enough, yes.
Fraser: Right. So, you’re gonna have all of these various punches that are gonna lead to various kinds of ballistic trajectories. He won’t go into orbit.
Pamela: Iron Man is fine.
Fraser: Right. Iron Man’s fine. He’s fine. He can’t go into orbit from these punches, right? He can only either return to hit the Earth again or escape the Earth entirely. So, now, Superman punches him and then flies up, because he can, and punches him again when he’s in space. What could he do with him now?
Pamela: So, with that double firing, it is possible to put Iron Man in orbit. So, the trick is you need Iron Man to get to a certain height and then stay at that height or at least stay away from the surface. And this is where I started to say had him on a pedestal because, if you had him on a pedestal, you could just punch and he continues going around the planet at the height of the pedestal if you have exactly the right punch. If you don’t have that pedestal, you have to go up from the surface, and then you have to turn, and that turn is what allows you to keep going around the planet instead of falling back down to the surface.
Fraser: Right. And so now that you are in orbit – Iron Man is in orbit. He’s fine. At every point, he’s perfectly fine. They’re just having fun. He’s got all kinds of inertial dampeners on board. He can handle this an infinite amount. Every time Superman gives him a punch, it’s going to change his orbit around the Earth, but it’s not going to cause him to, say, spiral outwards or spiral inward unless he goes back to that –
Pamela: He could.
Fraser: How could it be a spiral with one hit?
Pamela: No, it’s gonna have to be repeated hits to be a spiral.
Fraser: Right. Yes, yeah. It’s gonna have to be repeated hits. All right. So, then, Iron Man is orbiting around the Earth. Superman comes up, punches him in the direction opposite to his orbit. What happens to him?
Pamela: So, if you decelerate, you’re going to – this is where it gets [inaudible] [00:11:00], and the astronauts –
Fraser: Let’s say the exact amount of his orbital velocity.
Pamela: The exact amount of his orbital velocity, then he’s going to go plunging back down to the surface of the planet.
Fraser: And if he is able to punch him in the direction of his orbit another, say, 20 kilometers per second, what happens to him?
Pamela: That will ultimately end up changing the shape of the orbit and making it more and more elliptical unless it’s at the exact right point in the orbit that the punch occurs and it was already elliptical, in which case, you can make it circular. So, you’re going to change the shape of the orbit.
Fraser: Right. But at a certain point, he’s putting him into orbit around the sun and no longer into orbit around the Earth.
Pamela: Yes. And this is something we’ve done with the Spitzer Space Telescope.
Fraser: Yes. Remember when Superman punched the Spitzer Space Telescope into orbit around the sun. Yeah, I remember that.
Pamela: All right, yes.
Fraser: Yeah, no, I understand. This analogy is starting to get a little tortured, so we’ll shift gears and actually talk about actual objects. So, the setup, really – and I think it’s really important to understand this. There are objects on escaped trajectories that are leaving the solar system, that are leaving the Milky Way. How is this possible? What did it take to get these things into these new runaway trajectories?
Pamela: Well, we assume that it wasn’t Superman punching them. We don’t actually have evidence that it wasn’t Superman punching them, but we’re pretty sure that –
Fraser: I think it’s safe to say.
Pamela: – wasn’t the case. And so, quite often, with the things that we believe are escaping our solar system or have escaped other solar systems, it’s due to interactions with two other objects. There’s what’s called the three-body problem, and we did an entire episode on it long, long, long ago. And there are certain systems of three bodies that can end up ejecting one of the bodies, quite violently, outta the system. This quite often happens when you have two similarly massed objects and then a little one gets involved. And if that little one isn’t in a dead locked-in orbit around one or the other of the objects, if it’s shared between the two of them, it can just get flat-out ejected in all sorts of wild and crazy ways.
Fraser: Now, you say fairly violent, but it doesn’t actually have to be because, when you’ve got, say, the Oort Cloud surrounding the sun, the objects that are out in the Oort Cloud have almost no additional escaped velocity that’s required. They’re teetering right on the edge, and almost any amount of three-body interaction will kick them over the edge and cause them to drift away from the sun.
Pamela: And this is where I have to admit I’m thinking of stars and globular clusters as my baseline for this at all times.
Fraser: You could have a star come relatively close and the three-body interaction between the star and the sun, and the Oort Cloud object causes it to drift away from the solar system. And now, it’s gone interstellar. So, it doesn’t have to be catastrophic. And you could have a situation where a comet falls into the inner solar system. And it should return back out to the Oort Cloud, but it interacts Jupiter on its way through, goes near Ganymede, gets a kick, and now, it’s over the – it, now, has enough speed, on the way back out again, that it will overcome the sun’s escaped velocity, and away it goes, again, into the larger galaxy.
Pamela: And this is one of the really awesome things to think about where we have these comets that we see coming in, on hyperbolic or parabolic orbits where we know they’re leaving our solar system. And so we know that objects that have passed near our sun, have been influenced by our sun, weathered by our sun are, now, going to go and eventually make their way into another solar system out there. We also know that, if there are objects that are getting flung inwards, there’s objects that are getting flung outwards. So, there are also icy bodies that have never interacted with our sun that are getting flung outwards. And if we’re flinging things outward, there must be other solar systems doing the exact same thing. And now, we have observed that occurring with ‘Oumuamua. It’s a trading of icy bodies all over the galaxy.
Fraser: Yeah. I’ve read a couple of papers on this. It’s estimated that there are thousands, maybe even tens of thousands, of objects of these transferred Oort Cloud objects from other star systems passing through the solar system at any time. And so tens of thousands of our objects have left the solar system.
It’s mind-bending when you think about that, and then you think about the age of the Milky Way, how long – or the age of the sun – this process has been happening and how much material is getting from star system to star system. Now, you were sort of getting a little more violent, and so let’s kick that back up a notch and talk about how planets can leave on a hurry, very rapidly. So, what can cause a planet to leave a solar system very quickly?
Pamela: Well, a planet can leave the solar system if there is some kind of a three-body interaction between it and a couple of other planets in the solar system. It is thought that our solar system may have had many more planets than what it currently has, depending on your value of planet. And three-body interactions between Saturn, Jupiter, and those objects led to them getting flung not just further out in the solar system, but out of the solar system entirely. It’s kinda cool to think that there may be these isolated planets that got stuff from the early Earth, stuff from the early Mars flung on to them and are now carrying those samples away to other places in our galaxy.
Fraser: And this idea of rogue planets, I mean, a few have now been found through gravitational microlensing. It’s estimated that there could be as many rogue planets in the Milky Way as there are regular planets. That’s probably a high estimate, but still, they’re gonna be in the hundreds of millions or billions. So, we’ve talked about planets and how they can, either through three-body interactions of within the system or – you could obviously have a black hole or a star get very close and kick a bunch of planets out into space as well. So, I wanna talk about stars that are on an escaped trajectory. All right, let’s talk about stars. So, what can cause a star to go on a runaway trajectory?
Pamela: So, three-body problems are still a thing. There’s research that shows that globular clusters somewhat beat like a heart because they will condense down, over time, getting more and more combined systems. But then, as those systems get towards the core, they’ll undergo interactions that fling things out into much larger orbits, so they’ll expand back out. And so you have these star-star interactions that will fling an additional star. But the thing that is new, once we get to the stellar level of interactions, is you can also get things flung out during supernovae. And this is one of the things where we’ve seen many different runaway stars.
And what is thought to happen is you have the giant star. You have its companion, and the bigger of them evolves first. Evolves, evolves, gets big, and hits the point where it’s no longer generating energy in its core, collapses down, and the collapse drives nuclear explosion, essentially. And the force of this can fling that companion star at a runaway velocity. You can also end up with asymmetric supernovae that just fling stars on their own.
Fraser: So, the blast of the supernova is hurling the star out? Because my assumption was that it was like a slingshot. You’ve got the star going around this other star, and then, suddenly, the star is no longer there. And now, the star is on a slingshot trajectory.
Pamela: So, to say it’s not still there depends on the kind of supernova. So, it is this combination of –
Fraser: Oh, that’s a great point, yeah. Right, because you could have it either completely disappear or end up with a neutron star or black hole.
Pamela: Exactly. So, you have the leftover relic. You have the explosion outward is going to exert force on things, and you have the change in the distribution of mass that’s going to change the interaction of things. End of the day, that companion star is, in many cases, gone. Not all cases. It’s deeply confusing that we keep finding systems that have an advanced star and a companion star and we know supernovae have occurred, but they’re still together. So, we’re still figuring out all of the dynamics here. But sometimes, you end up just with a supernova remnant and nothing inside, in which case the runaway object is running away from itself, which we all understand at a fundamental level.
Fraser: Yeah, possibly. So, the supernova goes off, and it’s not equal, and so it’s almost like it fires off thruster.
Pamela: Exactly. Yes.
Pamela: And so that asymmetric supernova explosion will leave you with your supernova remnant and the stars running away from it. And I just love the conceptual idea of this where the supernova basically makes a giant mess, explodes all over the place, and runs away from the mess it made.
Fraser: So, with stars, we can get this situation where you’ve got – the supernova is symmetrical or not symmetrical, and so the star is given a giant thrust. You can have the situation where the star goes off and it blasts away its companion star with enormous explosion. You can have a situation where the star vaporizes completely. And now, the star is on a slingshot trajectory and in all cases. There’s one last scenario that I think is interesting is with a supermassive black hole. You can have a situation where a supermassive black hole could be kicked out on an escaped velocity, right?
Pamela: So, this is another multibody problem where, in this case, it’s the two galaxies coming in to merge that is leading to this situation, and so you have these two supermassive black holes. And instead of orbiting and merging, the collision and all the physics involved, as they pass to and fro, can end up flinging one of the black holes entirely away. And we think this is what explains the fact that there are a lot of merged systems out there that don’t have evidence for two supermassive black holes in their centers.
Fraser: So, the supermassive black hole just bounces off the other one and is off.
Pamela: I wouldn’t use the word bounce.
Fraser: I don’t know if bounce is the right word, but instead of it – if it’s not large enough, then it doesn’t merge. It just gets kicked out when it gets too close.
Pamela: It’s all of the multiple passes and gravitational tugs to and fro and everything else that is going on can just fling it away.
Fraser: That’s crazy. All right, thank you, Pamela.
Pamela: Thank you, Fraser. And thank you to all of our patrons out there. This week, I would like to thank, in particular, Matthew Horstman, Alex Cohen, Phillip Walker, David Gates, Claudia Mastroianni, Matthias Heyden, Kseniya Panfilenko, Justin Procter, Jim Schooler, Scott Bieber, Scott Kohn, Daniel Loosli, Gregory Singleton, Disasterina, Jeff Wilson, Tim McMackin, Kenneth Ryan, Cooper, Omar Del Rivero, Omar Del Rivero, Allan Mohn, Eran Segev, NinjaNick, Steven Shewalter, Paul D Disney, Don Mundis, Janelle aka Veronica_Cure, Michelle Cullen, Micheal Regan, Benjamin Müller, J. AlexAnderson, Dean McDaniel, Matt Rucker, Scott Briggs, Anitusar, Frode Tennebø, schercm, Bruce Amazeen, Benjamin Carryer, Peter, Moose and Deer, Jim McGihon, Philip Grand, Mark Steven Rasnake, Father Prax, Brent Kreinop, Dustin A Ruoff, Abraham Cottrill. Thank you, all, so very much for everything you do. To all our editors who have their work cut out for them today to be paid and to keep our show going, thank you.
Fraser: Thanks, everyone.
Pamela: Buh-bye. Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a creative comments attribution license. So, love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, astronomycast.com.
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