Dark Comets with Darryl Seligman

0:00So Chuck, I love having spanking brand new PhDs come here as a guest. Because we get spanking brand new information. Yes! And to learn that there's a whole new kind of object in the solar system, dark comets. Yes. I think he coined the phrase, too. Yeah, without a doubt. And of course, when you encounter a dark comet, you get superpowers. Wishful thinking. Coming up, new information about what's flying around our solar system on StarTalk. Welcome to StarTalk. your place in the universe where science and pop culture collide. StarTalk begins right now. This is StarTalk. Neil deGrasse Tyson here, your personal astrophysicist.

0:45Got Chuck Nice with me. Chuck? What's up, Neil? All right, man. I hear you're working on a stand-up special. Yes, we are trying to sell it right now. It's all done. It's done. If you're at Amazon or Netflix, hi. Okay. Well, good luck with that. Yeah, thanks. Good luck with that. So you know what we're going to talk about today? What's that? Dark comets. Ooh. Sounds like a Marvel movie. I'm dark comet. I'm dark comet. Dark comet. I've seen the term, and I don't know how they all define it. Okay. Because my specialty is like galaxies and cosmology and like what's going on in the solar system. That's too small for you.

1:26No. Comets. We can't be bothered with comets, can we? Dark comets. We have one of the world's experts on this subject, Daryl Seligman. Daryl, welcome to StarTalk. Thank you for having me. Yeah. So you are an assistant professor at Michigan State University? Yeah, well, starting next year, I'll be an assistant professor. Right now, I'm an NSF fellow. Oh, that's between your PhD and your first tenure-track job. Yes, exactly. So he's brand new. Wow, look at that. Look at him. Just birthed by the National Science Foundation. Just birthed. Look at that. So the National Science Foundation has what's called a postdoctoral fellowship where once you get your PhD, it's money.

2:07You just do whatever the hell you want. And it's like freedom that wouldn't otherwise be there if you immediately went right into academia. So, Daryl, what was the title of your thesis, if you can remember it? Because I barely remember the title of my thesis because there's a lot of words and a subtitle, but it's about comets. Well, it was actually about interstellar objects. Interstellar objects. Okay, well, let's start there. For the longest while, we presumed there was stuff flying between the stars. That's right. But nobody ever saw one because occasionally you'd expect them to pay us a visit.

2:41Or at least pass through. Pass through, that's what I mean. They'd just come in and go out, and we've been looking for centuries. Yeah. And your main topic of study is something that doesn't exist yet. Oh, well, no, I was pretty lucky because when I was a third-year graduate student, I was doing just pure fluid dynamics for astrophysics. And then as a third-year graduate student, Oumuamua was discovered. So just to be clear, so a fluid to an astrophysicist is what? A gas. A gas, exactly. Okay. Yes. So liquids and gases are both fluids to a physicist. Right. Right. Okay. So then why does Oumuamua have anything to do with fluid dynamics?

3:25Well, it doesn't have too much to do with fluid dynamics, to be honest, but it was such an exciting opportunity that we just kind of dropped everything else. Yes. We dropped everything else we were doing and we started working on that. So I don't know how closely you were following back in 2017, but it was one of the most exciting times for astronomy because basically this thing got discovered and it was also moving super fast. Right. So it was only observable for a couple of weeks from ground-based telescopes and then upwards of three or four weeks. Wouldn't you expect it to be moving fast if it's coming in from interstellar space?

3:59Yes, you would expect it to be moving fast. And why would it be faster than anything else? So because the planets are all moving in the same direction around the sun, Oumuamua comes in and it's coming from a totally different direction. And then it looks like it's moving really fast compared to our motion. Because we're going in opposite directions to each other. Yeah. Okay. Got it. Got it. So, Oumuamua, we found this thing. Yes. Called it Oumuamua, which sounds Hawaiian. It does. It is Hawaiian. I think it's, was it discovered by a telescope in Hawaii? So, it was picked up by PanSTARS, which is a telescope in Hawaii that is funded by the Department of Defense.

4:37And they're looking for near-Earth objects. Okay. So, small kilometer-scale objects that some of which, yes. Kilometer-scale. Small, yes. It'll knock you on your ass. Exactly. So some of those objects are potentially hazardous. So it's very important that we find them with ample time to do something. If they're going to hit us. Exactly. With ample time to kiss your ass goodbye. You need enough time to get that out of the way. So it seems to me, from what you say, Pan-STARRS had no expectation of finding interstellar objects, but it has the ability to do so regardless. Is that a fair statement?

5:18Well, that is true, but I think they originally designed PanSTARs hoping to find interstellar objects. Because for the last couple of decades, I mean, we know the solar system has ejected a good amount of material in interstellar objects. So it's not much of a stretch to imagine that other planetary systems would also be ejecting material and we would see them. And the only difference is you just see a small body, but it comes across the night sky and looks hyperbolic. But the fact that we hadn't seen any, we were trying to put limits on the number of them in the galaxy based on not seeing any.

5:52But the hope was that Pan-Stars would find one. Right. So if you don't see any for a certain amount of time, you can say there can't be more than this number, otherwise we would have seen it statistically. Yes, that's exactly right. Now you said it was hyperbolic. Tell me what you mean there. Because some people are hyperbolic. A different sense of the word hyperbolic. Yes. So it's the flavor-flav of comics. of comets. We knew it was hyperbolic because it was like, yeah, boy! So it came into the solar system and left and it's never coming back because hyperbolic means that its orbit is not bound to the sun.

6:30Got it. So everybody else has an orbit that is bound to the sun. That's right. It could either be like a circle or an ellipse. Wow. Right, okay. So these comets that we see every once in a while, like a Halley's, right? So that's bound to the sun. That's why we see them every once in a while because they're going around and coming back. That's exactly right. Halley's coming is like 170, 76 years. So you've only seen that once. Why you got to do that to my life? Living to 152. Come on. Cut a brother a break. You will not be 152 years old. I know I have hypertension, but damn. Just saying. Yeah.

7:09So hyperbolic. So that would be the best evidence that it was not a member of the solar system. Because comets are coming in from far away all the time. That's right. So why isn't it just an asteroid? Why do you know it's a comet? So there is kind of a continuum between comets and asteroids. Uh-oh. So now we get into word issues. That's right. Word definitions. Uh-oh. Yeah. So the things that formed at large distances from the sun, they have ice in them. Which hasn't evaporated away. Exactly. Over the years. Eons. Yeah. Exactly. So when a comet gets close to the sun, the ice will heat up and sublimate, producing a dust tail.

7:45And that produces a bright, beautiful cometary tail. You can't say evaporate. Yeah, I can't say. Right, it's sublimates. You've got to use the real word. Of course, yes. To regular folk, it's evaporating. Yeah, exactly. But no. Sublimates. Remind us what sublimate means. So that is when the ice, well, not just in a comet, but when ice transitions straight to the gas form. Straight away, without melting in between. That's right. Yeah, so what they call dry ice. Oh, okay. That just smokes away. Right. You've never seen liquid dry ice, have you? No, you haven't. Right. It goes right to the, and now, ladies and gentlemen, coming to the stage.

8:21It goes straight from the ice to the smoke. Yeah. In fact, if you have an ice tray in your freezer, you go away on a long vacation and come back, the ice cubes are littler. Yeah. Have you ever checked this out? I have seen that. Yeah. Yeah, you see them. They shrink up inside the actual tray. Yeah, they're actually sublimating. Cool. Okay, so keep going. I interrupted you. We're distinguishing between... Comets and asteroids. On the continuum of asteroids and comets. Right. We expected that the interstellar objects that get ejected from other planetary systems, that they most likely would have formed at large distances and then had a lot of ice in them.

8:58So we'd expect them to be comets. Is that because if they form at a large distance, they're not strongly held by the gravity of their host star? So they can be knocked off? That's exactly right. Okay. And if they form at far distances, then they haven't been close enough to their host stars. To lose the ice. Yeah, so you get a double force going there. So may I ask, if they are forming at such large distances from their host star, what happens to get them to be kicked out so that they're now slingshotted out into, hey, I see you, I'm on my way? Yeah, no, that's a great question. And it's a good question because we also don't know the answer.

9:39Stop asking our guests questions he can't answer. But the most likely thing is you basically have something like a deep space maneuver for a spacecraft where you slingshot off of a planet, but you have that with something as big as Jupiter. You mean their version of a Jupiter? Yeah, their version of a Jupiter. So something with Jupiter's mass at around five astronomical units. Then if a small body gets close to that planet, it could get slingshot out. So Jupiter is like a proxy sun or a Jupiter type. It acts like it would be the sun. Well, it messes with the orbits and things that arrive. Yeah, it's more like when you get close to Jupiter, you start feeling its gravitational influence much more than the sun, and then that will change your orbit.

10:24Gotcha, gotcha. Okay, now I get it. I get it. I get it. Jupiter intervenes. Intervenes. That's right. But also, presumably, given enough time, this other star system may get tugged on by other stars that happened to wander too close. Yes, that's exactly right. Yeah. That also can happen. Okay. So again, why are you thinking this was a comet instead of an asteroid? So Oumuamua, in every way we expected, acted the exact opposite. So the biggest thing was it had no cometary tail whatsoever. So then why call it a comet at all? Well, then it was initially called a comet because they made a mistake actually at the Minor Planet Center.

11:01Then they changed it to say that it was an asteroid because it looked inactive, inactive, which means that it doesn't look like a comet because it doesn't look like it has a tail. It doesn't reveal fluff. That's exactly right. But then we realized as we were trying to get follow-up observations with the Hubble Space Telescope and the Spitzer Space Telescope that the object had a non-gravitational acceleration. So Spitzer specializes in infrared, if I remember correctly. Yes, that's exactly right. Okay, good. So you want to combine different tools out of the toolkit of the observational astronomer to see it in as many ways as you can.

11:36Yeah. Okay. And Spitzer is really great. Like looking in the infrared is really great for comet science for some of the same reasons. You use it for things like exoplanets, but because you can see typical cometary gases like CO2 and CO that you wouldn't be able to see from the ground. So Spitzer named for Lyman Spitzer. Oh, okay. Former chair of the astrophysics group at Princeton University. Ah. And I have one of his books on the show. I was going to say, you sound like you know him. Yeah. So I keep interrupting you. No, it's okay. Keep going, go. Essentially, it had this non-gravitational acceleration, but no apparent dust coma.

12:09And then an important point is that these non-gravitational accelerations, we see them on... So first of all, what that means is that the object, you look over time at its position in the sky, and then you figure out its orbit, and then you realize that its orbit could not be explained just from the sun's gravitational influence. So there must be an additional force acting on it. And we measure that. That's what you mean by non-gravitational. That's what I mean by non-gravitational. So we measure that in comets all of the time. So there are many comets with non-grav, and I might even slip up and just call them non-gravs, but these non-gravs have been measured, and that's happening because as that ice sublimates, it produces like a rocket-type recoil effect.

12:48Oh, okay. So every little bitty jetty thing that comes off, that's a force operating on its trajectory. Yes, that's exactly right. So that alone is not unusual or uncommon. That alone is not uncommon. But it didn't have any fuzz. That's right. Exactly. And you want there to be fuzz if you're going to be non-gravitational. That's right. So you're in a conundrum. Yes. And there is another kind of important point, which is that you will also see asteroids accelerating non-gravitational. It's just that their non-gravitational accelerations, for example, they can be caused by solar pressure, radiation pressure.

13:22They're much, much weaker. So a MUA MUA is non-gravitational acceleration. The weird thing is not that it had one. It's that it had one with no dust coma, but it was much stronger than those that could be operating on an asteroid. So, because I think asteroids, many of them are very dark so that they would absorb sunlight. Yes. And that if you're absorbing momentum of photons. So it's like a solar sail at that point. Oh, yeah. Well, the solar sail would reflect light. Just moving extremely slowly. But in either case, the light pressure is acting on the system. Yeah, it's great. Okay, so it's a nice conundrum.

13:56And by the way, because I'm a big reader of the history of science, why wouldn't you have a conundrum for an object that has never been detected before? Yes. A class of object that we've never seen. Yes. Why wouldn't it be filled with things that - That don't make sense. That don't make sense. Well, it basically made me think like, I am really grateful for Oumuamua because I feel like Oumuamua, if you look back for the last couple of decades, it was the most intensely studied small body over a very short period of time. So there were something like 700 and something observations over the span of a couple weeks, just of this one object.

14:32That's evidence of how unusual it was. Right. But because of that, and because we then had to point Spitzer and Hubble at it, we then had to get the trajectory really, really accurate. And then we noticed this non-gravitational acceleration. So it made me think, maybe there are just these other objects in the solar system that are doing the same thing. It's just we had never noticed them before, because how do you detect comets? You see their cometary tails. Right. You don't see the non-gravitational acceleration. Hence, dark comet. Yes. Ah, get it. There we go. Okay.

15:12I'm Nicholas Costella, and I'm a proud supporter of StarTalk on Patreon. This is StarTalk with Neil deGrasse Tyson.

15:28Is there a consensus? Because we're now eight years into it, right? So you should know something by now. So I would say that, I would like to say that me and Jenny Bergner's recent theory about Oumuamua is the consensus, but unfortunately there are a lot of theories. Einstein had a theory. You had an hypothesis. We had a hypothesis. So say that again. Neil gets very touched about his theories and hypothesis. Okay. Fair enough. I respect that. You're in my office here at the Hague Planetarium, the regime of natural history. You're going to say it right. You're in very good company, by the way, of people who are like, no, you have a hypothesis.

16:05Okay, go ahead. So Jenny Bergner and I have this recent hypothesis. This is a collaborator. A collaborator of mine who's a professor at Berkeley. Essentially, a muamua was similar to a dark comet that the acceleration was driven by the outgassing of volatile-like, ice-like material, but it just had very little dust coma. And that's why we didn't see the dust coma. Okay, so that would explain it. Yes, that would explain it. A little bit after the fact, it'd be better if you had predicted that such a thing would exist. Right. But now you're just forced to sort of kludge an answer, which sounds plausible.

16:40Yeah. But have others followed behind it? So there's a lot of argument about what the type of volatile that was driving it. Explain to people what volatile is because any normal person thinks if something volatile, it means it'll burst into flames. Explode. Right, right. So geologists have a whole separate usage of the term. So catch us up on that. Yeah, it just is about what type of ice could be driving the acceleration. A volatile thing. A volatile thing is something that has ice in it. Oh, only that? I thought it just... When I say volatiles, I just mean ices, basically. Because when you put them in space, they will sublimate.

17:14Okay, so I thought anything that can turn to gas is volatile. Is that not true? That is technically true. Is it water in space volatile? Yes, but so water in its ice phase is what I mean by volatiles. Okay, and you wouldn't find a swimming pool of liquid water out there anyway. Got it. Okay, so in practice - It all starts as ice. It's ice. It's ice that can sublimate. Yes. These are your volatiles. Ice of any kind. Yes. Not just water ice, but like ammonia ice, I guess. Yes, yes. Maybe some more CO2 ice. CO2, CO. Dry ice, right. Water ice. Nitrogen. Nitrogen ice. Okay. Yeah, so they have that on Pluto.

17:47So there is nitrogen ice on Pluto. So to back it up, essentially in the solar system comets, the primary driver of activity, the main thing you see is water ice. And then you also see CO2 and CO, which is carbon dioxide and carbon monoxide. So those are the typical ones. But with the MUAMUA-SPITZER, the reason they looked at that is you would identify CO2 and CO. Because those have spectral signatures in the infrared with SPITZER especially capable of detecting it in ways that Hubble would not be able to. That's right. So another issue is that without getting too much in the details, essentially water could not be providing the acceleration because there was not enough energy from the sun.

18:28To sublimate the water. Yeah, that's right. Sublimate enough water to provide enough push to get the acceleration. Cool. But a much better accelerant is hydrogen. So our hypothesis, not theory. He thought about it too. He was like, oh, Howard. I'm going to say it because I'm here. Okay. The hypothesis is that there was water ice in Oumuamua, but then as it travels through the interstellar medium, it's exposed to these very high energy particles called cosmic rays, and that will break down some water and trap hydrogen in there. And then as the ice heats up, as Oumuamua comes through the solar system, it will just gently release this trapped hydrogen.

19:14Which has a greater impulse to accelerate the object than just evaporating water would. That's right. So it's both the fact that hydrogen is a lot lighter than water, but also it takes much less energy to sublimate hydrogen than water. Wow. Okay. But you said there's nearly a thousand papers on this object. That's right. And you're saying, but you and your colleague have this hypothesis. Right. Are you lone wolves out there? No, no, no. So essentially, it comes down to, can a hypothesis explain all of these mysterious properties? Another thing is you could have the acceleration driven by radiation pressure.

19:51It's just for that to work, it would have to be ultra thin or ultra low density. So Amaya Moore Martin at Space Telescope has this idea that it was an ultra low density fractal aggregate, which basically means it's a very large snowflake. So it's large, but very low density. That's right. Right, so there's a lot of... So you can push it. That's right. You can push it. That's right. So the solar photons, which very subtly push asteroids, could suddenly push this thing. Push this a lot harder. So because it's not... A lot harder. Because it doesn't have an orbital companion, there's no way you can know the mass of this thing.

20:26You have to make assumptions about it. So yes, that's totally right. Yes. That is a detail that I intentionally swept under the rug, but yes. No, this rug here is clean. All right. So is that the betting person's explanation? The way to test this was, is there another one of these objects? And I seem to remember there was. Yeah, so there was another one, another interstellar object, but this was 2I Borisov. So that was detected in 2019. And that was very clearly a comet. So that had a big dust tail. Okay. But the exciting thing is that the Vera Rubin Observatory and LSST, which I'll probably refer to it as, is coming online just next year.

21:07Farrah Rubin, another astrophysicist, died a few years back. She discovered dark matter in galaxies back in the 70s. So we named a whole observatory after her. And this is an observatory unlike any other in what important way? Right. So it is coming online right now at the beginning of next year. That would be 2025. 2025, right. What's the observatory going to do? So it's going to observe the entire southern hemisphere almost every night. So it'll be like Pan-STARRS now, which is in the Northern Hemisphere, but it'll look almost every night to see anything moving in the sky, but it goes almost five orders of magnitude more sensitive than Pan-STARRS.

21:49So five powers of 10. Yeah. So we'll be finding many more moving objects in the solar system, and we should be finding many more interstellar objects. I think people don't appreciate that if you just go to a telescope and get images and bring them home, you don't know if anything is moving on that image. Right. Like, you know, you got to get it over a period of time. They're basically snapshots. Right. You need exactly those kind of data. Right. To see if something goes bump in the night. That's right. So the Rubin telescope should blow open the whole field. Yes, absolutely. Gotcha. Wow. That's pretty cool.

22:20So it's pretty exciting. It is. Are there any other explanations for this? Aliens. Come on, guys. Why are we trying to act like we're not going to talk about the elephant in the room? That's the easiest explanation. That's what it's got to be, right? Like, ever. Or ancient astronauts or something. Right. Yeah, exactly.

22:59So I feel a little jealous because if Amu Amu 'a got kicked out of some other solar system, star system, and we're a star system, red-blooded star system, we ought to have some Oumuamuas of our own. I've been kicked out of better star systems than this. Yeah. Essentially, in the last couple of years, since Oumuamua, we've discovered these seven objects within the solar system, which are near-Earth objects, actually, which is what Oumuamua was initially... They thought it was a Neo? They thought it was a Neo, that's right. You're picking up on the abbreviation also. So they thought it was a near-Earth object.

23:35So these are near-Earth objects which are bound to the solar system, unlike Oumuamua. So they have elliptical orbits, not hyperbolic orbits. That's correct. We discovered significant non-gravitational accelerations on all seven of these objects, which are also inactive, which means that they don't have comet-like tails. Oh, okay. So therefore you can't credit the normal comet non-gravitational outgassing. That's right. But the accelerations we found on these dark comets, I mean, dark comets is just a name I made up. I think people like it, but - How could you not? So these objects, essentially their non-gravitational accelerations are too strong to be explained by the, similar to Oumuamua, the accelerations you see on other asteroids.

24:21So we have the hypothesis - Yeah! That's awesome. That's awesome. We have the hypothesis that these objects are like comets and that they have icy material in them, even though they are close to the sun and have been there for a long time. But they are, for whatever reason, not producing a large dust coma. Could you have a layer of something other than ice that encases ice within it? Oh. Yeah. So that prevents a full-up exposure to sun's energy? Yeah, absolutely. And so that was actually an idea. That was an idea that I wrote a paper on that could have, for Oumuamua, like the interstellar medium, the cosmic rays basically could have produced an insulating shell around it.