In 2017, the Solar System was visited by an object named 'Oumuamua, which came from another star. The unusual properties of this first interstellar visitor led some to suggest it may be an alien spacecraft - but the truth is that its oddness is already teaching us lessons about how solar systems form.
This lecture also considers the prospects of discovering more unusual objects in the Solar System, and what we might do about asteroids that threaten the Earth.
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This lecture was recorded by Chris Lintott on 24th January 2024 at Barnard's Inn Hall, London
The transcript and downloadable versions of the lecture are available from the Gresham College website:
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One of the things I really want to get across in these Gresham lectures is the many different ways that science happens. Sometimes, just occasionally it happens in the way you read about in books where somebody comes up with an idea, designs an experiment, and goes and tests it. But a lot of the time things are more chaotic than that. Uh, I see a few nods in the audience. So clearly people ag agree. Um, sometimes science happens to us. Uh, things occur that change our view of, in this case, the universe. And that's the story of tonight's lecture, this, uh, small object UA that shot through the solar system in 2017 and left us, uh, with new ideas and an awful lot of new questions. Uh, uh, as well. The Gresham College lecture that you're listening to right now is giving you knowledge and insight from one of the world's leading academic experts, making it takes a lot of time, but because we want to encourage a love of learning, we think it's well worth it. We never make you pay for lectures, although donations are needed. All we ask in return is this. Send a link to this lecture to someone you think would benefit. And if you haven't already, click the follow or subscribe button from wherever you are listening right now. Now, let's get back to the lecture. Um, I did think I'd start though, uh, with some of the traditional form of science, the very methodical, careful, uh, work carried out, for example, by people, uh, dressed like this. Uh, this is the clean room at the Jet Propulsion Laboratory, and this is a bunch of scientists trying to work out. Well, if you've ever had a problem getting a jam jar open, this is the scientific equivalent. This, uh, spacecraft, that's what it is sitting in front of them, is the Azaris Rex spacecraft, which journeyed to an asteroid called Benu, which is, uh, a near earth asteroid. It crosses the Earth's orbit, and in fact, has, uh, the highest chance of any large asteroid that we know about of hitting us in the foreseeable future. Uh, the foreseeable future being sometime in the 22nd century. Um, I don't think you have to worry at all until possibly the year 2080 something. So, um, looking at most of you, we are fine for now. Even I will be be long gone, I suspect. Um, but as Iix went to Benu, here it is. Here's a closeup of the asteroid, and on the right is this sampling tool that was deployed to take samples from the asteroid and bring them back to Earth. And it was designed to work on a solid surface. But as you can see in a second, as it hit the surface of this asteroid, it found basically, rather than a solid object, this body, which is a few kilometers across, turned out to be a rubble pile, essentially about as structurally sound as a sand castle that you might have built on the beach. Uh, and this caused problems. It did indeed manage to sample lots of the material and return it to earth. Here's the capsule sitting at the end of last year, uh, on the Utah desert, looking brilliantly, like something out of science fiction. Um, but instead of getting a few large lumps, what they got was a pile of rubble. And so there were good things about that. One of them was that even by scraping the outside of the collector, this is the outside of that jam jar, you could see that there's dust and material on the right there. They actually got 70 grams of material from the outside of the collector, which is more than they were trying to get of the asteroid in total. Um, some of that material's now here in London at the Natural History Museum, it's been distributed to scientists around the world who will study it. Um, but until yesterday, they hadn't managed to get into the collector itself. There was so much material it had jammed, uh, and, you know, they couldn't hit it against the side or, you know, we've all done that and, and had things go wrong. So they had to build special tools, but I wanted to celebrate them because yesterday this happened. This is the inside, uh, and we now have this pristine asteroid sample, uh, here, uh, on Earth. And about a third of this will be distributed to, um, labs around the world for analysis. We want to understand Ben's composition. We might, after all want to divert it, uh, away from us one day. Um, but we're also interested because it's a pristine relic of a time about four and a half billion years ago when the planets in our solar system were forming, they may well have formed out of building blocks like this asteroid. And so as well as defending ourselves, um, we can learn, uh, from objects like this one. Let me take those two ideas in turn. I think having mentioned that Benu might hit us, I should reassure you that we're actually doing okay. In terms of planetary defense in particular, um, there have been many ideas about how if we saw an asteroid coming towards us, what we might do. Some of them, for example, uh, in Hollywood films where they suggest for some reason you send, um, Bruce Willis and a team of oil executives to blow it up, that spoiler alert doesn't work very well. Uh, or another film where they just send Bruce Willis, uh, I dunno why it's always him. He's clearly a, a clearly a specialist. Um, my favorite idea for years was that if you saw an asteroid coming towards you, as you could see, this material is quite dark. Uh, it's, it's blackish and that's quite common, uh, for Nero asteroids. And so my favorite suggestion is you still send Bruce Willis. Uh, we wanna make sure he's still got a job. Um, but instead of blowing it up or attaching a rocket motor, all you do is paint the asteroid white. That then changes the, uh, properties of the interaction of the, the asteroid in the sun. Uh, it gets a change of momentum. And if you do that early enough, the asteroid will deflect and not hit the earth. Uh, unfortunately, recent calculations say that you'd need to do it thousands of years ahead of any impact, and we don't have the ability to predict that far ahead. So, uh, Bruce really is out of a job. What we could do instead, I is brute force. And this was demonstrated a couple of years ago by a mission called dart, which, um, brutally attacked a small asteroid called Dimorphic. Dimorphic is the thing on the right here. This is a, a, a double asteroid, and they'll be important later. It's a binary. They go round each other. The large one's called didymo, uh, and the small one is dimorphic. It's a, a few hundred meters across. Um, and what DART did was take a spacecraft about the size of a mini, um, and head directly for, uh, the smaller of the moons. And so these are images from the Dark probe, which was launched, uh, to impact this small asteroid. This is really a test of technology. It's, can we, can the, the spacecraft steer itself accurately enough to hit this small moon. As you can see, they look rather like better. It's this rubbery surface, uh, not really solid, but Dart scored an absolute bullseye. These are the last images that it sent back, um, as you can see. And then that's the last image that came back. We only got the top bit, and we had the strange experience of watching a mission control full of scientists who built this spacecraft celebrating wildly as their craft destroyed itself. Um, the reason that this was done, it seems slightly odd. I mean, okay, we want to show that we can hit the thing, but the idea was also to show how much an impact like this can deflect the asteroid. That seems a bit odd After all, you know, I set questions for students that say, okay, we've got two snooker balls. If I hit one snooker ball with another, if you know the mass of the snooker balls, you can count the speed, you can calculate what's gonna happen to the other snooker ball. I'm very, very good at snooker in theory. Uh, the practice I I leave to others. Um, however, because the asteroid is a rubble pile, when you hit it, it's not like a solid ball hitting another solid ball. You get material thrown up into space, um, that can help you. Um, it can have a sort of rocket effect or it could hinder you. Um, and actually the impact of data into Dimorphic was much more spectacular than anyone thought it was going to be. Um, the team were rather reluctant for people to go out and look with back garden telescopes, but many of those who did saw the asteroid Brighton. Uh, and with the Hubble Space Telescope, we got this spectacular view of a long tail of material. This is dust that's thrown up from the asteroid streaming through the solar system over many thousands of kilometers for many months afterwards. And by studying the orbit of dimorphic around DIDs, the larger moon, we're able to measure the deflection. And it turns out that we're pretty good at this. So if we'd seen an asteroid of this size, a few hundred meters coming towards us, and we had 10 years notice, we'd be able to launch a copy of Dart, uh, and deflect it and stop it hitting us. So the question is whether we'd have 10 years notice, and I'll come back to that at the end of the talk. Uh, you can live in peril and possible doom until then, I figure that will make sure everyone listens, uh, to the rest of the talk. But I did say that these asteroids were useful, not just, uh, for not, or interesting, not just because of their threat to us, but because they tell us about our origins as well. They, they're the building blocks from which we build planets. We've actually got quite good as well of seeing that process unfold. Um, one place we could look to do that is actually really prominent in the night sky at the minute. This is a beautiful picture by my friend Will Gator, uh, of the Orion constellation, which perhaps some of you recognize. You've got the three stars in the belt, got Beetlejuice or Bettger at the top, and then hanging down from the belt. Um, there is the sword of Orion. Um, and in that sword, if you go out tonight, I'm sure it's clear we're giving a lecture inside. So it must be clear outside. If you go out and look up even from the middle of London, if you look at the stars in the sword, you'll see that one of them is fuzzy. It's not an actual star. And this has been long known. It's this one, it's about halfway down. This is actually the great Orion Nebula. It's our nearest star forming nursery. This is a spectacular Hubble space telescope that shows nitrogen, hydrogen, oxygen, grass glowing in this, uh, stellar nursery that's about 1500 light years away, relatively nearby in terms of the structure of the galaxy. This nebula's lit up, not because the gas itself is intrinsically hot, but because it's heated by a small cluster of stars that's visible at the center of the nebula. Um, they're called the trapezium. They're visible in a small telescope. Uh, and even through, uh, the six center reflector I have in my garden, I spend ages looking at this, tele this object, because you could see this three dimensional structure, this gas wrapped around these stars that have just formed in the heart of this nebula. William Herschel, the great English observer who discovered with his sister the planet Uranus, um, was the first one of the first to write about this as a star forming region. And he described the nebula as being composed of the chaotic material of a thousand future suns. And I think that captures something of the drama of these objects. You've got new stars forming their light, the winds from those stars sculpt the gas around them, that can trigger further star formation and allow pe that allow stars to form out of the gas. Um, and we learn a lot by studying these things. But we can also look very closely at the stars that are forming. This is something that the Hubble Space Telescope in particular specialized in. And these four dots, this is I'm afraid going to be a talk about blobs. So you've had the nice images. Now. So relax in, we're gonna talk about blobs. Uh, at the heart of each of these dots is a newly formed star. These are stars which have just switched on. So nuclear reactions at their center have just got going. Um, and what you can see in each of these cases, there's that silhouette against the bright background of the gas in the nebula, there is a shadow, a silhouette, uh, which is a disc of material around that star. Now, these are big discs. Um, on average, these four are about 60 billion miles across, which is about seven and a half times our solar system. If you think of Neptune as the, the edge of the solar system, an idea we'll come back to, um, so that they, they may be big compared to our solar system, but we think these are in fact solar systems in the making as well as gas In the nebula, we have dust, silicon, carbon, tiny grains, maybe 10th the size of sand grains. Um, and as they get caught up in the formation of the star, they form because of their angular momentum into these discs. Um, and the discs we now know have structure. So if we look not with the Hubble Space telescope, but with a telescope that's tuned to the microwave region of the spectrum to very short wave radio, really, but what astronomers call the sub-millimeter or microwave, then we can see some of these discs in exquisite detail. We use that frequency 'cause it gets us to the cool parts of the solar system. So here are three planet forming discs, stars at the center. Um, ignore the, well, there's a zoom in on the right there that in the middle that shows detail. But what I want you to see is that these discs have structure, they have gaps in them, they have rings, and that structure is caused by planets that are forming within the disc. So once you form a planet, it can sweep up the dust in its orbit. And so we now think these, all these stars are a few million years old. And so we are looking at places where planets have formed within the last few million years, probably big planets, Jupiter sized or more, but we're seeing their effects on the disc. I'm a particular fan of the middle one, which is a star called tw hy. Um, because that first gap that you can see is one astronomical unit from the star. In other words, it's the same distance from the star as the earth is from the sun. So you can imagine looking at our inner soul system four and a half billion years ago and seeing exactly this, we can also see that this is an evolutionary process that this forming of planets and sweeping up a dust takes time. These are, uh, a recent view just from last year of 20 such discs all from this telescope called Alma up in the high Chilean Andes. Uh, it's an amazing place and an amazing telescope, but they're a range from youngest in the top left to oldest in the bottom right. And you can see what starts off as quite a amorphous, diffuse, diffuse structure becomes a regular disc. And then those gaps appear by the time you get down to the bottom right. And so planets form after the discs. So you have the star collapse, you have the disc, and you have planets. And the fact that we can watch this happen is one of, I think, the unsung triumphs of astronomy in the last 20 years. And I don't think anyone, even those building Elma, uh, 20 years ago realized we were gonna get this sort of grandstand view on planet formation. Now, to explain what we're seeing, we have a couple of options. Um, one option, which is beloved by, by many of my colleagues, is of course to build a, a high resolution computer simulation to take the physics that we understand of these quite complicated systems, which involve fluid dynamics and the heating of the star plus, uh, friction between grains in the disc, plus the gravitational interaction of the newly forming planets. But we can throw all that in a very large computer and we could build a convincing model with music apparently, um, that, I dunno if we have copyright clearance for the music. So we'll stick to the image. Um, and what you could see in this beautiful, this is a simulation. It's not an an animation, but what you can see is that these gaps grow naturally and then you start to gain some planets and it's rather beautiful and it's all very, very convincing. The trouble is that the simulation's cheap. Um, there is a fundamental mystery at the heart of our understanding of planet formation, and that's going to be a key part of this evening's lecture. So let's ignore the simulation. Just talk about first principles, what we're building these planets out of, of things like this. This is a actual interstellar dust grain. It was captured by a spacecraft called Stardust. There was sort of a, a precursor to azaria's. Rex actually visited, not an asteroid, but a comet. It flew through, uh, the coma, the atmosphere of Comet Vilt two and returned to earth with lots of nice commentary particles. But by accident, without planning, some particles got stuck on the back of the detector. Now a few of those were comet bits that had bounced off the spacecraft and ended up there, but it was also the back was exposed to interstellar space. And so six particles, I think that were passing through the solar system, happened to be captured by the detector. And they were found by a bunch of citizen scientists online in a product called Stardust at home that spent their life looking at dust grains and looking for ones that looked unusual. Uh, the unusual ones were extracted, tested in the lab. And so this is a piece of another solar system. Um, as you can see, it's pretty small. There's a scale bar at the top and it's this irregular structure. And the reason it looks like this is that it started life as a bunch of little spherical grains, again, thinks grains. And when they collide in a disc like those we see around these young stars, they stick together and they stick together because of chemical reactions, but also 'cause they often in the outer bits of the solar system have ice, water, ice or carbon monoxide ice surrounding them. And you get two nice icy grains, you stick them together. My friend Helen Fraser does this, uh, on a zero gravity on a plane that does this. So she gets a few minutes each time to try and stick dust grains together. Um, I think I've probably underestimated the, or not explained the complexity of her experiment, but that's fundamentally what she's doing. Um, and this works. And so over time what's happening in those discs is you're slowly sticking together bigger and bigger things to, you go from dust grains to aggregates like this one to maybe as much as a pebble. And then if you start sticking pebbles together, they're moving together, that can happen between the friction. Then you end up with things maybe the size of a boulder. So I have a, I have my own simulation, uh, of this process for you, which I, I think is as high tech as the previous ones. So this is a bowl of water. Uh, it's being stirred to simulate the disc. So this is the rotating disc. And then we're going to introduce some grains in this case of pepper, uh, into the bowl. And this will be utterly unspectacular.'cause initially you won't see the pepper, but as it rotates, friction brings them together, the pepper grains starts sticking together and they become visible in the video because they've grown. So this is exactly the same process. Um, I'm not saying planets are made of pepper, but you, you get the idea. So this works fine until you get to boulders. And the trouble is, once you go from here to a boulder, when I collide to boulders together, I don't get a bigger boulder. I get rubble, I go backwards, I get smaller things. And so this is a problem. I've broken the chain of getting larger and larger and growing towards the planet. Now, once I get to things roughly the size of, I don't know, one of the buildings around us here in the city, when I climb two skyscraper size things together, I get rubble, but I get a lot of it. And gravity can pull that back together. So once you get to big things and they collide, gravity is important. And so we can grow from skyscrapers to planets. We understand how that happens. And we can go from dust grains to boulders, but we don't know how to quickly jump that gap in the middle as long as you let me click my fingers and cross that gap. I know how to form planets. But there's this fundamental problem in the middle. And that's the problem that I think a solution may have appeared in our solar system. We weren't looking for it. This wasn't the result of a targeted search for a solution to this problem, nor was it particularly, uh, a targeted search for objects like the one I'm going to talk about. Um, the discovery was made by telescopes in these two small domes, which are in an observatory on top of the Hawaiian island of Maui. Um, it's a experiment called pan stars, which is funded by consortium of academic institutions and the US Air Force to look for threatening near earth asteroids. So the fact that we know that there's nothing more threatening than Benu that's large and heading our way is primarily due to pan stars and, and surveys like it. What they do is they scan the whole sky and they look for moving objects. And in October, 2017, they found such an object. This is, these are images from the 19th of October on the left. The red lines, by the way, are areas where there's no data available. The US Air Force having funded this mission, realize that it, if you're looking for moving objects in the sky, you also find things the US Air Force doesn't want you to find. And so they remove bits of the sky from our data, uh, to hide what they're doing. Then they have to remove bits of the sky that don't hide what they're doing.'cause otherwise, so, so the red lines are just that. But I dunno if you can see in the red circle, which I added. Uh, it would be really nice if the Air Force would circle the things we're looking for, but we haven't got there yet. Um, there's a very faint smudge. It's a little line. Um, and that's a moving object. It seemed unusual. It was followed up by a larger telescope on the neighboring big islander Hawaii on monarchy by CFHT, the Canada France, Hawaii telescope. And that image is much clearer on the right now, this time they're moving the telescope to follow whatever this new object is. So the.is the new object. The lines here are stars that move relative to the thing. So we know it's moving relative to the stars. Um, in the bottom there's also a closeup image. Um, and all it really tells you is that this is a point like source. It's not a big thing. It's not a comet at this stage with a tail. It's just a.in the sky. And at this point, most people are thinking this is in fact an interloper from the outer solar system. So our familiar model of the solar system with the planets, with Mercury, Venus, earth, Mars, Jupiter, Saturn, Eunice, Neptune, um, that's just the, the inner bit of our solar system. Most objects in the solar system, uh, live out in what's called the AUT cloud, which is about a third of the way to the nearest star. And this is the vast reservoir of icy bodies left over from when the solar system was forming. Um, that feeds into our store of comics that we see icy bodies that come into the inter inner solar system. So people are thinking this as a comet and they began to plot its trajectory and it was immediately unusual. So there's the solar system. Um, the dotted line shows the path on which this object traveled. It came down from up in the north, came rapidly into the solar system, slingshot it around the sun, and by the time it was discovered, was already heading off into the distant galaxy back into deep space. And we found it after it had been through the inner solar system. There's an animated version here. It's got a few other objects in. Uh, but it's this red line coming in here, uh, and there's this slingshot and then it heads off out into deep space. And this trajectory was unusual enough that we realized, it was realized that this was the first interstellar object, the first large object bigger than a dust brain that we'd seen coming from another solar system, or at least from beyond ours. And so the question at the beginning about the name, well, it was originally classified as an asteroid, then it was named then it was decided it was a comet. And eventually they gave it the designation. The International Astronomical Union gave it the designation one, i the first interstellar object and the name Ua Mu. Now it's a Hawaiian name because it was discovered in a Hawaiian observatory. Uh, in Hawaiian. Um, emphasis comes from repetition. So MUA means far away. So UA is really far away. And UA is U is a scout. Um, so it's the scout from really, really far away, which is rather nice. And I'm now gonna show you, and I want you to be impressed, please. This is lecture three, so I know at least some of you are up for this. Um, this is the best image that we have of this object that I'm basing an entire lecture on. Ready? Okay. Try to contain yourselves, <laugh>, it's the thing in the blue circle. And I added the circle. Um, we never saw it as more than a dot, sadly. Um, this is an image from the very large telescope, one of the most powerful instruments we have, hence the name. Um, again, tracking the object so that the stars blur in the background, but we never rea saw it as more than a dot. And so it became a bit of a challenge. What can astronomers learn about an object if all we see is a dot? Well, the first thing we can do is we can say, I, this is the kind of astronomy I like. We ask simple questions. We know it came from beyond the solar system 'cause we've seen how it's moving. Um, we can say, what color is it? It turns out it's slightly reddish. Uh, and that's typical of stuff in the outer solar system. So you could compare this with, I know Charan, one of the moons of Pluto, or, uh, perhaps some of the objects in the CO about the rubble that shares an orbit with Pluto, it wouldn't look out of place. And so that means it's probably icy and it's probably spent a lot of time in deep space, or at least formed whichever solar system it came from on the outer edges of its solar system. You could swap it there. So that, that makes sense. The other thing that we can do, if we measured color and we've seen it's a dot, we can keep looking at it. And it was followed for a few months and it actually showed remarkable variation in brightness. So this is, each.here is a measure of brightness. When it's high up, it's bright. When it's low down, its fa And this is from a whole cluster of the world's most powerful telescopes. But you can see there's this pattern where it goes up and down in brightness and then it goes up quickly and then down. But it's not a regular pattern. It's not, um, a simple periodic relationship. And so the change in brightness is quite big. Um, it's about a factor of 10, uh, between its brightest and its faint as it caused havoc. We're trying to plan to observe it. So we've got this unknown object that's changing brightness wildly. And so the astronomers who did this, uh, had to be really on it. And very quickly there were two hypotheses about what's causing this change in brightness. And they're illustrated. And normally for these lectures, I, I've been trying to make my own slides 'cause I want to give you the most original and, and interesting content, but I had to borrow this next one. This is an illustration by my friend Luke Duns about the two reasons why URA might have been changing in brightness. And he's using a local object to stand in for amu mua <laugh>. So the two options are on the right. You have, uh, str structure on the surface. We could have colored patches, we could have dark patches and light patches. And so if you imagine the cat rotating, the brightness of the cat would change over time. Um, I'm told by Luke, he tried to make the cat do this, but apparently difficult to coordinate. Um, so that's possible. You can imagine inventing a, a pattern that would match what we've seen. But if you look in our solar system with one honorable exception, Saturn's moon iapetus, which is half dark and half white, we don't see on small bodies. And we know that m is small because it's a dot, um, on small bodies, we don't see variations in, in color really. They tend to be pretty monochrome. So this is possible but unlikely. Uh, in the other example, we have a stretched object. And if you have a stretched object, and this was the, uh, press release image that you'll have seen perhaps as the most common, um, visualization of a, remember if you have a long object, then it's brightness depends very much on whether you are looking, uh, edge on or side on. So this will look very different from this, if that makes any sense. And if it's, if it's rotating, then you get a change over time. And in fact, it's not just rotating, it's tumbling. So it's not just go, it's going end over end, but twisting as it does. So. And if you model that, you can reproduce the light curve pretty well. Now, the original estimates were that this thing must be 400 meters long, 10 times or maybe even as 20 times as long as it is white. So it's this really extreme sort of cigar shaped thing. Later estimates, I think most people now would be a bit more restrained. They would now say it's more pancake than, um, cigar, maybe six times as long as it is white. But you've still got this elongated structure tumbling through the solar system. Why is it tumbling? Not sure. Maybe a violent start wherever it came from, something flicked it into, uh, into galactic space. We're not sure. Uh, but that explains the change of brightness. And before you stare too long at this artist's impression, I want to remind you that this is the best image that we've got, right?<laugh>, uh, not this or this, this, we're talking about the real stuff, but we think it might have looked like this. So, you know, we, we can imagine, but we must always remain grounded in reality. There were two other odd things about this object. So the color unusual but not that unusual. The shape pretty unusual compared to the asteroids that we've studied in the solar system. There were two other unusual things. One is that that trajectory where it came in originally seemed to be close to something called the local standard of rest. What that means is it seemed before it encountered the solar system to be moving along with the nearest stars to us. So is that unusual? Don't know. We've only seen one of these things, but if you were going to throw these things randomly around the galaxy, you wouldn't expect it, uh, to be following the nearby stars. So maybe it comes from one of those nearby stars. And then the real mystery is that as we watched it recede from the solar system, we're expecting it to slow down because of the gravity of the sun. Leaving the solar system takes energy. You have to climb out of the sun's gravity. Well, and escape umo did slow down a bit, but much less than we thought. There was some accelerating force pushing it out of the solar system. This would make sense if it was a comet. We've seen comets. This is the closeup nucleus of Cher Mary of Garca, which was visited by, I mostly put it in. So I could say Cher Garca, because once you've learn to do that, you use that skill as often as possible.<laugh>, it's like trying to pronounce the Icelandic volcano that caused, uh, the dust storm more than a decade ago. Esther Fuk, if you were, uh, asking, anyway, char Garca, the other chairman Goenka story is that it was, uh, nicknamed chewy, gooey by the scientists who built the probe that went there until they worked out that both Cher and Goenka were still around and we'd prefer not to have their discovery shortened. So, and we can call it 67 p uh, this is the come we know best Rosetta spent, uh, more than a year in orbit around it. And as you can see, this is in active phase and there's this jet of materials shooting out. So comets, which are made of ice don't melt evenly when they come into the inner solar system. You get these dramatic jets. And so that will have a rocket effect pushing this thing and this image up. And so if Uma was only a comet, then this acceleration would make no sense, but it's only a point of light. How do we explain this acceleration? So we have a, a first visitor to the solar system. We dunno where it came from, but we know it came from somewhere else. It appeared unannounced, it moved through the solar system silently. And we know we checked, we pointed radio telescopes at it just in case there were signals being sent. Uh, there weren't, or at least none that we could hear, but now it's speeding up as it leaves almost as if it's fired it's engines. And all of these thoughts combined with the fact that Arthur C. Clark wrote a book called Rendezvous with Rama, uh, back, uh, 50 years or so ago, which subscribes the visit to the solar system of a mysterious long cylindrical object which passes silently through the solar system. And despite in the book, um, people going to visit it and exploring it inside, it's obviously artificial. Uh, it remains mysterious and disappears off into the, uh, rest of the galaxy firing its engines as it goes. So, you know, this idea that maybe this is exactly the behavior that one would expect, uh, from an alien spacecraft was something that occurred to to lots of us at the time. It also occurred to a Harvard astronomer called Avi Loeb, uh, who believes, uh, very seriously that this is clearly an alien spacecraft and has written a 250 page book explaining why the rest of us are too stupid to agree with him <laugh>. Um, but it's a serious scientific point. We have an unusual set of properties should we consider, uh, an alien spacecraft as a possibility. It's so tempted to say yes and leave <laugh>, but actually I think there are good explanations for the unusual things that we know about, um, mu So I'm gonna tell you about them and then I'm going to try and convince you that it's interesting. Anyway, the trouble with mentioning aliens is that people always want it to be aliens. I try to write a book, um, called It's Never Aliens, which was gonna be a story about all the things that turn out not to be. And I got a a one line review from a potential publisher that just said, but we wish it was, which is fair, but this is still an exciting object. But let's deal with the unusual things. Firstly, the shape this, this elongated shape and its color. Well, if you talk to people who study the small bodies of the solar system, people like my friend Michelle Banister, they'll tell you that the most unusual thing about this object is that we've studied it in this detail at all. If you ask the people who control the very large telescope to look at a random asteroid that's only 200 meters across of which there are millions and perhaps billions in the solar system, they're gonna tell you they've got better things to do. Looking at Jupiter and distant galaxies and planets around other stars and all the other things that we are doing as astrophysicists. So if you took it and put it amongst its peers in the soul system, it may not appear that unusual in shape or color. The fact that it's close to the local standard of rest, that it's moving with the stars, it turns out it's not that close and that you could have got, uh, that alignment by, by luck. Um, and then there's this acceleration, this fact that it fired its engines or, or sped up as it left the solar system. Well we think we've got a couple of explanations for that as well. One is that, um, things are changed by time in space. Next time you get a chance to look at the full moon, you'll see the familiar seas, the dark areas which are are lava, and then the brighter areas on the moon, which are the highlands. And on the whole, those are older areas, but where are crater is punched into the surface. For example, the large crater at the bottom of this image here, you can see I hope that there are bright rays of material that shoot out across the surface. And they're bright because they're fresh. They're material that's been dug up from underneath the surface and distributed across a large percentage of the, of the lunar surface. They're now exposed to particles called cosmic rays, which come from the sun, but also come from the galactic environment. And these high energy particles hit the surface of the moon all the time. We are protected down here on earth for the most part by our magnetic field. But on the moon, uh, the surface is exposed to this cosmic rays and over time, over millions of years, they will darken. Now UA must have been exposed to cosmic rays as it traveled through the galaxy, perhaps for billions of years before it reached us. They would've had the effect of getting of exciting and getting rid of water or, um, other volatiles carbon monoxide carbon dioxide from the outer layer. And so you end up with an object, which is an icy core with a crispy crust around it, a kind of celestial magnum, uh, if you like, without the stick obviously. Um, they, uh, there's a marketing opportunity in there somewhere. They will tell the Gresham that we need our own ice creams. Uh, anyway, you get so, so this is the idea. So it's a hidden comet. And so when it comes into the solar system and warms up, you may get less activity than expected, but you may still get these jets. And so we wouldn't have seen the activity 'cause it wasn't dramatic enough for even our largest telescopes. But you can get this accelerating effect. Or if you don't like that idea, we have others. Um, there's a group of, uh, astronomers led by Darryl Seggerman who think that, who published papers like this one that claim that perhaps it was made of hydrogen. So perhaps it was a hydrogen iceberg that formed somewhere like the Orion Nebula in one of these dark star forming nurseries. Um, and has traveled through the galaxy. And the point here is that you can heat hydrogen up. You would've got these jets of material without um, us being able to detect it. So you can, you could sort of sneak away in this way personally. I think dehydrated comet, icy shell, crispy shell and IC center is the way to go. Um, but there are other arguments and that's all we know about Ur that's very satisfying, this lecture because we will never know anything else about this object. We can't go and chase it. It's gone too quickly and too far. Um, it's lost to our telescopes and very soon we won't even have the accurate accuracy to go and chase it, even if we, we wanted to in the future, if we invent, I dunno, the lin top drive that can go close to the speed of light, we won't know where it is interesting though, and I think probably luckily for all of our sanity when we're arguing, uh, about whether this first interstellar visitor, uh, really is an alien spacecraft or not, we have been visited by a second interloper. This is a humble space telescope image of an object called Comet Borisov, which is discovered in August, 2019, um, by a remarkable astronomer. Borisov is a, a technician, uh, at a large observatory, um, uh, in um, Kazakhstan I believe. Um, but in his spare time he's an astrophotographer. And so not during his day job, but in his night job, I guess as an imager of the skies, he discovered this comet, which turned out to be on a similar trajectory to . It's come from outside the solar system. It traveled through the solar system. Um, and it, this, this image is taken when it's about 300 million kilometers away. And I should say the thing on the left here, that's a galaxy, it just happens to be in the background. Um, but the blue thing is a comet. As you can see, it's got a tail. Uh, it's behaving much more like a normal comet. So we know that we can get normal in stellar objects as well. Borisov is exactly what was predicted. It had an interesting encounter with the solar system over time. These are images of the nucleus, the center of the comet. And you can see on March the 28th, it seemed to split up into two pieces, a few hundred kilometers apart. This is typical of comets that get too close to the sun and break up. And so we have an icy visitor from another solar system. And if you think about the geometry of the solar system that I showed you earlier, which has that alt cloud, that reservoir of comics in the outer solar system, it makes sense that in stellar objects would be comedy because it's easier to escape the sun's pull if you're already a third of the way to the nearest star than it is if you're the way down here in the inner solar system at the bottom of the gravity. Well, so rocky asteroids, which tend to be found near the sun, won't have escaped perhaps lots of these comets on the outskirts do. So we now have two of these things. I was convinced that this morning was gonna see the announcement of a third one, but no, so far, uh, we're still waiting five years on from our second interstellar object. But actually there are clues in our solar system that suggest that these things are very common indeed. And the first set of clues, the most important ones come from that outer edge of the solar system where, uh, Pluto lives. Um, Pluto of course definitely not a planet, as I've mentioned in all of my lectures so far, much to the distress of at least three people online. But it's not a, it's not not a planet because it's not interesting. Like we didn't decide it was boring. In fact, this image from the New Horizons probe, which flew past it in 2014, showed that it was much more interesting, uh, than we thought. It's a world with some differentiation. It has this heart-shaped plane of water, ice, now known as Sputnik ple pita. It actually has mounts of water, ice frozen solid. It has these dark, uh, patches actually rather similar to the color of Uma. And it has a complex interaction with, uh, its large moon charan, but it is only one of many millions of objects that live in the COI belt. Our solar systems outer asteroid belt, this region where planet formation, this process of adding, uh, grains together to form boulders and boulders together to form somehow skyscrapers and skyscrapers together to form planetesimals and then onto planets, didn't proceed as far as it did out in the Coer belt. What we have is the rubble of a failed construction project. There wasn't enough time to build a planet before the sun got going, uh, and heated up the disc and expelled most of it. And the dezens of the Coer belt are interesting in many ways, new horizons. When it went past Pluto headed off through the koi belt and the team looked for another object that they can visit. They actually used the Hubble Space telescope to look in the direction that their probe was going and tried to discover an object to order something close enough that they could divert to. And they found it in a small world called aov, which is only a few, again, a few hundred meters across. Um, this is the closest image color image we had of it from that fly by. Um, it flew by on New Year's Day 2019 from a distance it looked beautifully like a champagne bottle, which is great. As we got closer, it's much more snowman like. But the key thing here is that this is not really one object, it's two that have collided. You can see there's a bright ring between the two spheres between the snowman's head and body. And that's where these two objects have gently drifted into each other and stuck together. They're icy. It's slushy when you, when you combine the two things. And so this is a double object. And in fact, lots of the worlds that we know of in the are binary rather than solitary. Pluto even has its moon charan, which is much closer in mass and size to Pluto than any of the major planets. The next one down is the earth moon, but uh, Pluto and Charan are much closer. Big things like Eris, uh, another of the large co objects are double. Um, many of the small worlds seem to be, and of course the comets like 67 p Cher Garca that live out there. Um, this could well be a double object as well. It looks a lot like aov. And so somehow we formed these binary or double worlds. And the best explanation for that is that the Coyer belt was once a thousand times denser than it is today. Instead of having a few percent of the total mass of the earth spread out amongst these bodies, we might have had six or seven earth maths worth of stuff out in the solar system. So where did it go? Well, we have a theory for that as well. We have to abandon the idea that our solar system is a nice stable place. This is a simulation of the first billion years of our solar system's history. And we are looking here at the outer solar system. So the rings of the orbit of the four giant planets, Jupiter, Saturn, Uranus, and Neptune. And the green things are the COI about objects that are left over. So if I run this, uh, the time is in the top in millions of years, the first billion years, few, few hundred million years, nothing happens. Everything's nice and stable. You can see the planets are moving about very slightly. That's because of their mutual gravitational attraction. Jupiter's pulling on Saturn sa, Saturn, Uranus and Neptune have a collective effect on Jupiter u and Neptune tract, but nothing really happens for 600, 700 million years. But in this simulation, when we reach about 800 million years and we'll slow it down, a resonance is achieved. Saturn and Jupiter start lining up and all hell breaks loose. So what happens is Jupiter and Saturn get themselves into a pattern where they're, they line up in the same part of their orbit every few orbits, and that gives a regular kick to the rest of the solar system. So it's like if you're pushing somebody on a swing, if you're already always push at the same point, you get a much better, uh, response from the swing that causes havoc in the out soul system. Jupiter tends to move inward. Saturn tends to move outwards. Uranus and Neptune swapped places. I dunno if anyone noticed that in the animation. And the COI belt is scattered out into the galaxy. And so what we see in our COI belt, Pluto and Friends are the survivors of this most dramatic event. We think that our soul system donated 10 to the 16. So what's that? That's something like, let's say a hundred million billion objects to the galaxy. And if every other solar system does that, objects like ua, MUA are the most common macroscopic objects, the most common things in the galaxy. There are billions and billions and billions of them out there. They're passing throughout solar system all the time. Calculations show with a large error bar that there's almost always something the size of Ur muah within the orbit of Neptune. We are just extraordinarily bad at spotting them because they're small, they're dark, and they move fast. And so we shouldn't really be surprised that we saw Ur muah. We should be surprised that we've only found two of these things. And this is the answer to the mystery that I left open at the start of the talk. How do we jump from things that are bolder sized to skyscraper sized? Well, AMU few hundred meters across is a flying skyscraper. That disc of material around the young star will be peppered with these interstellar objects flying through from other solar systems. And Suzanne Felner and Michelle Banister have proposed the idea that the seeds of planet formation come from these interstellar objects. Uh, get started quick way of building a solar system, the equivalent of a ready meal. I guess if you start with the disc capturing these objects, then you can quickly grow a planet. Alright? You have to form planet somewhere the slow way. Somebody has to do it the hard way. Somewhere in the dim distant past of the Milky Way, there was a star with a long-lived disc that could have formed planets, which would then have scattered 10 to the 16 in stellar objects. Those would've kickstarted more planet formation. And suddenly you have this runaway process that spreads very quickly through the galaxy and allows us to form planets rapidly and makes it so that we live in a universe where planets are common and there are worlds to explore. So though UA isn't, I promise it's not an alien spacecraft, there's too many of the things. And the fact we found two and we'll find more tells us that it's still, I think, has a message for us, which is that what's happened here, that in explaining our solar system, we've got no choice but to consider ourselves as part of a galaxy connected by these scouts, these messengers that wander between the stars. Thank you very much Now by my book, Chris. Thank you very much. And let's all just take a moment Is on there. You see it's on the cover. So it is relevant in the, in the blue. Fantastic. Well, wow. Well we've had it all, we've had a world that's not a planet, but uh, it's sort of like one of those things. Like if it looks like a planet and it smells like a planet, why is it not a planet too small? Okay, it's too small. We're Gonna keep coming back to Pluto. I I promise to get it into every talk now. Excellent. And then we've also had, um, a Flying Magnum that was, that was a highlight. So loads to loads of those to talk about. Lots of questions. Um, and I think actually, right, okay, let's just hit the alien issue <laugh> right now.'cause um, I'm not gonna lie to you Chris, that is a feature and quite a lot. Hello. The internet of these, Of these questions. So just to pick up on that last point you made, and I think sort of collect some of these questions together. So obviously there's a degree to which some in humor, some not. So in humor there's a disappointment. This isn't an alien, this isn't, um, Arthur Clark's vision. This isn't a spaceship. But actually, um, I thought it was really interesting how you said, well, if we can get past some of our expectations, not just about what we should be looking for, what we should be seeing or what we would like to see, but actually it's really exciting to, like the, the implications of something like this could actually be really enormous for how we consider the universe. Is that about That's right. And I think one of the things Ur taught me, and I've changed what I'm working on to, to pay attention to, to things like this is that we should be ready to be surprised by the universe. This is our solar system, right? We've lived in it for a while. We've sent probes into it, we've mapped it, yes. All right. We find new asteroids daily, there's a catalog of a few million of them. Um, this is our backyard and yet Umu tells us that we can be caught unawares. That the unexpected can still happen. That actually in the solar system there may be types of object we haven't thought of yet. There may be a population of hidden comets in the distance solar system. There may be things in unusual orbits, there may be an alien spacecraft lurking between Mars and Jupiter that we have at value. We don't know. And so particularly I'm working on this thing, this is the Vera Rubbin observatory. This is going to do a new survey of the sky, which we'll find in its first year, we think 6 million asteroids. And one response to that is to say, great, we've got lots more asteroids. The other is say, okay, which 10 of those are most unusual and what are their properties and what do they tell us? Now my guess is that they'll tell us about details of how the solar system formed and uh, how it perhaps has evolved in the 4 billion years. But we should consider the possibility of looking for what we call techno signatures. So spacecraft in there as well. But we can do that by looking for weird astronomical things. And only when we've ruled out an astronomical origin, then I'll turn around and say, okay, maybe, maybe. But for amu muah, there, there's no evidence it's anything other than a really fascinating astronomical object. Great. So you're not quite, you're not taking aliens away from off. They're not off the table completely. Nope. They're just slightly to the side. And if we Well, they, they shouldn't be your first. I mean, I think where I, I disagree. So, so a avy Loeb who, who is the advocate of these things would say that they should be the first thing we think of that when we see something unusual, we should say, is this aliens before we do anything else? And that's fine. It doesn't really get me very far.'cause the answer is always maybe, uh, what I have to do is see if there's another alternative explanation first and then get driven to the, um, more extreme or, or or more unusual claims at the end. Okay. So if we maybe challenge 'em of our expectations or maybe aspirations, then we might be able to ask different kinds of questions, which is always really interesting for science. So let's take a question from the floor, A couple of observations. The, the, uh, those two objects, one eye and two eye, um, remain unique. Um, of course when the Vera Rubian telescope starts, starts operating, then we may find a lot if we don't, and in particular in the case of the, the more mysterious one, which is, which is one eye then, given that it is, and I'm told this by people who understand these things, it's is in principle possible to send a mission to it? Um, should we do so? Yeah. So, so there a few questions there. So yeah, so with the Rubin telescope, we think we're gonna find about a hundred of these interstellar objects, but that number could be 10 or it could be a thousand. It depends slightly on how optimistic we're feeling. Um, because we are trying to extrapolate from two, I should say we've work. I've got PhD student, Matthew Hopkins who's been working on this. And um, we can predict what these things will be like based on what we know about the galaxy. And we think that even if we find 50 of them, UR more may still look odd because it is on the extreme. It's not unusual. It's unusual, but it's not a complete outlier. So we shouldn't be surprised if the first 10 that we find all look like Borisov, they look like comets. We will find a population of the, we think we'll find a population of like things, but we don't know. So we'll have to find out. There's also a NASA mission, um, that's again looking for near earth asteroids that will launch in 2028, which we'll find lots of these things. So in 10 years time we should have a few hundred and then we can could say something sensible. Can, should we, should we launch Mission Tou? If we could. I definitely would be fascinating. There are all sorts of questions about it as you've seen. Um, I, for a sensible budget, um, and a sensible timescale. I don't think we can get there in time.'cause the problem is we only watched it for a few months and so the error on its trajectory is large. And so I think we're close to the point now that even if you had all the money in the world and you could launch with our best Rocket a satellite right now, you wouldn't necessarily find it even if you could hit the speed. So I think it's gone sadly. I think that's true. I would suggest you look at something called Project. Yeah, yeah. I know about, yeah. Thank you. I, I-I-I-I-I love their optimism.<laugh>, I'm gonna take one more question, Should say, by the way, we're going to do another episode of the Gresham podcast. Any questions? So we can answer more questions? So do send them in and I promise I'll get to them. Yeah. Um, What are the chances of New Horizon finding further objects and would we find out important things if It did? Yeah, it's a good question. So, um, the New Horizons team, um, spent a lot of time using Hubble, looking for a third object that they can go to. They don't have much fuel left on board, so they can't really divert. So they need to find something, um, in the direct, direct line. And they've more or less given up on that. What they're doing instead is they, they, it does have a telescope on board, so it's doing observations of, uh, relatively nearby objects. After all, it's much closer to them than we are. And so we're actually building up a library of images taken from quite a long distance, but much less than we have of these objects. So it's sort of become an astronomy satellite. And it's also monitoring the, the effect of the sun on the outer solar system. So it's still doing science, but I don't think we'll get another fly by like we did with Aof, unless we get really lucky. Okay. Great. Well, thank you so much. There's lots and lots of questions. Unfortunately we're out of time. But as Chris has just kindly volunteered to do another, any further questions podcast, look out for that. And I'm sure if your question's on here, you'll get to it. Even if it is about Aliens, it's still not aliens. I'm sorry. Chris then taught everyone.