In 1781, William Herschel, an astronomer in England, was scanning the skies making a map of where stars were in the sky. And he got to this one location, and he found something that he thought might be a comet, might be a nebula, he couldn't tell. It didn't quite look like a pin point star. It looked like a little fuzzy thing. Four days later, he went back and re-checked and it had moved. The fact that it moved, that one simple fact showed him that it wasn't a star. It wasn't a nebula, it wasn't a galaxy, it was in fact something in our solar system. Interestingly though it had moved very little, and the fact that it had moved very little suggested that it was quite far away. Herschel thought, at first, that he had found a comet. Those were the only things that were far away that we knew of. But very quickly, people started observing it more carefully and suggesting that, in fact, this might be a planet. A new planet, found beyond the orbit of Saturn. It's kind of an astounding thing to think of for people who had grown up all of human civilization not knowing that Saturn was the end of our solar system. There were continued debates whether it was a comet, whether it was a planet, what exactly was this for decades. And the debate was finally solved in 1820 by an astronomer named Alexis Bouvard. Alexis Bouvard was, well, he was many things. You can read all of his many titles here to see exactly what he was. But he was the director of the observatory in Paris and he realized that you could watch Uranus going around the sun. It had been discovered in 1781. It was now 1820, 40 years going around the sun. And you could follow it and you could also then try to predict where it should have been in the past. And Bouvard realized that you could go backwards in time, find other people who had been making maps of the sky. And see if perhaps they had inadvertently included Uranus in their maps. And he found them. Here's the critical table from this entire book that he published. If you look at this table here, well first you can look over here at all the equations. You don't have to look at the equations, but look at this table. You can see things like, here on date 1781, here is the discovery of Uranus right here. Here are observations all of the subsequent years, you can see in these tables. And in these columns, you can see how far Uranus is away from where it's supposed to be. If these were all zeroes, that will be good. If these are small numbers that will be good. But these were big enough numbers that, even at a time, there's something clearly going on. But what's more interesting is here's the discovery in 1781, and Bouvard was able to go back 1781, all the way to 1690, find all these old observations. And they really were important for extending the time baseline back by 90 years. And you can still see these uncertainties, these discrepancies between where Uranus should have been and where it was. Bouvard, as a theoretical astronomer, looked at his theory beautiful equations, looked at the observations, discrepancies and did what all good theoretical astronomers do. He blamed the observers. If you read the French in through here, he basically says, you know, my theory is perfect. And it's probably just that the observers didn't know what they were talking about. He does admit one other possibility down here at the end that maybe there's something else out there. Starting in 1820 when this was published, the observations started getting better and better. People were being extra careful to see where Uranus was and make sure they had very precise measurements. And from 1820 til 1845 there were still discrepancies like this. And in the end, Bouvard took his data of all these discrepancies and gave it to an astronomer also at the Paris Observatory named Urbain Le Verrier. And he said there must be a planet, go figure out where it is. Le Verrier, it took him awhile to do all the calculations, but Le Verrier found that if you had that the sun here and here was Uranus going around the Sun. And maybe sometimes when Uranus was here it was moving a little bit more slowly than it should have. And when Uranus was over it was moving a little more quickly than it should have. And a little more quickly on the way around and then a little more slowly. That those discrepancies in where it should have been suggested a planet right here. And he predicted that there should be an eighth planet out beyond Uranus. He sent his predictions to the observatory in Berlin, they opened up their telescope. And that very night they found Neptune, pretty amazing moment of discovery. You can imagine that if somebody finds Neptune by looking at perturbations of Uranus that the next obvious thing to do is to look for something else out there by the obvious perturbations of Neptune. Astronomers started those predictions from nearly the very first minute after Le Verrier announced his prediction of the eighth planet. And there were predictions of ninth planets for decades and decades and decades. The famous one, that most people have heard of of course, is the prediction of Planet X. And Planet X is associated with somebody whose name you might remember, Percival Lowell. He was trying to predict the existence of this Planet X based on perturbations to Neptune. And he thought he had an answer. He thought that Neptune was being nicely perturbed and he thought he could predict where this planet should be. And he had some people go and take an image and the image looked sort of like this. And well, there's no Planet X in there. At least there's no obvious large Planet X in the way that when Herschel found Uranus, he knew it was something, something big or something fuzzy. Here, there's nothing big or nothing fuzzy. Lowell, of course, built the Lowell Observatory. And the reason for building that observatory, a prime reason, was to search for his Planet X to prove himself right. And he didn't live to see the search take place. But in 1930 Clyde Tombaugh was hired to work at the observatory and to do the search for Planet X. And Clyde Tombaugh took this image that we're looking at. And he realised that like Herschel from 1781, what he needed to do was look for things that move. So Clyde Tombaugh took this picture and compared it with a picture a couple of days later and did you see it move? You didn't notice it. Okay, let's go back and look again. Here's the first night. Here's the next night. It's not easy because in these photographs, everything kind of moves around a little bit. They don't actually move around in the sky but it looks like they do just because their rotations and things got jostled. First night, second night. If you haven't seen it yet, I'll make it easy for you. First night, second night and for fun, first night, second night again. These are in fact the discovery images of Pluto. And Pluto, well, was it Planet X? Planet X was this thing that Lowell was looking for that was perturbing Neptune. And it was the reason it was supposed to be perturbing Neptune is because Planet X was supposed to be huge. So huge, that well, that doesn't look so huge. Doesn't matter, at the time of the announcement of the discovery of Pluto, this headline appeared in the New York Times. Ninth planet discovered on the edge of the solar system. Okay, so far so good. 25 years after search by late Percival Lowell, seen in Flagstaff, special photo-telescope, awesome. Here's the kicker though, astronomers hail finding, certainly true. The sphere, possibly larger than Jupiter meets predictions. Actually, it's the meets predictions that might even be the worst part. Possibly larger than Jupiter? Not actually true. It turns out that's wrong by a factor of 50,000. Meets predictions though is about true because people thought they were looking for something big. When they found Pluto, they declared it to be a planet even though it was wrong by a factor of 50,000. We have a name for that sort of thing these days. We call it Fake News. Okay, maybe that's not quite fair. They really did think it met the predictions. It had been predicted. It was found. It must have been true. But, it turns out that this little dot, and I've lost it already, where did it go? So small that I can never figure out which one it is. It's one of these in here somewhere. That little dot is not actually bigger than Jupiter. How big is it? Let's take a look. I'm showing you here a picture of the actual relative sizes of the actual planets. You almost never see something like this. You always see them at their wrong sizes with, with Mars almost as big as Jupiter. Turns out that's not true. Here's Jupiter, so big it doesn't even fit in the background. Saturn here, even without it's rings, as you know is quite large. Uranus and Neptune, ice giants, a good bit smaller. Mercury, Venus, Earth, Mars, the terrestrial planets hiding in through there. There's Ceres. Remember, we saw images of Ceres? It's the biggest object in the asteroid belt and one of the few that's big enough to even appear on this scale. There are ten others that are big enough to appear as a point on this scale. And what about Pluto? There it is. This is the real size of Pluto. Turns out it's not about the size of Jupiter. And also, as the New York Times said, at least as large as the Earth. It's actually smaller than our moon. The discovery was a coincidence. We now know that Neptune is not being perturbed by any massive object in the outer solar system. And the only reason people thought it was in the past is because we didn't quite know the mass of Uranus or Neptune well enough. And they weren't known well enough until 1989 when the Voyager spacecraft made it's final fly by of Neptune. Pluto just happened to be the largest member of the Kuiper Belt that could be seen, the brightest member of the Kuiper Belt that could be seen. It was close. It's very reflective so it was easy to see. If we put the other known members of the Kuiper Belt there to scale, well, you see some of your favorite dwarf planets that we talked about earlier. And then you see some of these other smaller one in through here too. And what you really see, I think quite obviously, is that this population is not the same as this population, which is not the same as this population. It's pretty hard to argue from looking at a picture like this that yeah, maybe that one right there. We should call it a planet, and that one. But, not the rest of these. And yeah, and we should classify it with that. It's always nice to look at the top-down view of the Kuiper Belt just to remember where the objects are. Here again, you see Jupiter, Saturn, Uranus, Neptune right here on the inner edge of the Kuiper Belt. Pluto, just if you want to single it out, is there in the white orbit. In each one of these things is the orbit of one of the many, many known Kuiper Belt objects. You can see that, for the most part, they're confined to this region in through here, and that they span the entire range of the outer solar system. One object is quite different. This is the orbit of the Kuiper Belt object that we call Sedna. And, if you remember, Sedna was on that list of the top ten largest Kuiper Belt objects. And look at the strange thing about it. Here's the rest of the Kuiper Belt we just looked at in here, and Sedna goes really far away. So, that's astounding to begin with. But, really the main astounding thing is it never gets close to the Kuiper Belt to begin with. Now if you remember, we talked about formation of planetesimals and things. Remember that everything needed to form in circular orbits so that their collisions could be quite slow and grazing. You can't form things on orbits like this. The only way to get things on to orbits like this is to perturb them with something like, well, maybe a planet. But the problem is, if you had an object in the Kuiper Belt and it got close to Neptune, maybe it could get shot out on to an orbit like this, but it would have to come back to where Neptune is. The hard part is getting it to be pushed out in this direction. This was a mystery back when Sedna was discovered in 2003. And if you wanted an interesting historic note, you can go to the end of this class and find the bonus lectures for the previous lecture, where I described what I thought was going on with Sedna. In 2012, a different group of astronomers found another object. 2012 VP113 is the official license plate number. Biden, because he was the Vice President at the time. I didn't make that name up. But, notice it, too, has that same characteristic. At the time of the discovery the astronomers who discovered it said, this must mean there's a planet out there. But, I remind you from about 1847 onward astronomers had been saying, there's a planet out there once every several months. Every time anything slightly unusual was found in the outer solar system, astronomers would jump to the conclusion that there must be a planet. My colleague Konstantin Batygin, who has an office right down the hall, and I started looking and realized something that we thought was pretty astounding. If you look at all of the most distant objects in the solar system, and Sedna in there, I think that's Sedna right there, in fact. And VP112 is probably that one. But, you look at all the rest of these. They're all swept off in this direction in an astounding way. After working on this for a year, we became convinced that the only explanation was there actually has to be a distant and massive planet in orbit around the sun, on a very eccentric orbit. A very eccentric orbit, we thought, probably went something like this, and essentially herded these little Kuiper Belt objects into place. To test that idea we tried out computer simulations. In these computer simulations you can see, you can't see because it's covered up. The sun is right here in the middle. Planet is on this orbit out here like this, and all of these other blue points are Kuiper Belt objects that we initially start randomly strewing around the sky. We let them interact gravitationally with this planet for four billion years and we see what happened. Now, and what happens? Well, the first thing you notice is that the planet very quickly clears out many of the objects that were nearby it. There are many fewer objects in the Kuiper Belt now, in this extended Kuiper Belt, than there were to begin with. What are we looking for? Well, we're looking to see if it captures objects into orbits that are like this. That's what we want to see. And the answer, well, I'll give you the preview. The answer is it never does. Our initial hypothesis on how a planet might actually do this was wrong. What's it doing? Well, lets watch very carefully and objects are going to get captured onto these orbits, these anti-aligned orbits with the giant planets. There they are. We're almost at three and a half billion years now, and this is starting to look a lot like the solar system that we see now. Over time it finally clears out, and finally clears out. And at four billion years, we really are left with something that looks a little bit familiar. So, let's go back to those objects in the Kuiper Belt that we've seen. If Kuiper Belt objects are going to be swept off in this direction by a planet, that planet is pointing in the opposite direction. What's interesting is I didn't show you this before, but if you look at those Kuiper Belt objects they're all actually tilted compared to the plane of the solar system too. And that shows us that that planet, which we now call Planet Nine must also be likewise tilted. So is it true? Is it really out there? In the time since we first proposed this we have found more and more evidence of different things from the solar system that are being perturbed by this massive planet out there. I would say, the probability that it's not true is much lower than the probability that it is true. If there is no planet out there, there are so many things that need to be explained in the outer solar system that we currently have no explanation for. Now, people have been saying this since 1847. So, it sounds a little bit crazy to sit here and say, well, everybody has been wrong for 170 years, but we're right. But, I do have something I'd like to say, which is that everybody had been wrong for 170 years. But we're right. Let me show you what this planet looks like. Okay, we don't actually know what it looks like. But we know how massive it is. The reason we know how massive it is, is because we do those computer simulations and we put in a less massive and a more massive planet. And we make the one that fits the best. And the answer is, it has to be something like 10 times the mass of the earth. It puts it between Earth which, is one times the mass of the Earth. And Neptune or Uranus, which are 15-17 times the mass of the Earth. 10 times the mass of the Earth, very interesting number if you recall. 10 times the mass of the Earth, well, we don't have anything that we know of in our solar system that mass, but it's one of the most common masses throughout the galaxy of the things that have been found by Kepler. So rather than Planet Nine making the solar system stranger, I would say it actually makes more sense. 10 Earth masses is not just common in the galaxy, as you will remember, 10 Earth masses is about the mass of a core of a giant planet. The most likely explanation for why Planet Nine is out there, not that we know the answer, but I think the most likely explanation is that Planet Nine formed in the same region as Uranus and Neptune and Jupiter and Saturn. And during that period of the dynamical instability that we talk so much about, planet nine was beginning to be a planet. It was beginning to be a gas giant and had formed a core. It started creating gas, and it got ejected to the outer part of the Solar System to stick around until the end of time. There's still some problems that we have to work out, we can't explain everything about Planet Nine, yet. But I think that this object, that looks, well, I don't know, we made it look like a little bit like Neptune. We even put lightning on the back of it, because that seemed cool. But we think this mini Neptune is lurking out there. So let's go find it. Here are my two favorite telescopes in the world, the twin Keck telescopes on the summit of Mauna Kea. You can see Maui there in the background. We're not going to use my favorite telescopes in the world, we're actually going to use this one, the Subaru telescope right next door. And in the next lecture, we'll actually take a trip up to the Subaru telescope. And what are we going to do? Well, you know what we're gonnado? We're going to take pictures of the sky, and we're going to come back the next day and take pictures of the sky, and we're going to look for things that move, we're going to do Herschel All over again. We're going to do Clyde Tombaugh all over again. This time we're looking for something that's much further away, that's much fainter, that requires using some of these biggest telescopes in the world. We're going to be able to answer some of these biggest questions that we have in the world. Where are we going to look? Well, we can actually come up with a pretty good prediction for where Planet Nine is. We know how the orbit is tilted compared to the solar system. We know where it's closest to the Sun. We know where it's furthest from the Sun. We also know that if it were closest to the Sun, we should have seen it by now. So there's a swath where it's furthest from the Sun, hardest to see, but well within our reach that we think we're going to find it. It's in one of my favorite areas of the sky, if you look up at the sky and find Orion. Orion is one of the constellations that I think may be the most visible constellation, recognizable for people around the entire world. It would have been fun if it went right through the middle of Orion. But the path of Planet Nine, it turns out, goes right through not Orion. But next door is Taurus. It goes right through here, like this. Here's Taurus, if I did this right. Let's see. There's the. Somewhere in here are the horns. I can't quite draw the horns. I think they must be like this. This is Aldebaran. If it's up tonight, go find Orion, looks right next door. Find [INAUDIBLE] on the bright star, find the Plates, they're pretty unmistakable. Look halfway in between there, that's about the path of Planet Nine. Where in its path is it? Well, I think the answer is right in through here. In fact, the swath that we're planning on looking at is literally right here. If we could look through all of these areas of the sky one night, the next night, look for something that move. Look for something that's very faint. Look for something that's very far away. We will have covered the search area for Planet Nine. It's easily in range of some of these biggest telescopes, of the Subaru Telescope in particular. And with enough time, we'll get there. Also, since we have announced the presence of Planet 9, dozens of other teams have started looking too. So I actually think that there's a good chance that somebody else may find it first. That's fine with me. I'm actually just very excited to see if we can use these same techniques that were used by Le Verrier in 1846 to realize that there really are perturbations going on in the outer solar system. And to finally be the first one is to get it right in 170 years, and say, hey, there's a planet out there, and we go look, and there's a planet out there.
