WEBVTT 00:00:01.000 --> 00:00:03.792 I'm going to tell you a story from 200 years ago. 00:00:04.792 --> 00:00:08.059 In 1820, French astronomer Alexis Bouvard 00:00:08.083 --> 00:00:13.309 almost became the second person in human history to discover a planet. 00:00:13.333 --> 00:00:16.643 He'd been tracking the position of Uranus across the night sky 00:00:16.667 --> 00:00:18.393 using old star catalogs, 00:00:18.417 --> 00:00:20.726 and it didn't quite go around the Sun 00:00:20.750 --> 00:00:23.184 the way that his predictions said it should. 00:00:23.208 --> 00:00:25.309 Sometimes it was a little too fast, 00:00:25.333 --> 00:00:27.018 sometimes a little too slow. 00:00:27.042 --> 00:00:30.768 Bouvard knew that his predictions were perfect. 00:00:30.792 --> 00:00:34.059 So it had to be that those old star catalogs were bad. 00:00:34.083 --> 00:00:36.143 He told astronomers of the day, 00:00:36.167 --> 00:00:38.601 "Do better measurements." 00:00:38.625 --> 00:00:39.893 So they did. 00:00:39.917 --> 00:00:42.059 Astronomers spent the next two decades 00:00:42.083 --> 00:00:46.143 meticulously tracking the position of Uranus across the sky, 00:00:46.167 --> 00:00:49.851 but it still didn't fit Bouvard's predictions. NOTE Paragraph 00:00:49.875 --> 00:00:52.018 By 1840, it had become obvious. 00:00:52.042 --> 00:00:55.101 The problem was not with those old star catalogs, 00:00:55.125 --> 00:00:57.976 the problem was with the predictions. 00:00:58.000 --> 00:00:59.518 And astronomers knew why. 00:00:59.542 --> 00:01:03.726 They realized that there must be a distant, giant planet 00:01:03.750 --> 00:01:05.434 just beyond the orbit of Uranus 00:01:05.458 --> 00:01:07.268 that was tugging along at that orbit, 00:01:07.292 --> 00:01:09.851 sometimes pulling it along a bit too fast, 00:01:09.875 --> 00:01:11.542 sometimes holding it back. NOTE Paragraph 00:01:12.750 --> 00:01:14.768 Must have been frustrating back in 1840 00:01:14.792 --> 00:01:18.184 to see these gravitational effects of this distant, giant planet 00:01:18.208 --> 00:01:21.976 but not yet know how to actually find it. 00:01:22.000 --> 00:01:24.059 Trust me, it's really frustrating. NOTE Paragraph 00:01:24.083 --> 00:01:25.559 (Laughter) NOTE Paragraph 00:01:25.583 --> 00:01:27.809 But in 1846, another French astronomer, 00:01:27.833 --> 00:01:29.143 Urbain Le Verrier, 00:01:29.167 --> 00:01:30.434 worked through the math 00:01:30.458 --> 00:01:33.184 and figured out how to predict the location of the planet. 00:01:33.208 --> 00:01:36.184 He sent his prediction to the Berlin observatory, 00:01:36.208 --> 00:01:37.643 they opened up their telescope 00:01:37.667 --> 00:01:40.726 and in the very first night they found this faint point of light 00:01:40.750 --> 00:01:42.851 slowly moving across the sky 00:01:42.875 --> 00:01:44.143 and discovered Neptune. 00:01:44.167 --> 00:01:48.292 It was this close on the sky to Le Verrier's predicted location. NOTE Paragraph 00:01:49.875 --> 00:01:54.393 The story of prediction and discrepancy and new theory 00:01:54.417 --> 00:01:57.476 and triumphant discoveries is so classic 00:01:57.500 --> 00:02:00.393 and Le Verrier became so famous from it, 00:02:00.417 --> 00:02:03.143 that people tried to get in on the act right away. 00:02:03.167 --> 00:02:05.684 In the last 163 years, 00:02:05.708 --> 00:02:11.309 dozens of astronomers have used some sort of alleged orbital discrepancy 00:02:11.333 --> 00:02:15.250 to predict the existence of some new planet in the solar system. NOTE Paragraph 00:02:16.292 --> 00:02:19.000 They have always been wrong. NOTE Paragraph 00:02:20.125 --> 00:02:22.309 The most famous of these erroneous predictions 00:02:22.333 --> 00:02:23.768 came from Percival Lowell, 00:02:23.792 --> 00:02:28.518 who was convinced that there must be a planet just beyond Uranus and Neptune, 00:02:28.542 --> 00:02:30.518 messing with those orbits. 00:02:30.542 --> 00:02:33.101 And so when Pluto was discovered in 1930 00:02:33.125 --> 00:02:34.768 at the Lowell Observatory, 00:02:34.792 --> 00:02:39.226 everybody assumed that it must be the planet that Lowell had predicted. 00:02:39.250 --> 00:02:41.643 They were wrong. 00:02:41.667 --> 00:02:45.768 It turns out, Uranus and Neptune are exactly where they're supposed to be. 00:02:45.792 --> 00:02:47.351 It took 100 years, 00:02:47.375 --> 00:02:49.143 but Bouvard was eventually right. 00:02:49.167 --> 00:02:52.768 Astronomers needed to do better measurements. 00:02:52.792 --> 00:02:54.559 And when they did, 00:02:54.583 --> 00:02:57.768 those better measurements had turned out that 00:02:57.792 --> 00:03:02.809 there is no planet just beyond the orbit of Uranus and Neptune 00:03:02.833 --> 00:03:05.518 and Pluto is thousands of times too small 00:03:05.542 --> 00:03:08.184 to have any effect on those orbits at all. NOTE Paragraph 00:03:08.208 --> 00:03:11.851 So even though Pluto turned out not to be the planet 00:03:11.875 --> 00:03:13.476 it was originally thought to be, 00:03:13.500 --> 00:03:16.934 it was the first discovery of what is now known to be 00:03:16.958 --> 00:03:21.684 thousands of tiny, icy objects in orbit beyond the planets. 00:03:21.708 --> 00:03:24.601 Here you can see the orbits of Jupiter, 00:03:24.625 --> 00:03:27.143 Saturn, Uranus and Neptune, 00:03:27.167 --> 00:03:30.184 and in that little circle in the very center is the Earth 00:03:30.208 --> 00:03:33.184 and the Sun and almost everything that you know and love. 00:03:33.208 --> 00:03:35.018 And those yellow circles at the edge 00:03:35.042 --> 00:03:37.809 are these icy bodies out beyond the planets. 00:03:37.833 --> 00:03:40.101 These icy bodies are pushed and pulled 00:03:40.125 --> 00:03:42.268 by the gravitational fields of the planets 00:03:42.292 --> 00:03:44.809 in entirely predictable ways. 00:03:44.833 --> 00:03:49.542 Everything goes around the Sun exactly the way it is supposed to. NOTE Paragraph 00:03:50.958 --> 00:03:52.226 Almost. NOTE Paragraph 00:03:52.250 --> 00:03:54.309 So in 2003, 00:03:54.333 --> 00:03:56.226 I discovered what was at the time 00:03:56.250 --> 00:03:59.934 the most distant known object in the entire solar system. 00:03:59.958 --> 00:04:02.434 It's hard not to look at that lonely body out there 00:04:02.458 --> 00:04:04.518 and say, oh yeah, sure, so Lowell was wrong, 00:04:04.542 --> 00:04:06.476 there was no planet just beyond Neptune, 00:04:06.500 --> 00:04:09.059 but this, this could be a new planet. 00:04:09.083 --> 00:04:10.559 The real question we had was, 00:04:10.583 --> 00:04:12.893 what kind of orbit does it have around the Sun? 00:04:12.917 --> 00:04:14.851 Does it go in a circle around the Sun 00:04:14.875 --> 00:04:16.434 like a planet should? 00:04:16.458 --> 00:04:20.351 Or is it just a typical member of this icy belt of bodies 00:04:20.375 --> 00:04:24.434 that got a little bit tossed outward and it's now on its way back in? NOTE Paragraph 00:04:24.458 --> 00:04:26.976 This is precisely the question 00:04:27.000 --> 00:04:31.601 the astronomers were trying to answer about Uranus 200 years ago. 00:04:31.625 --> 00:04:35.393 They did it by using overlooked observations of Uranus 00:04:35.417 --> 00:04:37.768 from 91 years before its discovery 00:04:37.792 --> 00:04:39.518 to figure out its entire orbit. 00:04:39.542 --> 00:04:41.559 We couldn't go quite that far back, 00:04:41.583 --> 00:04:46.184 but we did find observations of our object from 13 years earlier 00:04:46.208 --> 00:04:48.893 that allowed us to figure out how it went around the Sun. NOTE Paragraph 00:04:48.917 --> 00:04:50.184 So the question is, 00:04:50.208 --> 00:04:52.934 is it in a circular orbit around the Sun, like a planet, 00:04:52.958 --> 00:04:54.351 or is it on its way back in, 00:04:54.375 --> 00:04:56.268 like one of these typical icy bodies? 00:04:56.292 --> 00:04:57.976 And the answer is 00:04:58.000 --> 00:04:59.268 no. NOTE Paragraph 00:04:59.292 --> 00:05:02.059 It has a massively elongated orbit 00:05:02.083 --> 00:05:06.018 that takes 10,000 years to go around the Sun. 00:05:06.042 --> 00:05:08.059 We named this object Sedna 00:05:08.083 --> 00:05:09.934 after the Inuit goddess of the sea, 00:05:09.958 --> 00:05:14.018 in honor of the cold, icy places where it spends all of its time. 00:05:14.042 --> 00:05:15.643 We now know that Sedna, 00:05:15.667 --> 00:05:17.434 it's about a third the size of Pluto 00:05:17.458 --> 00:05:19.518 and it's a relatively typical member 00:05:19.542 --> 00:05:22.351 of those icy bodies out beyond Neptune. 00:05:22.375 --> 00:05:26.226 Relatively typical, except for this bizarre orbit. 00:05:26.250 --> 00:05:28.018 You might look at this orbit and say, 00:05:28.042 --> 00:05:30.768 "Yeah, that's bizarre, 10,000 years to go around the Sun," 00:05:30.792 --> 00:05:32.726 but that's not really the bizarre part. 00:05:32.750 --> 00:05:34.941 The bizarre part is that in those 10,000 years, 00:05:34.965 --> 00:05:38.934 Sedna never comes close to anything else in the solar system. 00:05:38.958 --> 00:05:41.268 Even at its closest approach to the Sun, 00:05:41.292 --> 00:05:43.601 Sedna is further from Neptune 00:05:43.625 --> 00:05:45.833 than Neptune is from the Earth. NOTE Paragraph 00:05:47.042 --> 00:05:49.184 If Sedna had had an orbit like this, 00:05:49.208 --> 00:05:51.809 that kisses the orbit of Neptune once around the Sun, 00:05:51.833 --> 00:05:54.851 that would have actually been really easy to explain. 00:05:54.875 --> 00:05:56.643 That would have just been an object 00:05:56.667 --> 00:05:58.934 that had been in a circular orbit around the Sun 00:05:58.958 --> 00:06:00.393 in that region of icy bodies, 00:06:00.417 --> 00:06:02.941 had gotten a little bit too close to Neptune one time, 00:06:02.965 --> 00:06:05.799 and then got slingshot out and is now on its way back in. NOTE Paragraph 00:06:07.333 --> 00:06:12.059 But Sedna never comes close to anything known in the solar system 00:06:12.083 --> 00:06:14.476 that could have given it that slingshot. 00:06:14.500 --> 00:06:16.518 Neptune can't be responsible, 00:06:16.542 --> 00:06:19.643 but something had to be responsible. 00:06:19.667 --> 00:06:22.601 This was the first time since 1845 00:06:22.625 --> 00:06:27.559 that we saw the gravitational effects of something in the outer solar system 00:06:27.583 --> 00:06:29.083 and didn't know what it was. NOTE Paragraph 00:06:30.208 --> 00:06:33.101 I actually thought I knew what the answer was. 00:06:33.125 --> 00:06:37.143 Sure, it could have been some distant, giant planet 00:06:37.167 --> 00:06:38.434 in the outer solar system, 00:06:38.458 --> 00:06:40.809 but by this time, that idea was so ridiculous 00:06:40.833 --> 00:06:42.684 and had been so thoroughly discredited 00:06:42.708 --> 00:06:44.518 that I didn't take it very seriously. 00:06:44.542 --> 00:06:45.809 But 4.5 billion years ago, 00:06:45.833 --> 00:06:50.684 when the Sun formed in a cocoon of hundreds of other stars, 00:06:50.708 --> 00:06:51.976 any one of those stars 00:06:52.000 --> 00:06:54.643 could have gotten just a little bit too close to Sedna 00:06:54.667 --> 00:06:58.643 and perturbed it onto the orbit that it has today. 00:06:58.667 --> 00:07:02.559 When that cluster of stars dissipated into the galaxy, 00:07:02.583 --> 00:07:06.351 the orbit of Sedna would have been left as a fossil record 00:07:06.375 --> 00:07:08.851 of this earliest history of the Sun. NOTE Paragraph 00:07:08.875 --> 00:07:10.684 I was so excited by this idea, 00:07:10.708 --> 00:07:12.184 by the idea that we could look 00:07:12.208 --> 00:07:14.434 at the fossil history of the birth of the Sun, 00:07:14.458 --> 00:07:16.059 that I spent the next decade 00:07:16.083 --> 00:07:18.809 looking for more objects with orbits like Sedna. 00:07:18.833 --> 00:07:22.268 In that ten-year period, I found zero. NOTE Paragraph 00:07:22.292 --> 00:07:23.309 (Laughter) NOTE Paragraph 00:07:23.333 --> 00:07:26.851 But my colleagues, Chad Trujillo and Scott Sheppard, did a better job, 00:07:26.875 --> 00:07:29.893 and they have now found several objects with orbits like Sedna, 00:07:29.917 --> 00:07:31.684 which is super exciting. NOTE Paragraph 00:07:31.708 --> 00:07:33.232 But what's even more interesting 00:07:33.256 --> 00:07:36.018 is that they found that all these objects 00:07:36.042 --> 00:07:39.934 are not only on these distant, elongated orbits, 00:07:39.958 --> 00:07:45.309 they also share a common value of this obscure orbital parameter 00:07:45.333 --> 00:07:49.292 that in celestial mechanics we call argument of perihelion. 00:07:50.250 --> 00:07:53.184 When they realized it was clustered in argument of perihelion, 00:07:53.208 --> 00:07:54.976 they immediately jumped up and down, 00:07:55.000 --> 00:07:57.976 saying it must be caused by a distant, giant planet out there, 00:07:58.000 --> 00:08:01.059 which is really exciting, except it makes no sense at all. NOTE Paragraph 00:08:01.083 --> 00:08:03.601 Let me try to explain it to you why with an analogy. 00:08:03.625 --> 00:08:06.934 Imagine a person walking down a plaza 00:08:06.958 --> 00:08:10.292 and looking 45 degrees to his right side. 00:08:11.125 --> 00:08:13.184 There's a lot of reasons that might happen, 00:08:13.208 --> 00:08:15.143 it's super easy to explain, no big deal. 00:08:15.167 --> 00:08:16.976 Imagine now many different people, 00:08:17.000 --> 00:08:20.893 all walking in different directions across the plaza, 00:08:20.917 --> 00:08:24.184 but all looking 45 degrees to the direction that they're moving. 00:08:24.208 --> 00:08:26.226 Everybody's moving in different directions, 00:08:26.250 --> 00:08:28.393 everybody's looking in different directions, 00:08:28.417 --> 00:08:31.726 but they're all looking 45 degrees to the direction of motion. 00:08:31.750 --> 00:08:33.583 What could cause something like that? 00:08:34.917 --> 00:08:36.184 I have no idea. 00:08:36.208 --> 00:08:39.934 It's very difficult to think of any reason that that would happen. NOTE Paragraph 00:08:39.958 --> 00:08:41.309 (Laughter) NOTE Paragraph 00:08:41.333 --> 00:08:44.184 And this is essentially what that clustering 00:08:44.208 --> 00:08:47.601 in argument of perihelion was telling us. NOTE Paragraph 00:08:47.625 --> 00:08:51.184 Scientists were generally baffled and they assumed it must just be a fluke 00:08:51.208 --> 00:08:52.559 and some bad observations. 00:08:52.583 --> 00:08:54.351 They told the astronomers, 00:08:54.375 --> 00:08:56.768 "Do better measurements." 00:08:56.792 --> 00:08:59.893 I actually took a very careful look at those measurements, though, 00:08:59.917 --> 00:09:01.184 and they were right. 00:09:01.208 --> 00:09:03.101 These objects really did all share 00:09:03.125 --> 00:09:05.601 a common value of argument of perihelion, 00:09:05.625 --> 00:09:07.018 and they shouldn't. 00:09:07.042 --> 00:09:09.250 Something had to be causing that. NOTE Paragraph 00:09:11.125 --> 00:09:15.434 The final piece of the puzzle came into place in 2016, 00:09:15.458 --> 00:09:17.976 when my colleague, Konstantin Batygin, 00:09:18.000 --> 00:09:20.643 who works three doors down from me, and I 00:09:20.667 --> 00:09:23.268 realized that the reason that everybody was baffled 00:09:23.292 --> 00:09:28.018 was because argument of perihelion was only part of the story. 00:09:28.042 --> 00:09:30.101 If you look at these objects the right way, 00:09:30.125 --> 00:09:34.184 they are all actually lined up in space in the same direction, 00:09:34.208 --> 00:09:37.934 and they're all tilted in space in the same direction. 00:09:37.958 --> 00:09:42.309 It's as if all those people on the plaza are all walking in the same direction 00:09:42.333 --> 00:09:45.768 and they're all looking 45 degrees to the right side. 00:09:45.792 --> 00:09:47.059 That's easy to explain. 00:09:47.083 --> 00:09:49.559 They're all looking at something. 00:09:49.583 --> 00:09:53.708 These objects in the outer solar system are all reacting to something. 00:09:55.000 --> 00:09:56.726 But what? NOTE Paragraph 00:09:56.750 --> 00:09:59.726 Konstantin and I spent a year 00:09:59.750 --> 00:10:04.559 trying to come up with any explanation other than a distant, giant planet 00:10:04.583 --> 00:10:05.851 in the outer solar system. 00:10:05.875 --> 00:10:11.309 We did not want to be the 33rd and 34th people in history to propose this planet 00:10:11.333 --> 00:10:13.667 to yet again be told we were wrong. 00:10:14.792 --> 00:10:16.559 But after a year, 00:10:16.583 --> 00:10:17.893 there was really no choice. 00:10:17.917 --> 00:10:20.059 We could come up with no other explanation 00:10:20.083 --> 00:10:22.583 other than that there is a distant, 00:10:22.625 --> 00:10:25.893 massive planet on an elongated orbit, 00:10:25.917 --> 00:10:27.976 inclined to the rest of the solar system, 00:10:28.000 --> 00:10:30.726 that is forcing these patterns for these objects 00:10:30.750 --> 00:10:32.768 in the outer solar system. NOTE Paragraph 00:10:32.792 --> 00:10:34.934 Guess what else a planet like this does. 00:10:34.958 --> 00:10:36.809 Remember that strange orbit of Sedna, 00:10:36.833 --> 00:10:39.768 how it was kind of pulled away from the Sun in one direction? 00:10:39.792 --> 00:10:43.518 A planet like this would make orbits like that all day long. 00:10:43.542 --> 00:10:45.851 We knew we were onto something. NOTE Paragraph 00:10:45.875 --> 00:10:48.851 So this brings us to today. 00:10:48.875 --> 00:10:53.059 We are basically 1845, Paris. NOTE Paragraph 00:10:53.083 --> 00:10:54.268 (Laughter) NOTE Paragraph 00:10:54.292 --> 00:10:59.601 We see the gravitational effects of a distant, giant planet, 00:10:59.625 --> 00:11:01.851 and we are trying to work out the calculations 00:11:01.875 --> 00:11:04.893 to tell us where to look, to point our telescopes, 00:11:04.917 --> 00:11:06.184 to find this planet. NOTE Paragraph 00:11:06.208 --> 00:11:08.583 We've done massive suites of computer simulations, 00:11:09.292 --> 00:11:11.226 massive months of analytic calculations 00:11:11.250 --> 00:11:13.809 and here's what I can tell you so far. NOTE Paragraph 00:11:13.833 --> 00:11:17.018 First, this planet, which we call Planet Nine, 00:11:17.042 --> 00:11:19.625 because that's what it is, 00:11:20.750 --> 00:11:24.018 Planet Nine is six times the mass of the Earth. 00:11:24.042 --> 00:11:26.268 This is no slightly-smaller-than-Pluto, 00:11:26.292 --> 00:11:29.018 let's-all-argue-about- whether-it's-a-planet-or-not thing. 00:11:29.042 --> 00:11:32.351 This is the fifth largest planet in our entire solar system. 00:11:32.375 --> 00:11:36.018 For context, let me show you the sizes of the planets. 00:11:36.042 --> 00:11:40.184 In the back there, you can the massive Jupiter and Saturn. 00:11:40.208 --> 00:11:42.851 Next to them, a little bit smaller, Uranus and Neptune. 00:11:42.875 --> 00:11:46.352 Up in the corner, the terrestrial planets, Mercury, Venus, Earth and Mars. 00:11:46.376 --> 00:11:47.726 You can even see that belt 00:11:47.750 --> 00:11:50.893 of icy bodies beyond Neptune, of which Pluto is a member, 00:11:50.917 --> 00:11:52.775 good luck figuring out which one it is. 00:11:52.799 --> 00:11:55.215 And here is Planet Nine. 00:11:56.583 --> 00:11:59.018 Planet Nine is big. 00:11:59.042 --> 00:12:00.309 Planet Nine is so big, 00:12:00.333 --> 00:12:02.934 you should probably wonder why haven't we found it yet. 00:12:02.958 --> 00:12:04.226 Well, Planet Nine is big, 00:12:04.250 --> 00:12:06.351 but it's also really, really far away. 00:12:06.375 --> 00:12:11.059 It's something like 15 times further away than Neptune. 00:12:11.083 --> 00:12:14.351 And that makes it about 50,000 times fainter than Neptune. 00:12:14.375 --> 00:12:17.309 And also, the sky is a really big place. 00:12:17.333 --> 00:12:19.476 We've narrowed down where we think it is 00:12:19.500 --> 00:12:22.018 to a relatively small area of the sky, 00:12:22.042 --> 00:12:23.934 but it would still take us years 00:12:23.958 --> 00:12:26.309 to systematically cover the area of the sky 00:12:26.333 --> 00:12:28.184 with the large telescopes that we need 00:12:28.208 --> 00:12:31.518 to see something that's this far away and this faint. 00:12:31.542 --> 00:12:34.684 Luckily, we might not have to. NOTE Paragraph 00:12:34.708 --> 00:12:39.601 Just like Bouvard used unrecognized observations of Uranus 00:12:39.625 --> 00:12:42.393 from 91 years before its discovery, 00:12:42.417 --> 00:12:46.184 I bet that there are unrecognized images 00:12:46.208 --> 00:12:49.083 that show the location of Planet Nine. 00:12:50.000 --> 00:12:53.059 It's going to be a massive computational undertaking 00:12:53.083 --> 00:12:55.393 to go through all of the old data 00:12:55.417 --> 00:12:58.333 and pick out that one faint moving planet. 00:12:59.292 --> 00:13:00.643 But we're underway. 00:13:00.667 --> 00:13:02.976 And I think we're getting close. NOTE Paragraph 00:13:03.000 --> 00:13:05.518 So I would say, get ready. 00:13:05.542 --> 00:13:09.518 We are not going to match Le Verrier's 00:13:09.542 --> 00:13:10.809 "make a prediction, 00:13:10.833 --> 00:13:12.726 have the planet found in a single night 00:13:12.750 --> 00:13:14.934 that close to where you predicted it" record. 00:13:14.958 --> 00:13:18.893 But I do bet that within the next couple of years 00:13:18.917 --> 00:13:21.309 some astronomer somewhere 00:13:21.333 --> 00:13:23.559 will find a faint point of light, 00:13:23.583 --> 00:13:25.851 slowly moving across the sky 00:13:25.875 --> 00:13:29.184 and triumphantly announce the discovery of a new, 00:13:29.208 --> 00:13:31.643 and quite possibly not the last, 00:13:31.667 --> 00:13:34.143 real planet of our solar system. NOTE Paragraph 00:13:34.167 --> 00:13:35.434 Thank you. NOTE Paragraph 00:13:35.458 --> 00:13:39.083 (Applause)