0:00:01.000,0:00:03.792 I'm going to tell you a story[br]from 200 years ago. 0:00:04.792,0:00:08.059 In 1820, French astronomer Alexis Bouvard 0:00:08.083,0:00:13.309 almost became the second person[br]in human history to discover a planet. 0:00:13.333,0:00:16.643 He'd been tracking the position[br]of Uranus across the night sky 0:00:16.667,0:00:18.393 using old star catalogs, 0:00:18.417,0:00:20.726 and it didn't quite go around the Sun 0:00:20.750,0:00:23.184 the way that his predictions[br]said it should. 0:00:23.208,0:00:25.309 Sometimes it was a little too fast, 0:00:25.333,0:00:27.018 sometimes a little too slow. 0:00:27.042,0:00:30.768 Bouvard knew that[br]his predictions were perfect. 0:00:30.792,0:00:34.059 So it had to be that those[br]old star catalogs were bad. 0:00:34.083,0:00:36.143 He told astronomers of the day, 0:00:36.167,0:00:38.601 "Do better measurements." 0:00:38.625,0:00:39.893 So they did. 0:00:39.917,0:00:42.059 Astronomers spent the next two decades 0:00:42.083,0:00:46.143 meticulously tracking the position[br]of Uranus across the sky, 0:00:46.167,0:00:49.851 but it still didn't fit[br]Bouvard's predictions. 0:00:49.875,0:00:52.018 By 1840, it had become obvious. 0:00:52.042,0:00:55.101 The problem was not[br]with those old star catalogs, 0:00:55.125,0:00:57.976 the problem was with the predictions. 0:00:58.000,0:00:59.518 And astronomers knew why. 0:00:59.542,0:01:03.726 They realized that there must be[br]a distant, giant planet 0:01:03.750,0:01:05.434 just beyond the orbit of Uranus 0:01:05.458,0:01:07.268 that was tugging along at that orbit, 0:01:07.292,0:01:09.851 sometimes pulling it along a bit too fast, 0:01:09.875,0:01:11.542 sometimes holding it back. 0:01:12.750,0:01:14.768 Must have been frustrating back in 1840 0:01:14.792,0:01:18.184 to see these gravitational effects[br]of this distant, giant planet 0:01:18.208,0:01:21.976 but not yet know how to actually find it. 0:01:22.000,0:01:24.059 Trust me, it's really frustrating. 0:01:24.083,0:01:25.559 (Laughter) 0:01:25.583,0:01:27.809 But in 1846, another French astronomer, 0:01:27.833,0:01:29.143 Urbain Le Verrier, 0:01:29.167,0:01:30.434 worked through the math 0:01:30.458,0:01:33.184 and figured out how to predict[br]the location of the planet. 0:01:33.208,0:01:36.184 He sent his prediction[br]to the Berlin observatory, 0:01:36.208,0:01:37.643 they opened up their telescope 0:01:37.667,0:01:40.726 and in the very first night[br]they found this faint point of light 0:01:40.750,0:01:42.851 slowly moving across the sky 0:01:42.875,0:01:44.143 and discovered Neptune. 0:01:44.167,0:01:48.292 It was this close on the sky[br]to Le Verrier's predicted location. 0:01:49.875,0:01:54.393 The story of prediction[br]and discrepancy and new theory 0:01:54.417,0:01:57.476 and triumphant discoveries is so classic 0:01:57.500,0:02:00.393 and Le Verrier became so famous from it, 0:02:00.417,0:02:03.143 that people tried to get in[br]on the act right away. 0:02:03.167,0:02:05.684 In the last 163 years, 0:02:05.708,0:02:11.309 dozens of astronomers have used[br]some sort of alleged orbital discrepancy 0:02:11.333,0:02:15.250 to predict the existence[br]of some new planet in the solar system. 0:02:16.292,0:02:19.000 They have always been wrong. 0:02:20.125,0:02:22.309 The most famous[br]of these erroneous predictions 0:02:22.333,0:02:23.768 came from Percival Lowell, 0:02:23.792,0:02:28.518 who was convinced that there must be[br]a planet just beyond Uranus and Neptune, 0:02:28.542,0:02:30.518 messing with those orbits. 0:02:30.542,0:02:33.101 And so when Pluto was discovered in 1930 0:02:33.125,0:02:34.768 at the Lowell Observatory, 0:02:34.792,0:02:39.226 everybody assumed that it must be[br]the planet that Lowell had predicted. 0:02:39.250,0:02:41.643 They were wrong. 0:02:41.667,0:02:45.768 It turns out, Uranus and Neptune[br]are exactly where they're supposed to be. 0:02:45.792,0:02:47.351 It took 100 years, 0:02:47.375,0:02:49.143 but Bouvard was eventually right. 0:02:49.167,0:02:52.768 Astronomers needed to do[br]better measurements. 0:02:52.792,0:02:54.559 And when they did, 0:02:54.583,0:02:57.768 those better measurements[br]had turned out that 0:02:57.792,0:03:02.809 there is no planet just beyond[br]the orbit of Uranus and Neptune 0:03:02.833,0:03:05.518 and Pluto is thousands of times too small 0:03:05.542,0:03:08.184 to have any effect on those orbits at all. 0:03:08.208,0:03:11.851 So even though Pluto[br]turned out not to be the planet 0:03:11.875,0:03:13.476 it was originally thought to be, 0:03:13.500,0:03:16.934 it was the first discovery[br]of what is now known to be 0:03:16.958,0:03:21.684 thousands of tiny, icy objects[br]in orbit beyond the planets. 0:03:21.708,0:03:24.601 Here you can see the orbits of Jupiter, 0:03:24.625,0:03:27.143 Saturn, Uranus and Neptune, 0:03:27.167,0:03:30.184 and in that little circle[br]in the very center is the Earth 0:03:30.208,0:03:33.184 and the Sun and almost everything[br]that you know and love. 0:03:33.208,0:03:35.018 And those yellow circles at the edge 0:03:35.042,0:03:37.809 are these icy bodies[br]out beyond the planets. 0:03:37.833,0:03:40.101 These icy bodies are pushed and pulled 0:03:40.125,0:03:42.268 by the gravitational fields of the planets 0:03:42.292,0:03:44.809 in entirely predictable ways. 0:03:44.833,0:03:49.542 Everything goes around the Sun[br]exactly the way it is supposed to. 0:03:50.958,0:03:52.226 Almost. 0:03:52.250,0:03:54.309 So in 2003, 0:03:54.333,0:03:56.226 I discovered what was at the time 0:03:56.250,0:03:59.934 the most distant known object[br]in the entire solar system. 0:03:59.958,0:04:02.434 It's hard not to look[br]at that lonely body out there 0:04:02.458,0:04:04.518 and say, oh yeah, sure,[br]so Lowell was wrong, 0:04:04.542,0:04:06.476 there was no planet just beyond Neptune, 0:04:06.500,0:04:09.059 but this, this could be a new planet. 0:04:09.083,0:04:10.559 The real question we had was, 0:04:10.583,0:04:12.893 what kind of orbit[br]does it have around the Sun? 0:04:12.917,0:04:14.851 Does it go in a circle around the Sun 0:04:14.875,0:04:16.434 like a planet should? 0:04:16.458,0:04:20.351 Or is it just a typical member[br]of this icy belt of bodies 0:04:20.375,0:04:24.434 that got a little bit tossed outward[br]and it's now on its way back in? 0:04:24.458,0:04:26.976 This is precisely the question 0:04:27.000,0:04:31.601 the astronomers were trying[br]to answer about Uranus 200 years ago. 0:04:31.625,0:04:35.393 They did it by using[br]overlooked observations of Uranus 0:04:35.417,0:04:37.768 from 91 years before its discovery 0:04:37.792,0:04:39.518 to figure out its entire orbit. 0:04:39.542,0:04:41.559 We couldn't go quite that far back, 0:04:41.583,0:04:46.184 but we did find observations[br]of our object from 13 years earlier 0:04:46.208,0:04:48.893 that allowed us to figure out[br]how it went around the Sun. 0:04:48.917,0:04:50.184 So the question is, 0:04:50.208,0:04:52.934 is it in a circular orbit[br]around the Sun, like a planet, 0:04:52.958,0:04:54.351 or is it on its way back in, 0:04:54.375,0:04:56.268 like one of these typical icy bodies? 0:04:56.292,0:04:57.976 And the answer is 0:04:58.000,0:04:59.268 no. 0:04:59.292,0:05:02.059 It has a massively elongated orbit 0:05:02.083,0:05:06.018 that takes 10,000 years[br]to go around the Sun. 0:05:06.042,0:05:08.059 We named this object Sedna 0:05:08.083,0:05:09.934 after the Inuit goddess of the sea, 0:05:09.958,0:05:14.018 in honor of the cold, icy places[br]where it spends all of its time. 0:05:14.042,0:05:15.643 We now know that Sedna, 0:05:15.667,0:05:17.434 it's about a third the size of Pluto 0:05:17.458,0:05:19.518 and it's a relatively typical member 0:05:19.542,0:05:22.351 of those icy bodies out beyond Neptune. 0:05:22.375,0:05:26.226 Relatively typical,[br]except for this bizarre orbit. 0:05:26.250,0:05:28.018 You might look at this orbit and say, 0:05:28.042,0:05:30.768 "Yeah, that's bizarre,[br]10,000 years to go around the Sun," 0:05:30.792,0:05:32.726 but that's not really the bizarre part. 0:05:32.750,0:05:34.941 The bizarre part is[br]that in those 10,000 years, 0:05:34.965,0:05:38.934 Sedna never comes close[br]to anything else in the solar system. 0:05:38.958,0:05:41.268 Even at its closest approach to the Sun, 0:05:41.292,0:05:43.601 Sedna is further from Neptune 0:05:43.625,0:05:45.833 than Neptune is from the Earth. 0:05:47.042,0:05:49.184 If Sedna had had an orbit like this, 0:05:49.208,0:05:51.809 that kisses the orbit of Neptune[br]once around the Sun, 0:05:51.833,0:05:54.851 that would have actually been[br]really easy to explain. 0:05:54.875,0:05:56.643 That would have just been an object 0:05:56.667,0:05:58.934 that had been in[br]a circular orbit around the Sun 0:05:58.958,0:06:00.393 in that region of icy bodies, 0:06:00.417,0:06:02.941 had gotten a little bit[br]too close to Neptune one time, 0:06:02.965,0:06:05.799 and then got slingshot out[br]and is now on its way back in. 0:06:07.333,0:06:12.059 But Sedna never comes close[br]to anything known in the solar system 0:06:12.083,0:06:14.476 that could have given it that slingshot. 0:06:14.500,0:06:16.518 Neptune can't be responsible, 0:06:16.542,0:06:19.643 but something had to be responsible. 0:06:19.667,0:06:22.601 This was the first time since 1845 0:06:22.625,0:06:27.559 that we saw the gravitational effects[br]of something in the outer solar system 0:06:27.583,0:06:29.083 and didn't know what it was. 0:06:30.208,0:06:33.101 I actually thought I knew[br]what the answer was. 0:06:33.125,0:06:37.143 Sure, it could have been[br]some distant, giant planet 0:06:37.167,0:06:38.434 in the outer solar system, 0:06:38.458,0:06:40.809 but by this time,[br]that idea was so ridiculous 0:06:40.833,0:06:42.684 and had been so thoroughly discredited 0:06:42.708,0:06:44.518 that I didn't take it very seriously. 0:06:44.542,0:06:45.809 But 4.5 billion years ago, 0:06:45.833,0:06:50.684 when the Sun formed in a cocoon[br]of hundreds of other stars, 0:06:50.708,0:06:51.976 any one of those stars 0:06:52.000,0:06:54.643 could have gotten[br]just a little bit too close to Sedna 0:06:54.667,0:06:58.643 and perturbed it onto the orbit[br]that it has today. 0:06:58.667,0:07:02.559 When that cluster of stars[br]dissipated into the galaxy, 0:07:02.583,0:07:06.351 the orbit of Sedna would have been[br]left as a fossil record 0:07:06.375,0:07:08.851 of this earliest history of the Sun. 0:07:08.875,0:07:10.684 I was so excited by this idea, 0:07:10.708,0:07:12.184 by the idea that we could look 0:07:12.208,0:07:14.434 at the fossil history[br]of the birth of the Sun, 0:07:14.458,0:07:16.059 that I spent the next decade 0:07:16.083,0:07:18.809 looking for more objects[br]with orbits like Sedna. 0:07:18.833,0:07:22.268 In that ten-year period, I found zero. 0:07:22.292,0:07:23.309 (Laughter) 0:07:23.333,0:07:26.851 But my colleagues, Chad Trujillo[br]and Scott Sheppard, did a better job, 0:07:26.875,0:07:29.893 and they have now found several objects[br]with orbits like Sedna, 0:07:29.917,0:07:31.684 which is super exciting. 0:07:31.708,0:07:33.232 But what's even more interesting 0:07:33.256,0:07:36.018 is that they found that all these objects 0:07:36.042,0:07:39.934 are not only on these distant,[br]elongated orbits, 0:07:39.958,0:07:45.309 they also share a common value[br]of this obscure orbital parameter 0:07:45.333,0:07:49.292 that in celestial mechanics we call[br]argument of perihelion. 0:07:50.250,0:07:53.184 When they realized it was clustered[br]in argument of perihelion, 0:07:53.208,0:07:54.976 they immediately jumped up and down, 0:07:55.000,0:07:57.976 saying it must be caused[br]by a distant, giant planet out there, 0:07:58.000,0:08:01.059 which is really exciting,[br]except it makes no sense at all. 0:08:01.083,0:08:03.601 Let me try to explain it[br]to you why with an analogy. 0:08:03.625,0:08:06.934 Imagine a person walking down a plaza 0:08:06.958,0:08:10.292 and looking 45 degrees to his right side. 0:08:11.125,0:08:13.184 There's a lot of reasons[br]that might happen, 0:08:13.208,0:08:15.143 it's super easy to explain, no big deal. 0:08:15.167,0:08:16.976 Imagine now many different people, 0:08:17.000,0:08:20.893 all walking in different[br]directions across the plaza, 0:08:20.917,0:08:24.184 but all looking 45 degrees[br]to the direction that they're moving. 0:08:24.208,0:08:26.226 Everybody's moving[br]in different directions, 0:08:26.250,0:08:28.393 everybody's looking[br]in different directions, 0:08:28.417,0:08:31.726 but they're all looking 45 degrees[br]to the direction of motion. 0:08:31.750,0:08:33.583 What could cause something like that? 0:08:34.917,0:08:36.184 I have no idea. 0:08:36.208,0:08:39.934 It's very difficult to think of any reason[br]that that would happen. 0:08:39.958,0:08:41.309 (Laughter) 0:08:41.333,0:08:44.184 And this is essentially[br]what that clustering 0:08:44.208,0:08:47.601 in argument of perihelion was telling us. 0:08:47.625,0:08:51.184 Scientists were generally baffled[br]and they assumed it must just be a fluke 0:08:51.208,0:08:52.559 and some bad observations. 0:08:52.583,0:08:54.351 They told the astronomers, 0:08:54.375,0:08:56.768 "Do better measurements." 0:08:56.792,0:08:59.893 I actually took a very careful look[br]at those measurements, though, 0:08:59.917,0:09:01.184 and they were right. 0:09:01.208,0:09:03.101 These objects really did all share 0:09:03.125,0:09:05.601 a common value of argument of perihelion, 0:09:05.625,0:09:07.018 and they shouldn't. 0:09:07.042,0:09:09.250 Something had to be causing that. 0:09:11.125,0:09:15.434 The final piece of the puzzle[br]came into place in 2016, 0:09:15.458,0:09:17.976 when my colleague, Konstantin Batygin, 0:09:18.000,0:09:20.643 who works three doors down from me, and I 0:09:20.667,0:09:23.268 realized that the reason[br]that everybody was baffled 0:09:23.292,0:09:28.018 was because argument of perihelion[br]was only part of the story. 0:09:28.042,0:09:30.101 If you look at these[br]objects the right way, 0:09:30.125,0:09:34.184 they are all actually lined up[br]in space in the same direction, 0:09:34.208,0:09:37.934 and they're all tilted in space[br]in the same direction. 0:09:37.958,0:09:42.309 It's as if all those people on the plaza[br]are all walking in the same direction 0:09:42.333,0:09:45.768 and they're all looking[br]45 degrees to the right side. 0:09:45.792,0:09:47.059 That's easy to explain. 0:09:47.083,0:09:49.559 They're all looking at something. 0:09:49.583,0:09:53.708 These objects in the outer solar system[br]are all reacting to something. 0:09:55.000,0:09:56.726 But what? 0:09:56.750,0:09:59.726 Konstantin and I spent a year 0:09:59.750,0:10:04.559 trying to come up with any explanation[br]other than a distant, giant planet 0:10:04.583,0:10:05.851 in the outer solar system. 0:10:05.875,0:10:11.309 We did not want to be the 33rd and 34th[br]people in history to propose this planet 0:10:11.333,0:10:13.667 to yet again be told we were wrong. 0:10:14.792,0:10:16.559 But after a year, 0:10:16.583,0:10:17.893 there was really no choice. 0:10:17.917,0:10:20.059 We could come up with no other explanation 0:10:20.083,0:10:22.583 other than that there is a distant, 0:10:22.625,0:10:25.893 massive planet on an elongated orbit, 0:10:25.917,0:10:27.976 inclined to the rest of the solar system, 0:10:28.000,0:10:30.726 that is forcing these patterns[br]for these objects 0:10:30.750,0:10:32.768 in the outer solar system. 0:10:32.792,0:10:34.934 Guess what else a planet like this does. 0:10:34.958,0:10:36.809 Remember that strange orbit of Sedna, 0:10:36.833,0:10:39.768 how it was kind of pulled away[br]from the Sun in one direction? 0:10:39.792,0:10:43.518 A planet like this would make[br]orbits like that all day long. 0:10:43.542,0:10:45.851 We knew we were onto something. 0:10:45.875,0:10:48.851 So this brings us to today. 0:10:48.875,0:10:53.059 We are basically 1845, Paris. 0:10:53.083,0:10:54.268 (Laughter) 0:10:54.292,0:10:59.601 We see the gravitational effects[br]of a distant, giant planet, 0:10:59.625,0:11:01.851 and we are trying to work out[br]the calculations 0:11:01.875,0:11:04.893 to tell us where to look,[br]to point our telescopes, 0:11:04.917,0:11:06.184 to find this planet. 0:11:06.208,0:11:08.583 We've done massive suites[br]of computer simulations, 0:11:09.292,0:11:11.226 massive months of analytic calculations 0:11:11.250,0:11:13.809 and here's what I can tell you so far. 0:11:13.833,0:11:17.018 First, this planet,[br]which we call Planet Nine, 0:11:17.042,0:11:19.625 because that's what it is, 0:11:20.750,0:11:24.018 Planet Nine is six times[br]the mass of the Earth. 0:11:24.042,0:11:26.268 This is no slightly-smaller-than-Pluto, 0:11:26.292,0:11:29.018 let's-all-argue-about-[br]whether-it's-a-planet-or-not thing. 0:11:29.042,0:11:32.351 This is the fifth largest planet[br]in our entire solar system. 0:11:32.375,0:11:36.018 For context, let me show you[br]the sizes of the planets. 0:11:36.042,0:11:40.184 In the back there,[br]you can the massive Jupiter and Saturn. 0:11:40.208,0:11:42.851 Next to them, a little bit smaller,[br]Uranus and Neptune. 0:11:42.875,0:11:46.352 Up in the corner, the terrestrial planets,[br]Mercury, Venus, Earth and Mars. 0:11:46.376,0:11:47.726 You can even see that belt 0:11:47.750,0:11:50.893 of icy bodies beyond Neptune,[br]of which Pluto is a member, 0:11:50.917,0:11:52.775 good luck figuring out which one it is. 0:11:52.799,0:11:55.215 And here is Planet Nine. 0:11:56.583,0:11:59.018 Planet Nine is big. 0:11:59.042,0:12:00.309 Planet Nine is so big, 0:12:00.333,0:12:02.934 you should probably wonder[br]why haven't we found it yet. 0:12:02.958,0:12:04.226 Well, Planet Nine is big, 0:12:04.250,0:12:06.351 but it's also really, really far away. 0:12:06.375,0:12:11.059 It's something like[br]15 times further away than Neptune. 0:12:11.083,0:12:14.351 And that makes it about 50,000 times[br]fainter than Neptune. 0:12:14.375,0:12:17.309 And also, the sky is a really big place. 0:12:17.333,0:12:19.476 We've narrowed down where we think it is 0:12:19.500,0:12:22.018 to a relatively small area of the sky, 0:12:22.042,0:12:23.934 but it would still take us years 0:12:23.958,0:12:26.309 to systematically cover[br]the area of the sky 0:12:26.333,0:12:28.184 with the large telescopes that we need 0:12:28.208,0:12:31.518 to see something that's[br]this far away and this faint. 0:12:31.542,0:12:34.684 Luckily, we might not have to. 0:12:34.708,0:12:39.601 Just like Bouvard used[br]unrecognized observations of Uranus 0:12:39.625,0:12:42.393 from 91 years before its discovery, 0:12:42.417,0:12:46.184 I bet that there are unrecognized images 0:12:46.208,0:12:49.083 that show the location of Planet Nine. 0:12:50.000,0:12:53.059 It's going to be a massive[br]computational undertaking 0:12:53.083,0:12:55.393 to go through all of the old data 0:12:55.417,0:12:58.333 and pick out that one faint moving planet. 0:12:59.292,0:13:00.643 But we're underway. 0:13:00.667,0:13:02.976 And I think we're getting close. 0:13:03.000,0:13:05.518 So I would say, get ready. 0:13:05.542,0:13:09.518 We are not going to match Le Verrier's 0:13:09.542,0:13:10.809 "make a prediction, 0:13:10.833,0:13:12.726 have the planet found in a single night 0:13:12.750,0:13:14.934 that close to where[br]you predicted it" record. 0:13:14.958,0:13:18.893 But I do bet that within[br]the next couple of years 0:13:18.917,0:13:21.309 some astronomer somewhere 0:13:21.333,0:13:23.559 will find a faint point of light, 0:13:23.583,0:13:25.851 slowly moving across the sky 0:13:25.875,0:13:29.184 and triumphantly announce[br]the discovery of a new, 0:13:29.208,0:13:31.643 and quite possibly not the last, 0:13:31.667,0:13:34.143 real planet of our solar system. 0:13:34.167,0:13:35.434 Thank you. 0:13:35.458,0:13:39.083 (Applause)