0:00:01.008,0:00:04.341 I'm going to tell you a story[br]from 200 years ago. 0:00:04.785,0:00:08.039 In 1820, French astronomer Alexis Bouvard 0:00:08.063,0:00:12.895 almost became the second person[br]in human history to discover a planet. 0:00:13.330,0:00:16.648 He'd been tracking the position[br]of Uranus across the night sky 0:00:16.672,0:00:18.406 using old star catalogs, 0:00:18.430,0:00:20.728 and it didn't quite go around the Sun 0:00:20.752,0:00:23.172 the way that his predictions[br]said it should. 0:00:23.196,0:00:25.299 Sometimes it was a little too fast, 0:00:25.323,0:00:27.006 sometimes it was a little too slow. 0:00:27.030,0:00:30.258 Bouvard knew that[br]his predictions were perfect. 0:00:30.778,0:00:34.072 So it had to be that those[br]old star catalogs were bad. 0:00:34.096,0:00:36.158 He told astronomers of the day, 0:00:36.182,0:00:37.849 "Do better measurements." 0:00:38.620,0:00:39.778 So they did. 0:00:39.802,0:00:42.047 Astronomers spent the next two decades 0:00:42.071,0:00:46.142 meticulously tracking the position[br]of Uranus across the sky, 0:00:46.166,0:00:49.841 but it still didn't fit[br]Bouvard's predictions. 0:00:49.865,0:00:52.017 By 1840, it had become obvious. 0:00:52.041,0:00:55.093 The problem was not[br]with those old star catalogs, 0:00:55.117,0:00:57.584 the problem was with the predictions. 0:00:57.998,0:00:59.537 And astronomers knew why. 0:00:59.561,0:01:03.736 They realized that there must be[br]a distant, giant planet 0:01:03.760,0:01:05.416 just beyond the orbit of Uranus 0:01:05.440,0:01:07.275 that was tugging along at that orbit, 0:01:07.299,0:01:09.871 sometimes pulling it along a bit too fast, 0:01:09.895,0:01:11.561 sometimes holding it back. 0:01:12.768,0:01:14.776 Must have been frustrating back in 1840 0:01:14.800,0:01:18.179 to see these gravitational effects[br]of this distant, giant planet 0:01:18.203,0:01:21.361 but not yet know how to actually find it. 0:01:21.996,0:01:24.068 Trust me, it's really frustrating. 0:01:24.092,0:01:25.552 (Laughter) 0:01:25.576,0:01:27.828 But in 1846, another French astronomer, 0:01:27.852,0:01:29.133 Urbain Le Verrier, 0:01:29.157,0:01:30.329 worked through the math 0:01:30.353,0:01:33.069 and figured out how to predict[br]the location of the planet. 0:01:33.093,0:01:36.193 He sent his prediction[br]to the Berlin observatory, 0:01:36.217,0:01:37.662 they opened up their telescope 0:01:37.686,0:01:40.741 and in the very first night[br]they found this faint point of light 0:01:40.765,0:01:44.044 slowly moving across the sky[br]and discovered Neptune. 0:01:44.068,0:01:48.294 It was this close on the sky[br]to Le Verrier's predicted location. 0:01:49.862,0:01:54.410 The story of prediction[br]and discrepancy and new theory 0:01:54.434,0:01:57.388 and triumphant discoveries is so classic 0:01:57.412,0:02:00.403 and Le Verrier became so famous from it, 0:02:00.427,0:02:03.141 that people tried to get in[br]on the act right away. 0:02:03.165,0:02:05.680 In the last 163 years, 0:02:05.704,0:02:11.319 dozens of astronomers have used[br]some sort of alleged orbital discrepancy 0:02:11.343,0:02:15.498 to predict the existence[br]of some new planet in the Solar system. 0:02:16.292,0:02:19.006 They have always been wrong. 0:02:20.141,0:02:22.307 The most famous of these[br]erroneous predictions 0:02:22.331,0:02:23.769 came from Percival Lowell, 0:02:23.793,0:02:28.504 who was convinced that there must be[br]a planet just beyond Uranus and Neptune, 0:02:28.528,0:02:30.520 messing with those orbits. 0:02:30.544,0:02:33.107 And so when Pluto was discovered in 1930, 0:02:33.131,0:02:34.774 at the Lowell Observatory, 0:02:34.798,0:02:38.780 everybody assumed that it must be[br]the planet that Lowell had predicted. 0:02:39.244,0:02:41.180 They were wrong. 0:02:41.680,0:02:45.747 It turns out, Uranus and Neptune[br]are exactly where they're supposed to be. 0:02:45.771,0:02:47.363 It took 100 years, 0:02:47.387,0:02:49.133 but Bouvard was eventually right. 0:02:49.157,0:02:52.775 Astronomers needed to do[br]better measurements. 0:02:52.799,0:02:54.578 And when they did, 0:02:54.602,0:02:57.760 those better measurements[br]had turned out that 0:02:57.784,0:03:02.808 there is no planet just beyond[br]the orbit of Uranus and Neptune 0:03:02.832,0:03:05.506 and Pluto is thousands of times too small 0:03:05.530,0:03:08.180 to have any effect on those orbits at all. 0:03:08.204,0:03:11.863 So even though Pluto[br]turned out not to be the planet 0:03:11.887,0:03:13.466 it was originally thought to be, 0:03:13.490,0:03:16.918 it was the first discovery[br]of what is now known to be 0:03:16.942,0:03:21.704 thousands of tiny icy objects[br]in orbit beyond the planets. 0:03:21.728,0:03:24.601 Here you can see the orbits of Jupiter, 0:03:24.625,0:03:27.163 Saturn, Uranus and Neptune, 0:03:27.187,0:03:30.187 and in that little circle[br]in the very center is the Earth 0:03:30.211,0:03:33.185 and the Sun and almost everything[br]that you know and love. 0:03:33.209,0:03:35.028 And those yellow circles at the edge 0:03:35.052,0:03:37.789 are these icy bodies[br]out beyond the planets. 0:03:37.813,0:03:40.107 These icy bodies are pushed and pulled 0:03:40.131,0:03:42.250 by the gravitational fields of the planets 0:03:42.274,0:03:44.805 in entirely predictable ways. 0:03:44.829,0:03:49.549 Everything goes around the Sun[br]exactly the way it is supposed to. 0:03:50.950,0:03:52.108 Almost. 0:03:52.132,0:03:54.307 So in 2003, 0:03:54.331,0:03:56.227 I discovered, what was at the time, 0:03:56.251,0:03:59.592 the most distant known object[br]in the entire Solar system. 0:03:59.978,0:04:02.391 It's hard not to look[br]at that lonely body out there 0:04:02.415,0:04:04.436 and say, oh yeah sure,[br]so Lowell was wrong, 0:04:04.460,0:04:06.373 there was no planet just beyond Neptune, 0:04:06.397,0:04:08.669 but this, this could be a new planet. 0:04:09.089,0:04:10.617 The real question we had was, 0:04:10.641,0:04:12.879 what kind of orbit[br]does it have around the Sun? 0:04:12.903,0:04:14.839 Does it go in a circle around the Sun 0:04:14.863,0:04:16.441 like a planet should? 0:04:16.465,0:04:20.363 Or is it just a typical member[br]of this icy belt of bodies 0:04:20.387,0:04:23.886 that got a little bit tossed outward[br]and it's now on its way back in? 0:04:24.442,0:04:26.956 This is precisely the question 0:04:26.980,0:04:31.236 the astronomers were trying[br]to answer about Uranus 200 years ago. 0:04:31.616,0:04:35.412 They did it by using[br]overlooked observations of Uranus 0:04:35.436,0:04:39.506 from 91 years before its discovery[br]to figure out its entire orbit. 0:04:39.530,0:04:41.553 We couldn't go quite that far back, 0:04:41.577,0:04:46.172 but we did find observations[br]of our object from 13 years earlier, 0:04:46.196,0:04:48.879 that allowed us to figure out[br]how it went around the Sun. 0:04:48.903,0:04:50.101 So the question is, 0:04:50.125,0:04:52.934 is it in a circular orbit[br]around the Sun, like a planet, 0:04:52.958,0:04:54.332 or is it on its way back in, 0:04:54.356,0:04:56.276 like one of these typical icy bodies? 0:04:56.300,0:04:57.964 And the answer is 0:04:57.988,0:04:59.146 no. 0:04:59.170,0:05:02.057 It has a massively elongated orbit 0:05:02.081,0:05:05.634 that takes 10,000 years[br]to go around the Sun. 0:05:06.049,0:05:08.041 We named this object Sedna, 0:05:08.065,0:05:09.946 after the Inuit goddess of the sea 0:05:09.970,0:05:14.009 in honor of the cold, icy places[br]where it spends all of its time. 0:05:14.033,0:05:15.636 We now know that Sedna, 0:05:15.660,0:05:17.414 it's about a third the size of Pluto 0:05:17.438,0:05:19.612 and it's a relatively typical member 0:05:19.636,0:05:22.358 of those icy bodies out beyond Neptune. 0:05:22.382,0:05:26.229 Relatively typical,[br]except for this bizarre orbit. 0:05:26.253,0:05:28.022 You might look at this orbit and say, 0:05:28.046,0:05:30.776 "Yeah, that's bizarre,[br]10,000 years to go around the Sun," 0:05:30.800,0:05:32.737 but that's not really the bizarre part. 0:05:32.761,0:05:34.927 The bizarre part is[br]that in those 10,000 years 0:05:34.951,0:05:38.594 Sedna never comes close[br]to anything else in the Solar system. 0:05:38.967,0:05:41.285 Even at its closest approach to the Sun, 0:05:41.309,0:05:43.611 Sedna is further from Neptune 0:05:43.635,0:05:45.785 than Neptune is from the Earth. 0:05:47.053,0:05:49.164 If Sedna had had an orbit like this, 0:05:49.188,0:05:51.823 that kisses the orbit of Neptune[br]once around the Sun, 0:05:51.847,0:05:54.578 that would have actually been[br]really easy to explain. 0:05:54.895,0:05:56.634 That would have just been an object 0:05:56.658,0:05:58.952 that had been in a circular[br]orbit around the Sun 0:05:58.976,0:06:00.387 in that region of icy bodies, 0:06:00.411,0:06:02.944 had gotten a little bit[br]too close to Neptune one time, 0:06:02.968,0:06:05.784 and then got slingshot out[br]and is now on its way back in. 0:06:07.350,0:06:12.056 But Sedna never comes close[br]to anything known in the Solar system 0:06:12.080,0:06:14.477 that could have given it that slingshot. 0:06:14.501,0:06:16.514 Neptune can't be responsible, 0:06:16.538,0:06:18.938 but something had to be responsible. 0:06:19.672,0:06:22.609 This was the first time since 1845 0:06:22.633,0:06:27.553 that we saw the gravitational effects[br]of something in the outer Solar system, 0:06:27.577,0:06:29.444 and didn't know what it was. 0:06:30.196,0:06:32.434 I actually thought I knew[br]what the answer was. 0:06:33.141,0:06:37.139 Sure, it could have been some[br]distant, giant planet 0:06:37.163,0:06:38.443 in the outer Solar system, 0:06:38.467,0:06:40.824 but by this time, that idea[br]was so ridiculous 0:06:40.848,0:06:42.689 and had been so thoroughly discredited 0:06:42.713,0:06:44.514 that I didn't take it very seriously. 0:06:44.538,0:06:45.778 But 4.5 billion years ago, 0:06:45.802,0:06:50.696 when the Sun formed on a cocoon[br]of hundreds of other stars, 0:06:50.720,0:06:51.942 any one of those stars 0:06:51.966,0:06:54.641 could have gotten[br]just a little bit too close to Sedna 0:06:54.665,0:06:58.109 and perturbed it onto the orbit[br]that it has today. 0:06:58.653,0:07:02.541 When that cluster of stars[br]dissipated into the galaxy, 0:07:02.565,0:07:06.343 the orbit of Sedna would have been[br]left as a fossil record 0:07:06.367,0:07:08.398 of this earliest history of the Sun. 0:07:08.855,0:07:10.664 I was so excited by this idea, 0:07:10.688,0:07:12.197 by the idea that we could look 0:07:12.221,0:07:14.427 at the fossil history[br]of the birth of the Sun, 0:07:14.451,0:07:16.053 that I spent the next decade 0:07:16.077,0:07:18.807 looking for more objects[br]with orbits like Sedna. 0:07:18.831,0:07:22.276 In that ten-year period, I found zero. 0:07:22.300,0:07:23.301 (Laughter) 0:07:23.325,0:07:26.845 But my colleagues Chad Trujillo[br]and Scott Sheppard, did a better job, 0:07:26.869,0:07:29.886 and they have now found several objects[br]with orbits like Sedna, 0:07:29.910,0:07:31.680 which is super exciting. 0:07:31.704,0:07:33.243 But what's even more interesting 0:07:33.267,0:07:35.999 is that they found that all these objects 0:07:36.023,0:07:39.942 are not only on these distant,[br]elongated orbits, 0:07:39.966,0:07:45.323 they also share a common value[br]of this obscure orbital parameter 0:07:45.347,0:07:49.275 that in celestial mechanics we call[br]argument of perihelion. 0:07:50.243,0:07:53.196 When they realized it was clustered[br]in argument of perihelion, 0:07:53.220,0:07:54.974 they immediately jumped up and down, 0:07:54.998,0:07:57.958 saying it must be caused[br]by a distant, giant planet out there, 0:07:57.982,0:08:01.076 which is really exciting,[br]except it makes no sense at all. 0:08:01.100,0:08:03.641 Let me try to explain it[br]to you why, with an analogy. 0:08:03.665,0:08:06.934 Imagine a person walking down a plaza 0:08:06.958,0:08:10.299 and looking 45 degrees to his right side. 0:08:11.109,0:08:13.117 There's a lot of reasons[br]that might happen, 0:08:13.141,0:08:15.054 it's super easy to explain, no big deal. 0:08:15.078,0:08:16.960 Imagine now many different people, 0:08:16.984,0:08:20.873 all walking in different[br]directions across the plaza, 0:08:20.897,0:08:24.198 but all looking 45 degrees[br]to the direction that they're moving. 0:08:24.222,0:08:26.238 Everybody's moving[br]in different directions, 0:08:26.262,0:08:28.339 everybody's looking[br]in different directions, 0:08:28.363,0:08:31.727 but they're all looking 45 degrees[br]to the direction of motion. 0:08:31.751,0:08:33.847 What could cause something like that? 0:08:34.926,0:08:36.148 I have no idea. 0:08:36.172,0:08:39.942 It's very difficult to think of any reason[br]that that would happen. 0:08:39.966,0:08:41.291 (Laughter) 0:08:41.315,0:08:44.172 And this is essentially[br]what that clustering 0:08:44.196,0:08:47.212 in argument of perihelion was telling us. 0:08:47.633,0:08:51.196 Scientists were generally baffled[br]and they assumed it must just be a fluke 0:08:51.220,0:08:52.545 and some bad observations. 0:08:52.569,0:08:54.347 They told the astronomers, 0:08:54.371,0:08:56.038 "Do better measurements." 0:08:56.776,0:08:59.894 I actually took a very careful look[br]at those measurements, though, 0:08:59.918,0:09:01.125 and they were right. 0:09:01.149,0:09:03.117 These objects really did all share 0:09:03.141,0:09:05.593 a common value of argument of perihelion, 0:09:05.617,0:09:07.006 and they shouldn't. 0:09:07.030,0:09:09.230 Something had to be causing that. 0:09:11.141,0:09:15.417 The final piece of the puzzle[br]came into place in 2016, 0:09:15.441,0:09:17.990 when my colleague, Konstantin Batygin, 0:09:18.014,0:09:20.633 who works three doors down from me, and I 0:09:20.657,0:09:23.268 realized that the reason[br]that everybody was baffled 0:09:23.292,0:09:27.632 was because argument of perihelion[br]was only part of the story. 0:09:28.021,0:09:30.037 If you look at these[br]objects the right way, 0:09:30.061,0:09:34.172 they are all actually lined up[br]in space in the same direction, 0:09:34.196,0:09:37.939 and they're all tilted in space[br]in the same direction. 0:09:37.963,0:09:42.320 It's as if all those people on the plaza[br]are all walking in the same direction 0:09:42.344,0:09:45.439 and they're all looking[br]45 degrees to the right side. 0:09:45.812,0:09:46.998 That's easy to explain. 0:09:47.022,0:09:49.222 They're all looking at something. 0:09:49.585,0:09:53.704 These objects in the outer Solar system[br]are all reacting to something. 0:09:54.982,0:09:56.132 But what? 0:09:56.735,0:09:59.727 Konstantin and I spent a year 0:09:59.751,0:10:04.561 trying to come up with any explanation[br]other than a distant, giant planet 0:10:04.585,0:10:05.831 in the outer Solar system. 0:10:05.855,0:10:11.307 We did not want to be the 33rd and 34th[br]people in history to propose this planet 0:10:11.331,0:10:13.665 to yet again be told we were wrong. 0:10:14.784,0:10:16.545 But after a year, 0:10:16.569,0:10:17.879 there was really no choice. 0:10:17.903,0:10:20.044 We could come up with no other explanation 0:10:20.068,0:10:22.615 other than that that there is a distant, 0:10:22.639,0:10:25.885 massive planet on an elongated orbit, 0:10:25.909,0:10:27.988 inclined to the rest of the Solar system, 0:10:28.012,0:10:30.728 that is forcing these patterns[br]for these objects 0:10:30.752,0:10:32.485 in the outer Solar system. 0:10:32.792,0:10:34.918 Guess what else a planet like this does. 0:10:34.942,0:10:36.823 Remember that strange orbit of Sedna, 0:10:36.847,0:10:39.783 how it was kind of pulled away[br]from the Sun in one direction? 0:10:39.807,0:10:42.904 A planet like this would make[br]orbits like that all day long. 0:10:43.522,0:10:45.522 We knew we were onto something. 0:10:45.871,0:10:48.480 So this brings us to today. 0:10:48.863,0:10:53.073 We are basically 1845, Paris. 0:10:53.097,0:10:54.248 (Laughter) 0:10:54.272,0:10:59.621 We see the gravitational effects[br]of a distant, giant planet, 0:10:59.645,0:11:01.871 and we are trying to work the calculations 0:11:01.895,0:11:04.879 to tell us where to look,[br]to point our telescopes, 0:11:04.903,0:11:06.109 to find this planet. 0:11:06.133,0:11:08.425 We've done massive suits[br]of computer simulations, 0:11:09.307,0:11:11.227 massive months of analytic calculations, 0:11:11.251,0:11:13.823 and here's what I cal tell you so far. 0:11:13.847,0:11:17.014 First, this planet,[br]which we call Planet Nine, 0:11:17.038,0:11:19.621 because that's what it is, 0:11:20.768,0:11:23.998 Planet Nine is six times[br]the mass of the Earth. 0:11:24.022,0:11:26.268 This is no slightly-smaller-than-Pluto, 0:11:26.292,0:11:29.038 let's-all-argue-about[br]whether-it's-a-planet-or-not thing. 0:11:29.062,0:11:32.371 This is the fifth largest planet[br]in our entire Solar system. 0:11:32.395,0:11:36.037 For context, let me show you[br]the sizes of the planets. 0:11:36.061,0:11:39.633 In the back there,[br]you can the massive Jupiter and Saturn. 0:11:40.204,0:11:42.840 Next to them, a little bit smaller,[br]Uranus and Neptune. 0:11:42.864,0:11:46.355 Up in the corner, the terrestrial planets,[br]Mercury, Venus, Earth and Mars. 0:11:46.379,0:11:47.734 You can even see that belt 0:11:47.758,0:11:50.877 of icy bodies beyond Neptune,[br]of which Pluto is a member, 0:11:50.901,0:11:52.774 good luck figuring out which one it is. 0:11:52.798,0:11:55.212 And here is Planet Nine. 0:11:56.585,0:11:58.640 Planet Nine is big. 0:11:59.046,0:12:00.219 Planet Nine is so big, 0:12:00.243,0:12:02.847 you should probably wonder[br]why haven't we found it yet. 0:12:02.871,0:12:04.110 Well, Planet Nine is big, 0:12:04.134,0:12:06.347 but it's also really, really far away. 0:12:06.371,0:12:11.045 It's something like[br]15 times further away than Neptune. 0:12:11.069,0:12:14.347 And that makes it about 50,000 times[br]fainter than Neptune. 0:12:14.371,0:12:17.315 And also, the sky is a really big place. 0:12:17.339,0:12:19.475 We've narrowed down where we think it is 0:12:19.499,0:12:22.037 to a relatively small area of the sky, 0:12:22.061,0:12:23.918 but it would still take us years 0:12:23.942,0:12:26.324 to systematically cover[br]the area of the sky 0:12:26.348,0:12:28.192 with the large telescopes that we need 0:12:28.216,0:12:31.101 to see something that's[br]this far away and this faint. 0:12:31.474,0:12:33.990 Luckily, we might not have to. 0:12:34.729,0:12:39.618 Just like Bouvard used[br]unrecognized observations of Uranus 0:12:39.642,0:12:42.387 from 91 years before its discovery, 0:12:42.411,0:12:46.192 I bet that there are unrecognized images 0:12:46.216,0:12:49.078 that show the location of Planet Nine. 0:12:49.984,0:12:53.061 It's going to be a massive[br]computational undertaking 0:12:53.085,0:12:55.389 to go through all of the old data, 0:12:55.413,0:12:58.331 and pick out that one faint moving planet. 0:12:59.276,0:13:00.641 But we're underway. 0:13:00.665,0:13:02.990 And I think we're getting close. 0:13:03.014,0:13:05.514 So I would say, get ready. 0:13:05.538,0:13:09.513 We are not going to match Le Verrier's 0:13:09.537,0:13:10.705 "make a prediction, 0:13:10.729,0:13:12.633 have the planet found in a single night 0:13:12.657,0:13:14.919 that close to where[br]you predicted it" record. 0:13:14.943,0:13:18.882 But I do bet that within[br]the next couple of years 0:13:18.906,0:13:21.319 some astronomer somewhere 0:13:21.343,0:13:23.573 will find a faint point of light, 0:13:23.597,0:13:25.866 slowly moving across the sky 0:13:25.890,0:13:29.174 and triumphantly announce[br]the discovery of a new, 0:13:29.198,0:13:31.635 and quite possibly not the last, 0:13:31.659,0:13:33.792 real planet of our Solar system. 0:13:34.183,0:13:35.334 Thank you. 0:13:35.358,0:13:38.877 (Applause)