WEBVTT 00:01.160 --> 00:03.940 Prof: Okay now, we didn't have sections this 00:03.939 --> 00:07.329 week, in case you didn't notice, and therefore you didn't have 00:07.329 --> 00:10.719 an opportunity to discuss the problem set during section. 00:10.720 --> 00:14.370 So, I thought I would say a few words about it now - here is the 00:14.369 --> 00:15.179 problem set. 00:15.180 --> 00:20.360 You probably can't read it at this typeface but that's okay. 00:20.360 --> 00:23.970 So, let's see, problem zero is just a stupid 00:23.969 --> 00:27.159 way of making you read the policies. 00:27.160 --> 00:28.800 Never mind that. 00:28.800 --> 00:31.630 Problem set one, problem one: 00:31.627 --> 00:37.077 here are exercises in one-digit scientific notation. 00:37.080 --> 00:39.210 I don't have rules for this. 00:39.210 --> 00:42.380 As I mentioned last time, the only rule is common sense. 00:42.380 --> 00:46.880 I think there might be some difficulty with the last one. 00:46.880 --> 00:50.250 This business of taking things to the one third power is 00:50.246 --> 00:53.606 important because you keep ending up with a cubed equals 00:53.613 --> 00:55.513 something, and you have to figure out what 00:55.513 --> 00:56.123 to do about that. 00:56.120 --> 00:58.130 So, let me not do this particular problem, 00:58.127 --> 00:59.887 let me do a different one for you. 00:59.890 --> 01:04.830 Supposing you had (6 x 10^(4))^(1/3). 01:04.829 --> 01:08.709 And you might be tempted to say, well okay, 01:08.706 --> 01:11.656 that's 6^(1/3) times 10^(4/3). 01:11.659 --> 01:15.939 And that leads you to a bad place, because 10^(4/3) is not 01:15.935 --> 01:17.655 the notation we want. 01:17.659 --> 01:19.429 We want this to be an integer up there. 01:19.430 --> 01:23.320 What does it mean to be a 1 with 4/3 of a zero after it? 01:23.320 --> 01:24.860 And so, you don't like that. 01:24.860 --> 01:27.730 So, the way to deal with this is to regroup. 01:27.730 --> 01:32.830 This is (60 x 10^(3))^(1/3). 01:32.830 --> 01:37.530 That's 60^(1/3) x 10^(1). 01:37.530 --> 01:38.860 What's 60^(1/3)? 01:38.860 --> 01:41.630 Well, 4 times 4 times 4, as it happens, 01:41.627 --> 01:44.537 is 64, and that's close enough for me. 01:44.540 --> 01:50.790 So, this is 4 x 10^(1). 01:50.790 --> 01:53.540 So, that's just an example of how these kinds of things where 01:53.535 --> 01:55.315 you take things to fractional powers, 01:55.319 --> 01:58.479 either the square root, which is to the 1/2, 01:58.483 --> 02:00.693 or the cube root to the 1/3. 02:00.690 --> 02:02.420 All right, the next problem. 02:02.420 --> 02:06.790 Let's see, Neptune's moon Nereid has an orbital period of 02:06.791 --> 02:09.291 almost exactly one Earth year. 02:09.289 --> 02:11.359 If the mass of Neptune is something or other, 02:11.357 --> 02:12.717 what's its semi-major axis? 02:12.719 --> 02:16.369 So, you have P and M and you're asked for 02:16.367 --> 02:17.107 A. 02:17.110 --> 02:19.530 That's a completely straightforward plug-and-chug 02:19.532 --> 02:22.212 problem because there's an equation that relates these 02:22.208 --> 02:25.238 three things and the only tricky thing about this is that you 02:25.236 --> 02:27.706 have to make sure the units come out right. 02:27.710 --> 02:31.640 All right, the next one looks similar in form, 02:31.639 --> 02:32.949 but it isn't. 02:32.949 --> 02:36.189 Consider a Sun-like star orbited by a planet with a 02:36.190 --> 02:37.810 period of eighty years. 02:37.810 --> 02:40.650 So, we have P--the separation of the planet and the 02:40.653 --> 02:42.453 star appears to be 20 arc seconds. 02:42.449 --> 02:44.189 So we have an angle, that's α. 02:44.190 --> 02:46.250 How far away is the star? 02:46.250 --> 02:47.750 And you want to know D. 02:47.750 --> 02:51.280 Okay, there is no equation that contains all three of those 02:51.281 --> 02:54.351 things and so this, although it looks similar in 02:54.350 --> 02:57.360 form, is actually a substantially more difficult 02:57.356 --> 02:58.056 problem. 02:58.060 --> 03:02.290 Going on, problem four. 03:02.289 --> 03:04.649 Okay the important--in fact, there is such a star, 03:04.654 --> 03:07.604 blah, blah, does this fact make any difference in the forgoing 03:07.597 --> 03:08.367 calculation? 03:08.370 --> 03:09.280 Explain. 03:09.280 --> 03:11.730 The important thing to note about this is that I'm not 03:11.726 --> 03:13.246 asking you to do a calculation. 03:13.250 --> 03:15.060 This is not a calculation problem; 03:15.060 --> 03:16.090 this is a comment. 03:16.090 --> 03:18.400 You're supposed to say something about how the 03:18.398 --> 03:21.628 calculation would go if you were to do it, but you don't have to 03:21.630 --> 03:23.580 actually calculate anything there. 03:23.580 --> 03:25.850 And then, finally, this last problem. 03:25.849 --> 03:30.049 This is the sort of essay question here. 03:30.050 --> 03:33.970 The point here as I've written down on the--there's about 03:33.970 --> 03:34.530 Pluto. 03:34.530 --> 03:37.650 The point here as I've written down on the actual paper 03:37.651 --> 03:40.081 itself--there's no right answer to this. 03:40.080 --> 03:43.060 This is a thought question. 03:43.060 --> 03:45.740 There are some people I know from the course evaluations who 03:45.741 --> 03:48.241 get disturbed by this because they have the feeling that 03:48.241 --> 03:50.061 science ought to have right answers; 03:50.060 --> 03:51.690 that is, after all, the point. 03:51.690 --> 03:54.830 And I keep asking these sort of touchy-feely, 03:54.833 --> 03:57.123 humanities kinds of questions. 03:57.120 --> 04:00.850 Revel in it, go with it. 04:00.849 --> 04:04.999 And what we're looking for here is the same thing as your 04:05.000 --> 04:08.410 English teacher would be looking for, right? 04:08.409 --> 04:11.289 Although perhaps less emphasis on writing style, 04:11.290 --> 04:14.100 but, just have some thoughts, do some reading, 04:14.097 --> 04:17.497 understand something and say something intelligent and defend 04:17.499 --> 04:18.879 it, okay? 04:18.879 --> 04:23.059 And a paragraph or two, you know, two sentences 04:23.060 --> 04:25.060 probably too little. 04:25.060 --> 04:27.770 If it's more than two pages double-spaced or one page 04:27.772 --> 04:30.952 single-spaced I'm going to get really irritated because I have 04:30.953 --> 04:33.773 to read these things and there are eighty of them, 04:33.770 --> 04:38.280 and so don't go nuts. 04:38.279 --> 04:42.519 Again, just be sane about it and say something intelligent. 04:42.520 --> 04:44.960 That's all we ask. 04:44.959 --> 04:48.079 Questions about the problem set or about procedural aspects of 04:48.079 --> 04:49.459 the course at this stage? 04:49.460 --> 04:53.810 04:53.810 --> 04:57.650 If you do have some at some point, send them to us on the 04:57.645 --> 04:58.805 classes server. 04:58.810 --> 05:00.740 Yeah, go ahead. 05:00.740 --> 05:03.490 Student: What would you say is too short for the answer 05:03.485 --> 05:04.335 to that question? 05:04.339 --> 05:05.949 Prof: Oh the answer--well, 05:05.951 --> 05:08.401 a sentence is too short, two sentences probably too 05:08.397 --> 05:10.407 short unless they're pretty severe sentences. 05:10.410 --> 05:14.210 I mean, it's got to be a paragraph, you know; 05:14.209 --> 05:16.849 otherwise, you haven't really cleared your throat. 05:16.850 --> 05:20.030 But I don't want to say anything too precise about that. 05:20.029 --> 05:22.099 If you've got a really keen sentence, you know, 05:22.100 --> 05:23.180 that might do the job. 05:23.180 --> 05:26.000 Again, sanity ought to prevail. 05:26.000 --> 05:27.230 Yes. 05:27.230 --> 05:29.490 Student: Is the answer to a question like that typical 05:29.490 --> 05:30.620 of most of the problem sets? 05:30.620 --> 05:33.850 Prof: We'll probably have one of that nature. 05:33.850 --> 05:37.030 Not necessarily of this kind about scientific controversies, 05:37.033 --> 05:38.223 but about something. 05:38.220 --> 05:40.950 Oh, and I should say, the way the tests will work out 05:40.946 --> 05:43.616 is it's going to be probably slightly more than half 05:43.621 --> 05:46.611 calculation and slightly less than half other things. 05:46.610 --> 05:49.480 There won't be essays because there probably isn't time to do 05:49.477 --> 05:52.107 that on a fifty-minute test but there'll be short answer 05:52.105 --> 05:53.725 questions and stuff like that. 05:53.730 --> 05:56.800 Yeah, we'll have these kinds of discussion things. 05:56.800 --> 06:01.140 They won't be the dominant part of the problem sets. 06:01.139 --> 06:03.309 Just as in this case, it's six points out of--this 06:03.306 --> 06:05.536 adds up to twenty points; this one's six [points at 06:05.544 --> 06:06.134 problem set]. 06:06.130 --> 06:06.960 Other questions? 06:06.960 --> 06:10.100 06:10.100 --> 06:12.010 Okay, as I say, if you've got some, 06:12.005 --> 06:12.785 let us know. 06:12.790 --> 06:22.810 06:22.810 --> 06:27.250 Okay, so as I pointed out last time, we're talking about 06:27.253 --> 06:30.973 "exoplanets" – planets around other stars. 06:30.970 --> 06:35.380 And the problem with exoplanets is you can't see them directly. 06:35.379 --> 06:43.919 So, you can't see the exoplanets directly as blobs of 06:43.918 --> 06:50.648 light in the sky and so what do you do? 06:50.649 --> 06:53.879 So, you have to detect them obviously indirectly and 06:53.881 --> 06:57.621 here's--we've got to invoke another one of Newton's laws. 06:57.620 --> 07:01.540 So, the key law here is now Newton's Third Law. 07:01.540 --> 07:03.580 Newton's Second Law is F = ma; 07:03.579 --> 07:06.489 you may recall that explains all the Physics 180. 07:06.490 --> 07:13.500 Newton's Third Law is usually phrased in textbooks and stuff 07:13.502 --> 07:20.042 as "every action has an equal and opposite reaction." 07:20.040 --> 07:24.860 07:24.860 --> 07:29.980 I hate this formulation because it leads to really bad 07:29.983 --> 07:31.243 philosophy. 07:31.240 --> 07:34.350 You know, this is one of the whole problems with physics in 07:34.353 --> 07:34.893 general. 07:34.889 --> 07:37.979 There are these words that seem to mean something. 07:37.980 --> 07:41.700 Words like "force," words like "potential," and these have 07:41.699 --> 07:45.679 technical meanings in physics that are actually different from 07:45.681 --> 07:49.401 the intuitive meaning that you have in your head just from 07:49.401 --> 07:51.621 using them in everyday life. 07:51.620 --> 07:54.920 The big problem with introductory physics is getting 07:54.924 --> 07:59.014 people to use these words as if they mean the physics definition 07:59.007 --> 08:02.957 rather than as if they mean the everyday life definition. 08:02.959 --> 08:05.699 And this leads, as I say, in extreme cases, 08:05.704 --> 08:07.864 to all sorts of bad philosophy. 08:07.860 --> 08:11.920 In fact, the two most misused physics words that I know of 08:11.923 --> 08:15.203 are, first of all, "relativity"--"Everything is 08:15.202 --> 08:17.272 relative," said Einstein. 08:17.269 --> 08:20.099 Well, no, actually, he said nothing of the kind. 08:20.100 --> 08:24.380 And the other one is "uncertainty," which has to do 08:24.381 --> 08:29.691 with quantum mechanics and these are technical terms in physics 08:29.690 --> 08:34.570 and then they get sort of promoted to use in philosophy as 08:34.571 --> 08:38.511 if they meant what they mean in English. 08:38.509 --> 08:42.079 So, I don't like this "every action has an equal and opposite 08:42.076 --> 08:42.786 reaction." 08:42.789 --> 08:45.979 You can get into all sorts of bogus philosophy and I don't 08:45.977 --> 08:47.037 want to go there. 08:47.039 --> 08:51.019 So, I would rather phrase this as "conservation of momentum." 08:51.020 --> 08:56.440 08:56.440 --> 08:59.770 And momentum, as I say, is a technical term. 08:59.769 --> 09:06.059 It means M, mass, times velocity, 09:06.061 --> 09:09.611 that's all it means. 09:09.610 --> 09:13.280 And the kind of use that it's put to in electoral politics or 09:13.277 --> 09:17.247 other things has nothing to do with what's going on in physics. 09:17.250 --> 09:20.900 Oh, I should say: velocity, one should be aware 09:20.899 --> 09:23.199 of, consists of two things. 09:23.200 --> 09:25.390 It's not just the speed that something has, 09:25.394 --> 09:26.704 but also its direction. 09:26.700 --> 09:31.570 09:31.570 --> 09:32.840 And so direction counts. 09:32.840 --> 09:35.600 So, if you turn around and go at the same speed you've 09:35.603 --> 09:38.523 reversed your--you now have negative velocity compared to 09:38.522 --> 09:39.932 where you started with. 09:39.929 --> 09:43.219 This, technically speaking, makes this a vector quantity, 09:43.219 --> 09:45.039 but again, we won't go there. 09:45.039 --> 09:50.279 Okay, so how does this work in the case of a planetary orbit? 09:50.279 --> 09:53.059 So, here's a planet, it's on one side of its star, 09:53.062 --> 09:55.222 it's going this way [draws an arrow up] 09:55.219 --> 09:56.979 and it's got some momentum. 09:56.980 --> 10:00.460 And then half an orbit later, it's over here going the other 10:00.456 --> 10:00.806 way. 10:00.809 --> 10:04.279 Let's say it's orbiting some star in the middle and it's got 10:04.280 --> 10:07.810 more or less the same speed but its direction is reversed. 10:07.809 --> 10:18.039 So over half an orbit--so over 1/2 orbit, the momentum is 10:18.040 --> 10:20.050 reversed. 10:20.049 --> 10:23.139 So, it goes from being positive to negative, or something like 10:23.144 --> 10:23.504 that. 10:23.500 --> 10:27.000 Now, Newton's Third Law says that momentum is conserved. 10:27.000 --> 10:30.050 The total momentum of the system has to remain the same; 10:30.050 --> 10:32.970 so something else has to change. 10:32.970 --> 10:36.830 And here's the whole trick about finding exoplanets, 10:36.832 --> 10:39.712 and that is that the star moves too. 10:39.710 --> 10:44.870 So, when the planet's over here at this blue arrow--the star is 10:44.871 --> 10:49.451 here, and it's also moving in the opposite direction. 10:49.450 --> 10:53.130 And then half an orbit later, the planet has come over to 10:53.131 --> 10:53.921 this side. 10:53.919 --> 10:57.489 The star has moved in the opposite direction; 10:57.490 --> 10:59.940 it's now going this way. 10:59.940 --> 11:04.040 And so the star moves too, that's key. 11:04.040 --> 11:07.340 11:07.340 --> 11:09.150 How much does the star move? 11:09.149 --> 11:14.309 Well, the two momenta have to be equal and opposite. 11:14.309 --> 11:17.379 This is what is equal and opposite about Newton's Third 11:17.382 --> 11:20.002 Law is that these things have to cancel out. 11:20.000 --> 11:25.310 So what you find is that the mass of the planet times the 11:25.306 --> 11:30.986 velocity of the planet is equal to the mass of the star times 11:30.991 --> 11:33.741 the velocity of the star. 11:33.740 --> 11:36.840 And because the mass of the star is so much bigger, 11:36.840 --> 11:39.630 so huge compared to the mass of the planet, 11:39.629 --> 11:43.119 its velocity must be much, much smaller in order for these 11:43.119 --> 11:44.649 two things to be equal. 11:44.650 --> 11:49.960 Okay. 11:49.960 --> 11:55.180 What are they orbiting around then, if everything is moving 11:55.179 --> 11:56.169 together? 11:56.169 --> 12:05.369 They orbit around the "center of mass," so-called. 12:05.370 --> 12:07.980 This is just, you know, where the balance 12:07.982 --> 12:10.402 point of a seesaw would kind of be. 12:10.399 --> 12:14.489 And so here we have--here's the center of mass, 12:14.485 --> 12:19.635 here's the star moving this way, way over here somewhere is 12:19.637 --> 12:22.477 a planet moving much faster. 12:22.480 --> 12:26.400 And let's define two quantities: this is the distance 12:26.396 --> 12:29.406 from the planet to the center of mass. 12:29.409 --> 12:32.199 Here's the distance of the star to the planet of mass. 12:32.200 --> 12:34.550 Star is moving at V_star, 12:34.554 --> 12:37.314 planet is moving at V_planet. 12:37.309 --> 12:42.459 And you can define a total velocity, which is equal to 12:42.457 --> 12:45.757 these two things added together. 12:45.760 --> 12:46.920 And what does that mean? 12:46.919 --> 12:49.839 That's the velocity where if you're on one of these objects, 12:49.837 --> 12:52.257 the other appears to be moving relative to you. 12:52.259 --> 12:55.589 So, if I'm going this way and you're going that way, 12:55.593 --> 12:58.863 we have a relative velocity equal to the sum of our 12:58.860 --> 13:00.560 individual velocities. 13:00.559 --> 13:04.999 And there's also a total distance, which can be defined, 13:05.002 --> 13:08.432 which is, by analogy, the sum of the distance from 13:08.431 --> 13:11.741 the planet to the center of mass, and the star to the center 13:11.742 --> 13:12.362 of mass. 13:12.360 --> 13:16.630 And that's just the distance between these two objects. 13:16.629 --> 13:19.659 The distance and the velocity in an elliptical orbit can 13:19.663 --> 13:21.873 change during the course of the orbit. 13:21.870 --> 13:25.630 But for circular orbits or close to circular orbits they 13:25.629 --> 13:27.269 remain about the same. 13:27.269 --> 13:30.289 Orbits, you'll remember, are generally elliptical. 13:30.289 --> 13:34.539 And a way of saying this is that the maximum of the total 13:34.540 --> 13:38.490 distance between these two is this quantity a, 13:38.488 --> 13:40.308 the semi-major axis. 13:40.309 --> 13:43.879 Remember, the semi-major axis is the long slice through the 13:43.883 --> 13:44.503 ellipse. 13:44.500 --> 13:50.060 So, when this is at a maximum, that equals the semi-major 13:50.064 --> 13:50.764 axis. 13:50.759 --> 13:54.499 For orbits that are nearly circular you can sort of say in 13:54.504 --> 13:56.154 an offhand way, well this, 13:56.146 --> 13:59.756 D_total, doesn't change that much. 13:59.759 --> 14:03.109 So, it's the distance between the two objects. 14:03.110 --> 14:06.380 All right, now as I mentioned, V_p 14:06.380 --> 14:09.590 M_p is equal to V_star 14:09.585 --> 14:11.805 M_star. 14:11.809 --> 14:15.749 It is also true the way you figure out where the center of 14:15.753 --> 14:19.973 mass is, D_p M_p is equal to 14:19.972 --> 14:23.572 D_star M_star. 14:23.570 --> 14:27.600 And the whole point of this is that this is a large quantity 14:27.601 --> 14:30.131 compared to the mass of the planet, 14:30.129 --> 14:36.629 and therefore these are small quantities compared to the 14:36.627 --> 14:42.177 distances and velocities taken by the planet. 14:42.179 --> 14:46.139 Small, but as it turns out, measurable. 14:46.139 --> 14:48.769 In particular, the velocity is the thing--the 14:48.768 --> 14:52.228 velocity of the star turns out to be the thing that you can 14:52.233 --> 14:52.953 measure. 14:52.950 --> 14:56.610 14:56.610 --> 15:01.020 And so that's how you find--determine that there's an 15:01.022 --> 15:02.552 exoplanet there. 15:02.549 --> 15:06.759 What you do is you look for the reflex motion of the star. 15:06.759 --> 15:10.209 Planets going around the star, stars going around the center 15:10.212 --> 15:11.092 of mass also. 15:11.090 --> 15:15.880 And that is a motion that now, these days, can be observed. 15:15.879 --> 15:18.249 And you can see why this might have happened only very 15:18.245 --> 15:20.605 recently, because that motion is really very small. 15:20.610 --> 15:23.670 Let me give you some masses just to give you a sense of 15:23.672 --> 15:24.072 this. 15:24.070 --> 15:28.250 I've already written down that the Sun's mass is about 2 x 15:28.252 --> 15:29.722 10^(30) kilograms. 15:29.720 --> 15:35.810 Just for reference, the Earth's mass is 6 x 10^(24) 15:35.808 --> 15:41.408 kg, so down by almost a factor of a million. 15:41.409 --> 15:44.839 And so the Sun moves much slower than the Earth does, 15:44.840 --> 15:46.820 due to their mutual gravity. 15:46.820 --> 15:53.270 Jupiter is the most massive of the planets, and it's at about 2 15:53.270 --> 15:57.850 x 10^(27) kg, so 1,000 times smaller than the 15:57.847 --> 15:58.677 Sun. 15:58.679 --> 16:01.019 And so, of course, the Sun moves 1,000 times less 16:01.019 --> 16:03.699 because of Jupiter than it does because of the Earth. 16:03.700 --> 16:05.710 Now, of course, the Sun actually responds to 16:05.707 --> 16:08.137 all of these planets, so it's actually executing some 16:08.136 --> 16:11.296 complicated motion, which is the sum of the motions 16:11.301 --> 16:13.191 induced by all the planets. 16:13.190 --> 16:15.900 But in fact, Jupiter is significantly more 16:15.901 --> 16:18.481 massive than the rest of the planets, 16:18.480 --> 16:22.610 so by far the dominant motion that the Sun goes through has to 16:22.610 --> 16:24.710 do with the orbit of Jupiter. 16:24.710 --> 16:28.070 And so the consequence of this, because the masses are so much 16:28.069 --> 16:30.769 smaller, is that the velocity of the star is much, 16:30.768 --> 16:31.538 much less. 16:31.539 --> 16:33.339 These two less-than signs [<<] 16:33.343 --> 16:36.233 means much, much less than the velocity of the planet. 16:36.230 --> 16:40.320 But it can nevertheless be detected. 16:40.320 --> 16:45.190 16:45.190 --> 16:50.290 Okay, now, what do we expect to see? 16:50.289 --> 16:52.849 Supposing you can now go out and through means that we'll 16:52.847 --> 16:56.897 actually talk about on Thursday, actually measure the velocities 16:56.904 --> 16:59.574 of stars in response to planets. 16:59.570 --> 17:07.060 What do you expect to see in those--in other stars? 17:07.059 --> 17:09.299 And basically, the answer to that is, 17:09.296 --> 17:12.956 what you expect to see depends on what your expectations for 17:12.962 --> 17:14.332 Solar Systems are. 17:14.329 --> 17:17.839 We've got one example, or at least ten years ago, 17:17.839 --> 17:19.739 we had only one example. 17:19.740 --> 17:23.420 And so, you have to take what you know about our own Solar 17:23.415 --> 17:27.215 System and infer what other Solar Systems might want to look 17:27.219 --> 17:27.799 like. 17:27.799 --> 17:31.149 And so, at this point I want to show you some things about our 17:31.147 --> 17:33.667 own Solar System, so a little slide show of the 17:33.672 --> 17:34.882 Solar System here. 17:34.880 --> 17:42.590 All right, don't take notes, I'll tell you everything you 17:42.594 --> 17:49.624 need to know after we finish the pretty pictures. 17:49.619 --> 17:53.779 Okay, so starting from the innermost part of the Solar 17:53.783 --> 17:54.493 System. 17:54.490 --> 17:57.270 This is the innermost planet, this is the planet Mercury. 17:57.269 --> 18:00.809 Looks much like the Moon: it's basically a rock with 18:00.809 --> 18:01.919 craters on it. 18:01.920 --> 18:05.100 There was a time when we thought that its spin period was 18:05.103 --> 18:08.633 exactly the same as its orbital period, so it keeps one face to 18:08.627 --> 18:09.307 the Sun. 18:09.309 --> 18:11.849 There's all kinds of science fiction based on that--that 18:11.852 --> 18:13.102 turns out not to be true. 18:13.099 --> 18:17.209 But basically, it's kind of a hot rock, 18:17.210 --> 18:21.970 that's all you need to know about Mercury. 18:21.970 --> 18:24.690 Let's see, oh, here's a close up of a little 18:24.694 --> 18:28.244 piece of Mercury surface, and you would have a tough time 18:28.243 --> 18:31.853 telling that this was Mercury rather than the Moon or many 18:31.854 --> 18:35.344 other objects--rocky objects in the Solar System. 18:35.340 --> 18:36.370 Next one out is Venus. 18:36.369 --> 18:39.549 Venus looks quite different because it's got a very thick 18:39.546 --> 18:42.436 atmosphere, very thick carbon dioxide atmosphere. 18:42.440 --> 18:44.950 This is all clouds that you're looking at here. 18:44.950 --> 18:46.900 And, in fact, the greenhouse effect, 18:46.896 --> 18:49.506 which is supposed to be responsible, perhaps, 18:49.509 --> 18:51.979 for global warming, was first studied and 18:51.984 --> 18:55.084 identified on Venus, because it appears to have run 18:55.076 --> 18:56.186 amok on Venus. 18:56.190 --> 18:59.040 The surface of Venus is extremely hot and it's covered 18:59.043 --> 19:00.823 with these really thick clouds. 19:00.819 --> 19:02.669 So, it was quite hard, for a long time, 19:02.665 --> 19:05.525 to get a handle on what was going on down on the surface. 19:05.529 --> 19:08.169 This has now however been accomplished. 19:08.170 --> 19:11.370 They've put things into orbit that have radar, 19:11.372 --> 19:14.862 and can view the topography through the clouds. 19:14.859 --> 19:18.389 They've also dropped things onto the surface of Venus. 19:18.390 --> 19:21.870 The problem is at 700 degrees, and it rains sulfur down there, 19:21.874 --> 19:24.734 so it's an unpleasant environment for machinery. 19:24.730 --> 19:26.470 So things don't last very long. 19:26.470 --> 19:28.410 But nevertheless, they've gotten some 19:28.413 --> 19:29.173 information. 19:29.170 --> 19:32.100 Here's a little Venus landscape. 19:32.099 --> 19:34.849 It's entirely artificially colored, right? 19:34.849 --> 19:39.489 But the topography comes from these radar mapping missions. 19:39.490 --> 19:46.050 Here is a map of the whole of Venus made by these orbiting 19:46.053 --> 19:47.323 missions. 19:47.319 --> 19:51.399 And so Venus is important primarily for its atmosphere, 19:51.402 --> 19:55.792 and as a kind of warning for what might potentially one day 19:55.787 --> 19:58.657 happen here, if we're not careful. 19:58.660 --> 20:01.930 Okay, this is the third rock from the Sun. 20:01.930 --> 20:05.440 Ninety nine percent of all Yale courses deal with what's going 20:05.437 --> 20:07.907 on on this little piece of cosmic debris. 20:07.910 --> 20:10.850 I'm not going to say anymore about it, therefore, 20:10.854 --> 20:14.114 oh, except for one thing: it comes with this companion 20:14.105 --> 20:14.775 object. 20:14.780 --> 20:16.130 This is the Moon. 20:16.130 --> 20:18.570 The Moon is a very special thing because, 20:18.566 --> 20:20.756 relative to its planet, it's huge. 20:20.759 --> 20:23.119 This really shouldn't be thought of as a planet and a 20:23.117 --> 20:24.837 moon, but rather as a double planet. 20:24.839 --> 20:29.729 Here they are to scale, and that's much closer in size 20:29.730 --> 20:35.360 than any other moon-planet system around the major planets. 20:35.359 --> 20:39.149 Moving outwards, we come to Mars. 20:39.150 --> 20:43.000 This is about as good an image of Mars as you can get from the 20:43.001 --> 20:46.601 Earth, and you can see why people got excited about it. 20:46.599 --> 20:49.929 These blotchy things here turn out to change with time. 20:49.930 --> 20:51.980 And in fact, they change with the Martian 20:51.978 --> 20:52.488 seasons. 20:52.490 --> 20:54.980 So people got very excited, thought, "oh my goodness, 20:54.983 --> 20:57.093 it's vegetation," you know, the seasons come, 20:57.094 --> 20:57.434 go. 20:57.430 --> 21:00.900 And there's a polar ice cap up there, obviously. 21:00.900 --> 21:03.830 And 100 years ago, people somehow convinced 21:03.833 --> 21:07.333 themselves that there were canals and maybe cities, 21:07.326 --> 21:10.256 and maybe people all over this planet. 21:10.260 --> 21:11.610 This turns out to be wrong. 21:11.610 --> 21:12.690 It isn't vegetation. 21:12.690 --> 21:15.770 It's actually dust storms--that changes what you see. 21:15.769 --> 21:19.379 And by now we have some much more close up views from things 21:19.381 --> 21:23.301 like the Viking Missions and a number of more recent missions. 21:23.299 --> 21:27.269 And this is basically what the surface of Mars looks like. 21:27.269 --> 21:31.019 It has this slight reddish tint overall, and it's a bunch of 21:31.021 --> 21:31.531 rocks. 21:31.529 --> 21:34.959 It has an atmosphere, although it's less thick than 21:34.963 --> 21:36.683 the Earth's atmosphere. 21:36.680 --> 21:39.980 Now, one of the interesting things about Mars is you can see 21:39.983 --> 21:41.723 features that look like this. 21:41.720 --> 21:46.530 And this looks very much like river deltas. 21:46.529 --> 21:49.319 You know, you see these little tributaries coming into a big 21:49.320 --> 21:52.060 river, this kind of looks like Louisiana, or something like 21:52.064 --> 21:52.494 that. 21:52.490 --> 21:56.450 And so, people are pretty much convinced that there was once 21:56.449 --> 21:58.059 running water on Mars. 21:58.059 --> 22:01.829 And that's important, because it is thought that the 22:01.828 --> 22:06.478 existence of life as we know it is dependent on the existence of 22:06.483 --> 22:07.743 liquid water. 22:07.740 --> 22:11.090 For a long time, people thought that there was 22:11.089 --> 22:12.429 no liquid water. 22:12.430 --> 22:16.340 Now, on Mars--it turns out that the particular temperature and 22:16.343 --> 22:20.263 atmospheric pressure that exists on Mars means that water goes 22:20.256 --> 22:23.876 from the solid state, from ice and sublimes, 22:23.878 --> 22:29.018 directly into the gaseous state, much like carbon dioxide 22:29.017 --> 22:30.207 does here. 22:30.210 --> 22:33.190 That's why it's called dry ice: because carbon dioxide, 22:33.194 --> 22:35.684 when you freeze it, and then warm it up again, 22:35.682 --> 22:37.232 turns directly into gas. 22:37.230 --> 22:39.960 Water is supposed to do the same on Mars, 22:39.957 --> 22:42.067 but there was, just a month ago, 22:42.070 --> 22:44.730 this interesting picture published. 22:44.730 --> 22:48.090 This is from a satellite orbiting Mars that's been taking 22:48.089 --> 22:49.289 a lot of pictures. 22:49.289 --> 22:53.169 This is pictures of two identical parts of the Martian 22:53.170 --> 22:56.100 surface, one from 1999, one from 2005. 22:56.099 --> 22:59.129 And the claim is that there's new stuff down here, 22:59.126 --> 23:02.396 and that the way and the pattern of that new stuff, 23:02.400 --> 23:05.160 and the way it must have come on, is from stuff flowing 23:05.162 --> 23:07.262 downhill, down the side of this crater. 23:07.259 --> 23:09.759 And so now, people are thinking, maybe there is 23:09.758 --> 23:12.848 something flowing around on Mars, although clearly not all 23:12.853 --> 23:13.943 that much of it. 23:13.940 --> 23:16.390 But that would be exciting if it was confirmed. 23:16.390 --> 23:19.710 Okay, out beyond Mars is the asteroid belt, 23:19.706 --> 23:24.046 filled with rocky chunks of stuff that look vaguely like 23:24.049 --> 23:26.339 this--many, many of them. 23:26.339 --> 23:28.529 There are asteroids all over the Solar System. 23:28.529 --> 23:31.299 Most of them are between the orbits of Mars and Jupiter, 23:31.300 --> 23:33.820 but there are other families that are elsewhere. 23:33.819 --> 23:37.599 Some of these other families, it has been suggested, 23:37.604 --> 23:41.364 come from the asteroid belt, but they've had collisions or 23:41.364 --> 23:43.694 other catastrophes, and have been bumped into 23:43.691 --> 23:44.751 different orbits. 23:44.750 --> 23:47.870 But most of the asteroids are between Mars and Jupiter. 23:47.869 --> 23:50.789 Now, out beyond the asteroid belt are a number of other 23:50.792 --> 23:53.932 planets, and much of what we know about these other planets 23:53.931 --> 23:57.451 come from a couple of satellites that look kind of like this. 23:57.450 --> 24:00.630 These are the Voyager satellites that were launched in 24:00.630 --> 24:03.630 the 1970s and have been traveling through the Outer 24:03.630 --> 24:05.310 Solar System ever since. 24:05.309 --> 24:07.379 This is a clever thing that they did. 24:07.380 --> 24:11.340 It turned out that in the '70s and '80s, the outer planets, 24:11.340 --> 24:14.150 Jupiter, Saturn, Uranus, and Neptune were 24:14.153 --> 24:18.113 aligned in such a way that one satellite could catch them all 24:18.114 --> 24:19.504 as they went past. 24:19.500 --> 24:22.000 And each time it goes past one of these things, 24:22.002 --> 24:24.882 it uses the gravitational attraction of that planet to 24:24.884 --> 24:27.684 swing itself to the next one, and then to the next one, 24:27.684 --> 24:28.734 and then to the next one. 24:28.730 --> 24:31.990 And so, these wonderful satellites, for many years, 24:31.987 --> 24:35.047 gave us pictures of one planet after another, 24:35.049 --> 24:39.769 which we now know quite a lot more about than we used to. 24:39.769 --> 24:44.019 So, here's Jupiter, this is by far the most massive 24:44.015 --> 24:44.775 planet. 24:44.779 --> 24:46.849 Here you've got the famous red spot. 24:46.849 --> 24:50.009 All that you see here is atmosphere, and it's got very 24:50.008 --> 24:51.198 elaborate weather. 24:51.200 --> 24:54.700 And the red spot, it turns out to be a hurricane 24:54.703 --> 24:57.763 that has persisted for about 350 years. 24:57.759 --> 25:00.549 To us, it seems like an almost permanent feature, 25:00.552 --> 25:02.882 although it's gotten fainter recently. 25:02.880 --> 25:06.630 But it's sort of as if--supposing you were a race of 25:06.626 --> 25:09.926 creatures whose lifetime was about half a day, 25:09.932 --> 25:12.652 and you were observing the Earth. 25:12.650 --> 25:16.210 And you observed it for many lifetimes, and you saw the same 25:16.211 --> 25:19.351 hurricane sitting down somewhere in the Caribbean. 25:19.349 --> 25:22.009 You would think that that little spot was kind of a 25:22.007 --> 25:24.557 permanent feature, and that seems to be what this 25:24.557 --> 25:24.927 is. 25:24.930 --> 25:26.880 It's a sort of really long-lasting hurricane. 25:26.880 --> 25:29.990 If you have--they have time-lapse movies of this. 25:29.990 --> 25:32.900 You can find them on the Internet, where you can see that 25:32.899 --> 25:35.029 the wind is actually circulating there. 25:35.029 --> 25:38.069 Jupiter has moons, many of them--these--the four 25:38.073 --> 25:41.703 big ones you see here are the so-called "Galilean" moons, 25:41.700 --> 25:44.550 because they were discovered by Galileo. 25:44.549 --> 25:46.719 They've also included a little one. 25:46.720 --> 25:48.630 There are many dozens of moons this size. 25:48.630 --> 25:51.200 The moons--each of the moons has its own peculiar 25:51.204 --> 25:52.174 characteristics. 25:52.170 --> 25:55.300 I'm quite fond of this one, this is the innermost moon. 25:55.299 --> 25:57.759 It's called Io, sometimes referred to as the 25:57.762 --> 25:59.082 pepperoni pizza moon. 25:59.079 --> 26:03.469 And it's got the most elaborate volcanoes anywhere in the Solar 26:03.472 --> 26:04.112 System. 26:04.110 --> 26:07.630 It spews up sulfur all the time. 26:07.630 --> 26:10.310 And then, this sulfur sort of melts and flows all over the 26:10.309 --> 26:12.989 surface, and that's what gives it its particular color. 26:12.990 --> 26:15.770 Each of the other moons has interesting characteristics of 26:15.771 --> 26:16.261 its own. 26:16.259 --> 26:20.389 Here's an interesting thing that the Voyager satellites 26:20.388 --> 26:24.438 discovered: they discovered that Jupiter has rings. 26:24.440 --> 26:27.080 It was not thought that Jupiter had rings; 26:27.079 --> 26:28.349 from the Earth, you can't see them. 26:28.349 --> 26:31.239 But from close up, it became apparent that Jupiter 26:31.238 --> 26:33.418 has rings the same way Saturn does. 26:33.420 --> 26:35.530 But, of course, the Saturn rings are the most 26:35.528 --> 26:36.198 spectacular. 26:36.200 --> 26:38.240 Here's Saturn, the next planet out. 26:38.240 --> 26:41.180 You can see that it, too, has weather-banded things 26:41.175 --> 26:41.875 down here. 26:41.880 --> 26:45.020 And then it has these very spectacular rings, 26:45.015 --> 26:48.785 seen here from various different angles as Saturn goes 26:48.792 --> 26:50.362 through its orbit. 26:50.359 --> 26:52.299 And these rings, we know now, 26:52.302 --> 26:55.912 are made up of individual little chunks of things. 26:55.910 --> 26:57.890 The Voyager Mission, this is obviously 26:57.892 --> 27:01.052 artificially-colored so that you could see all the different 27:01.054 --> 27:01.594 rings. 27:01.589 --> 27:04.009 And each one of those rings is made up of many, 27:04.011 --> 27:05.381 many, many little rocks. 27:05.380 --> 27:08.070 Saturn too has moons. 27:08.069 --> 27:10.149 This is Titan, Saturn's big moon. 27:10.150 --> 27:12.910 And you can see from here, in this particular picture, 27:12.907 --> 27:14.517 that Titan has an atmosphere. 27:14.520 --> 27:15.990 That makes it very interesting. 27:15.990 --> 27:19.160 People have the feeling that at places where there are 27:19.158 --> 27:21.608 atmospheres are potential sources of life, 27:21.610 --> 27:24.540 and so people find Titan an interesting moon. 27:24.540 --> 27:26.520 I actually like this one better. 27:26.520 --> 27:27.860 This is Mimas. 27:27.859 --> 27:30.909 It's just a rocky moon, but you can see it's kind of 27:30.905 --> 27:34.005 got the great grandmother of all craters up there. 27:34.009 --> 27:37.229 This thing got slammed into by some asteroid that was just 27:37.234 --> 27:40.294 barely not big enough to blow the whole thing apart, 27:40.289 --> 27:43.989 but it raised this big pucker on the side of the moon. 27:43.990 --> 27:48.050 Moving out to the next planet, which is Uranus. 27:48.049 --> 27:50.789 In ordinary light, you can't actually see any 27:50.786 --> 27:51.466 features. 27:51.470 --> 27:52.850 Again, you're looking at the atmosphere; 27:52.850 --> 27:54.200 there are clouds. 27:54.200 --> 27:58.350 But this picture was taken in a particular kind of red light, 27:58.351 --> 28:00.981 which brings out the cloud features. 28:00.980 --> 28:04.420 And what you can see is, it's got a banded structure, 28:04.421 --> 28:07.661 the same as Jupiter and Saturn, but interestingly, 28:07.664 --> 28:09.654 the bands are on its side. 28:09.650 --> 28:13.870 One of the peculiar features of Uranus is that it rotates 28:13.871 --> 28:18.401 sideways, rather than kind of up and down, uniquely among the 28:18.395 --> 28:19.295 planets. 28:19.300 --> 28:22.010 This planet, too, has rings. 28:22.010 --> 28:23.110 It also has moons. 28:23.109 --> 28:27.149 Here's another favorite moon of mine, this is Miranda. 28:27.150 --> 28:29.750 And this looks like what happened was that it actually 28:29.753 --> 28:32.363 did get blown apart by some impact, but then fell back 28:32.356 --> 28:33.286 together again. 28:33.289 --> 28:36.509 And you can see that it looks like it's been chopped into 28:36.508 --> 28:39.898 pieces and then sort of thrown back into--together again. 28:39.900 --> 28:42.900 Moving out to the next planet, here's Neptune. 28:42.900 --> 28:44.720 Neptune, again, has weather. 28:44.720 --> 28:48.120 Here is the big, dark spot on Neptune, 28:48.115 --> 28:51.965 similar to the big, red spot on Jupiter. 28:51.970 --> 28:54.580 This little cloud here is called Scooter, 28:54.577 --> 28:58.097 because it moves faster than the rest of the weather on 28:58.097 --> 28:58.877 Neptune. 28:58.880 --> 29:01.230 It's actually kind of a mystery how come Neptune has all this 29:01.228 --> 29:02.948 weather, because it's very cold out there. 29:02.950 --> 29:05.180 There isn't a whole lot of energy that should be in the 29:05.176 --> 29:07.776 atmosphere, and nobody can quite figure out how this is supposed 29:07.775 --> 29:08.265 to work. 29:08.270 --> 29:10.290 Neptune has moons. 29:10.289 --> 29:12.699 Here's Neptune's biggest moon, this is Triton. 29:12.700 --> 29:15.820 You can see this sort of frontier here between two types 29:15.816 --> 29:17.966 of topography, sort of moves around, 29:17.970 --> 29:22.070 and that is thought to be due to weather of various 29:22.065 --> 29:25.255 kinds--methane snow, stuff like that. 29:25.259 --> 29:28.229 And then, by now, this was the last planet 29:28.232 --> 29:31.712 Voyager II examined, and then it went past and it 29:31.712 --> 29:35.632 took this--took a lovely shot looking back at the Solar 29:35.627 --> 29:36.567 System. 29:36.569 --> 29:41.579 Here is Neptune and Triton as the Voyager Mission moved out 29:41.579 --> 29:46.329 into the Outer Solar System beyond the large planets. 29:46.329 --> 29:51.109 Now, Pluto wasn't aligned properly to take part in this. 29:51.109 --> 29:53.329 This is--these are pictures of Pluto. 29:53.329 --> 29:54.449 This is a picture of Pluto from the ground. 29:54.450 --> 29:56.090 Here it is from the space telescope. 29:56.090 --> 29:57.990 As you can see, it's got a moon. 29:57.990 --> 30:00.740 This is kind of all we know about Pluto at the moment. 30:00.740 --> 30:04.230 There is a spacecraft that is currently on route to Pluto, 30:04.228 --> 30:07.658 and when it arrives there a couple of decades from now--a 30:07.655 --> 30:10.825 decade from now, I guess, we'll know much more. 30:10.829 --> 30:13.789 Now, you may be aware that there was a little bit of fuss 30:13.792 --> 30:15.382 about Pluto over the summer. 30:15.380 --> 30:16.780 Let me show you why. 30:16.780 --> 30:19.370 Here it is. 30:19.369 --> 30:20.679 This is what all the fuss is about. 30:20.680 --> 30:24.380 This is three pictures, sort of shown as a movie. 30:24.380 --> 30:25.860 See this thing over here that's moving? 30:25.859 --> 30:29.989 That's moving because, unlike all the other objects in 30:29.990 --> 30:33.110 this picture, that thing is not a distant 30:33.108 --> 30:33.808 star. 30:33.809 --> 30:36.449 That is a planet in the Outer Solar System. 30:36.450 --> 30:38.930 This is the one that for a while was called Xena, 30:38.931 --> 30:40.121 now it's called Eris. 30:40.119 --> 30:43.289 And these are the discovery photographs of this. 30:43.289 --> 30:47.049 Then as they kept taking pictures, they could plot the 30:47.052 --> 30:47.622 orbit. 30:47.619 --> 30:50.319 They determined the orbit of the thing using an equation 30:50.316 --> 30:51.686 you've already discovered. 30:51.690 --> 30:53.500 They figure out how far away it is. 30:53.500 --> 30:55.960 From how far away it is and how bright it is, 30:55.956 --> 30:57.906 you can figure out how big it is. 30:57.910 --> 31:01.180 And the problem that Eris presents is, it's bigger than 31:01.184 --> 31:01.674 Pluto. 31:01.670 --> 31:07.970 And there are a whole bunch of other things out there that have 31:07.973 --> 31:11.333 also been discovered, also as big, 31:11.328 --> 31:13.868 or bigger than Pluto. 31:13.869 --> 31:16.249 Here are the eight currently largest-known, 31:16.250 --> 31:19.480 so-called "trans-Neptunian" objects, sometimes also called 31:19.480 --> 31:20.840 Kuiper Belt objects. 31:20.839 --> 31:22.039 Here's Eris, here's Pluto, 31:22.042 --> 31:23.632 here are a bunch of other ones. 31:23.630 --> 31:26.000 They find these things, like, one or two of them a 31:25.999 --> 31:28.609 year, now, and so we can confidently expect that twenty 31:28.611 --> 31:30.801 years from now, there are going to 100 such 31:30.802 --> 31:31.182 things. 31:31.180 --> 31:33.890 So you have the problem--oh you should--it's worth nothing, 31:33.891 --> 31:37.361 these things also have moons, and some of them have a 31:37.360 --> 31:42.280 moon-to-planet ratio similar to the Earth and the moon. 31:42.280 --> 31:43.990 But here's the Earth for scale. 31:43.990 --> 31:46.100 All of them, including Pluto and Eris, 31:46.096 --> 31:48.086 very much smaller than the Earth. 31:48.089 --> 31:50.249 And so you've got yourself a problem. 31:50.250 --> 31:53.930 If you're going to count Pluto, how do you not count these 31:53.931 --> 31:54.901 other things? 31:54.900 --> 31:57.450 And so the first suggestion was, well, you ought to count 31:57.447 --> 32:00.447 Pluto, and you ought to count a bunch of these other things too. 32:00.450 --> 32:02.660 And so, there was going to be twelve planets, 32:02.662 --> 32:04.572 or something like that, and presumably, 32:04.573 --> 32:05.683 many more to come. 32:05.680 --> 32:08.050 And then people decided, you know, the heck with this 32:08.054 --> 32:09.154 whole bunch of things. 32:09.150 --> 32:10.840 We'll give them a different name; 32:10.839 --> 32:12.229 we won't call them planets at all. 32:12.230 --> 32:16.600 And that was what was finally determined, leading to all sorts 32:16.596 --> 32:19.436 of, you know, "Save Pluto" campaigns from 32:19.442 --> 32:22.072 disgruntled third-graders who--[laughter] 32:22.069 --> 32:25.809 yes, thank you very much--who were unhappy about having to 32:25.811 --> 32:29.821 re-memorize all the mnemonics that tell you the planets in the 32:29.816 --> 32:31.126 Solar System. 32:31.130 --> 32:33.670 Actually, if you included things like Quaoar, 32:33.672 --> 32:36.682 you'll get yourself into trouble trying to learn them 32:36.677 --> 32:37.137 all. 32:37.140 --> 32:40.910 And they're finding lots of lots of these things. 32:40.910 --> 32:45.020 Okay, so just a little about the geography of the Solar 32:45.015 --> 32:45.695 System. 32:45.700 --> 32:48.230 Here's the Inner Solar System out to the orbit of Jupiter, 32:48.226 --> 32:49.596 with the asteroids and stuff. 32:49.599 --> 32:52.509 Here's the Outer Solar System, starting with Jupiter. 32:52.509 --> 32:54.839 Here's Pluto, and you'll notice that Pluto 32:54.837 --> 32:58.187 really is an elliptical orbit, much more elliptical than the 32:58.187 --> 32:59.207 other planets. 32:59.210 --> 33:01.660 It also turns out the orbit is tipped. 33:01.660 --> 33:04.540 All the other planets--the orbits are in the same plane as 33:04.542 --> 33:05.202 each other. 33:05.200 --> 33:07.070 Pluto's orbit is tipped out of the plane. 33:07.069 --> 33:10.549 Here's one of the--these other trans-Neptunian objects. 33:10.549 --> 33:13.069 Here's its orbit, and you can see that it goes 33:13.066 --> 33:16.306 way, way out and is highly elliptical, very much out of the 33:16.310 --> 33:16.870 plane. 33:16.869 --> 33:18.859 This is, in general, true of these, 33:18.856 --> 33:22.066 what are now called "dwarf planets," or "trans-Neptunian 33:22.070 --> 33:24.350 objects," or "Kuiper Belt objects." 33:24.349 --> 33:27.659 But it's also true that there is a further component of the 33:27.662 --> 33:29.492 Solar System even beyond that. 33:29.490 --> 33:31.980 Here's Sedna's orbit--that again is Sedna's orbit, 33:31.983 --> 33:34.073 so we've now gone up a degree in scale. 33:34.069 --> 33:38.569 And then there's this whole cloud of stuff out there called 33:38.567 --> 33:43.217 the Oort cloud and this is--this is where the comets live. 33:43.220 --> 33:44.860 Comets are sort of ice balls. 33:44.859 --> 33:47.109 We can't see them individually in the Oort cloud, 33:47.106 --> 33:49.816 but sometimes they collide with each other, and one of them 33:49.821 --> 33:51.601 falls into the Inner Solar System. 33:51.600 --> 33:54.460 And as it does, it heats up. 33:54.460 --> 33:56.300 It's made out of ice, the ice melts, 33:56.299 --> 33:58.609 gets--streams backward, and we get these very 33:58.611 --> 33:59.821 spectacular things. 33:59.819 --> 34:03.499 Comets cause all kinds of mayhem, they kill dinosaurs, 34:03.496 --> 34:06.126 for example, and recently we've seen an 34:06.132 --> 34:07.522 example of this. 34:07.519 --> 34:10.229 This is a comet called Shoemaker-Levy that's broken up 34:10.231 --> 34:11.511 into a bunch of pieces. 34:11.510 --> 34:14.450 And in 1994, those pieces slammed into 34:14.449 --> 34:18.819 Jupiter, and here you see the various--the effect of the 34:18.818 --> 34:22.788 various pieces of this comet that hit Jupiter. 34:22.789 --> 34:26.589 And so, comets can be dangerous things when they land on your 34:26.590 --> 34:27.160 planet. 34:27.159 --> 34:31.319 Okay, this is the Outer Solar System again. 34:31.320 --> 34:33.320 Here's the tipped orbit of Pluto. 34:33.320 --> 34:34.570 That's the orbit of Neptune. 34:34.570 --> 34:37.510 Here's Voyager I, Voyager II traveling now 34:37.512 --> 34:38.232 outward. 34:38.230 --> 34:40.560 They are escaping from the Solar System. 34:40.559 --> 34:45.939 And the very edges of the Sun's influence, where you might say 34:45.936 --> 34:50.866 the Solar System ends and the interstellar medium begins, 34:50.872 --> 34:52.902 are indicated here. 34:52.900 --> 34:56.150 Each of these things means a slightly different boundary to 34:56.148 --> 34:57.378 the Sun's influence. 34:57.380 --> 35:00.020 But the Voyagers are going to get there sometime in our 35:00.019 --> 35:02.709 lifetime, so they will be the first man-made things that 35:02.707 --> 35:04.757 actually pass out of our Solar System. 35:04.760 --> 35:07.930 And that's as far as this goes. 35:07.929 --> 35:12.229 Let me turn the lights back on, here. 35:12.230 --> 35:17.260 Okay, so much for the Solar System. 35:17.260 --> 35:17.620 Now. 35:17.620 --> 35:22.940 Here's the important question: I just threw a whole bunch of 35:22.937 --> 35:24.377 facts at you. 35:24.380 --> 35:26.390 Facts, information, pretty pictures. 35:26.389 --> 35:28.559 We have a lot of facts, and information, 35:28.557 --> 35:29.777 and pretty pictures. 35:29.780 --> 35:33.440 The Voyagers took many thousands of pictures and 35:33.437 --> 35:37.637 other--acquired other forms of information, as well. 35:37.639 --> 35:39.849 There have been many other spacecraft flying around our 35:39.854 --> 35:41.294 Solar System for quite some time. 35:41.290 --> 35:43.690 We know a lot of things. 35:43.690 --> 35:46.050 Here's the question: now that you know all this 35:46.045 --> 35:48.395 stuff, what do you do with that information? 35:48.400 --> 35:58.590 So, what do you do with all this Solar System information? 35:58.590 --> 36:01.730 36:01.730 --> 36:04.430 One thing you could do is, write it all down and memorize 36:04.429 --> 36:04.669 it. 36:04.670 --> 36:08.220 I strongly advise against that. 36:08.219 --> 36:10.699 You know, this is what the Internet is for. 36:10.699 --> 36:13.439 There are a whole bunch of websites out there that'll tell 36:13.438 --> 36:16.078 you everything you could possibly want to know about all 36:16.081 --> 36:18.101 of the objects and all of the planets, 36:18.100 --> 36:19.590 moons, everything else. 36:19.590 --> 36:21.860 So that isn't actually a productive way, 36:21.864 --> 36:24.144 I don't think, of spending your time. 36:24.140 --> 36:25.730 So, what would you rather do? 36:25.730 --> 36:28.730 What would one--what would be a more productive way of thinking 36:28.734 --> 36:29.804 about this material? 36:29.800 --> 36:33.260 And now, I want to step back and remind you of how science 36:33.263 --> 36:33.753 works. 36:33.750 --> 36:35.530 Remember the scientific method? 36:35.530 --> 36:38.320 They probably taught you something about this when you 36:38.320 --> 36:40.480 were, like, eleven or twelve years old. 36:40.480 --> 36:42.160 And you'll remember how this works. 36:42.159 --> 36:47.439 So, scientific method--so, you have a hypothesis, 36:47.440 --> 36:51.400 which is a fancy word for a guess. 36:51.400 --> 36:55.010 Maybe you have competing hypotheses. 36:55.010 --> 36:59.540 And on the basis of this hypothesis you formulate some 36:59.543 --> 37:01.343 kind of experiment. 37:01.340 --> 37:05.490 Good experiments are sometimes called "controlled" experiments. 37:05.489 --> 37:07.979 And on the basis of the results of the experiment, 37:07.980 --> 37:10.470 you determine how much you believe the hypothesis, 37:10.470 --> 37:12.300 or which hypothesis you believe. 37:12.300 --> 37:15.100 And then, you know, you may modify your hypothesis 37:15.098 --> 37:18.408 or change it all together, and then do more experiments. 37:18.409 --> 37:22.109 And by iterating this procedure, you develop an 37:22.106 --> 37:27.166 understanding of whatever it is you're trying to think about. 37:27.170 --> 37:29.070 Okay. 37:29.070 --> 37:32.040 No. 37:32.039 --> 37:35.349 Not really, certainly not in astronomy. 37:35.349 --> 37:37.849 Astronomy doesn't work this way at all. 37:37.849 --> 37:40.799 Think about what an experiment would be in astronomy. 37:40.800 --> 37:44.980 Okay, so here's a ball of gas the size of the Sun made out of 37:44.976 --> 37:46.086 pure hydrogen. 37:46.090 --> 37:49.570 Here's another ball of gas the size of the Sun made out of pure 37:49.565 --> 37:50.065 helium. 37:50.070 --> 37:52.890 We stick them in the sky and watch them evolve for ten 37:52.889 --> 37:54.059 billion years, okay. 37:54.059 --> 37:56.709 No, you can't do experiments in astronomy. 37:56.710 --> 37:58.170 It doesn't work that way at all. 37:58.170 --> 38:01.420 So, there has to be some other way of approaching it. 38:01.420 --> 38:04.570 And the reason for this is, is that astronomy is not an 38:04.565 --> 38:07.705 experimental science, it's an observational science. 38:07.710 --> 38:09.030 This is true of many other sciences. 38:09.030 --> 38:12.520 A lot of biology, particularly environmental, 38:12.517 --> 38:16.477 or ecological aspects of biology, work this way. 38:16.480 --> 38:18.360 All of the social sciences. 38:18.360 --> 38:21.380 You can't do controlled experiments in child 38:21.381 --> 38:23.631 development, that's just ugly. 38:23.630 --> 38:30.240 And so, these are observational sciences, and this has a 38:30.243 --> 38:33.133 different methodology. 38:33.130 --> 38:36.350 And it starts, as its name would suggest, 38:36.349 --> 38:37.959 with observations. 38:37.960 --> 38:40.980 And you go out and you find a bunch of things, 38:40.977 --> 38:44.727 whether they're butterflies, or planets, or whatever they 38:44.733 --> 38:45.273 are. 38:45.269 --> 38:49.019 And what do you do when you've found a whole bunch of things? 38:49.020 --> 38:49.830 What's the next step? 38:49.830 --> 38:52.500 The next step is classification. 38:52.500 --> 38:55.120 You put them into categories. 38:55.119 --> 38:58.499 And it's important to get this right. 38:58.500 --> 39:00.300 If you're dealing with, for example, 39:00.295 --> 39:03.475 animals, and you classify them as things that live in the ocean 39:03.476 --> 39:06.836 and things that live on land, then you're going to have fish 39:06.841 --> 39:08.681 and whales in the same category. 39:08.679 --> 39:12.999 And you're going to have lizards and frogs and snakes in 39:13.001 --> 39:18.031 the same category with people and bears, and things like that. 39:18.030 --> 39:20.880 And this isn't going to lead you to a deep understanding, 39:20.884 --> 39:23.284 because you haven't got the categories right. 39:23.280 --> 39:25.610 And so the classification is very important. 39:25.610 --> 39:29.950 And what it leads to when you get it right is a useful 39:29.950 --> 39:32.980 interpretation of what is going on. 39:32.980 --> 39:36.320 This is--interpretation, it's just a fancy word for a 39:36.324 --> 39:37.164 good story. 39:37.159 --> 39:39.649 These are the kinds of things we write down in textbooks. 39:39.650 --> 39:44.040 The astronomers sometimes dignify this with the fancy word 39:44.039 --> 39:45.039 "scenario." 39:45.040 --> 39:47.600 But it's basically a story. 39:47.599 --> 39:49.579 And on the basis of this story, you say, well, 39:49.582 --> 39:51.702 we better check out, let's see whether this story 39:51.697 --> 39:52.267 holds up. 39:52.269 --> 39:58.189 Let's do more observations, and--all of these connections 39:58.188 --> 40:00.828 work in all directions. 40:00.829 --> 40:04.879 And so, this is actually a better description of how an 40:04.875 --> 40:09.665 observational science is done than this kind of thing up here. 40:09.670 --> 40:14.280 So now, having gone through that little piece of philosophy, 40:14.282 --> 40:16.552 let's apply it in practice. 40:16.550 --> 40:18.950 I've just described to you some observations, 40:18.954 --> 40:21.144 some observations of the Solar System. 40:21.139 --> 40:24.299 Now, what I'm going to do is, I'm going to take those objects 40:24.297 --> 40:26.397 that I just showed you and classify them, 40:26.402 --> 40:27.352 or attempt to. 40:27.349 --> 40:33.809 So let us classify--these are now going to be categories of 40:33.808 --> 40:36.368 Solar System objects. 40:36.370 --> 40:40.110 40:40.110 --> 40:41.640 So, I claim, or at least, 40:41.642 --> 40:45.472 it's the start of a good story, that there are six categories 40:45.474 --> 40:47.714 of objects in the Solar System. 40:47.710 --> 40:51.040 The first is the Sun, which I didn't actually show 40:51.040 --> 40:55.050 you, which is 99% of all of the mass in the Solar System, 40:55.050 --> 41:00.140 and almost all of the energy and heat, and so forth. 41:00.139 --> 41:05.419 Then there are the inner planets, sometimes called the 41:05.416 --> 41:10.396 terrestrial planets, because the Earth is the prime 41:10.395 --> 41:11.585 example. 41:11.590 --> 41:15.440 And these are rocks, basically that would be 41:15.438 --> 41:18.658 silicon, iron, elements like that, 41:18.659 --> 41:26.809 with a very thin surface coating of ice, 41:26.813 --> 41:36.433 in some cases melted, in some cases gaseous. 41:36.429 --> 41:39.709 What I mean by ice is not necessarily water-ice, 41:39.708 --> 41:42.568 but also things like ammonia, and methane, 41:42.568 --> 41:45.148 and other compounds of that kind. 41:45.150 --> 41:47.070 This coating is very thin. 41:47.070 --> 41:49.350 If you had a scale model of the Earth that was six feet across 41:49.349 --> 41:50.769 and you put your hand up against it, 41:50.769 --> 41:53.599 you wouldn't even feel that it was wet from the oceans. 41:53.600 --> 41:55.180 So, not very much. 41:55.179 --> 41:59.039 And these have masses, these things, 41:59.043 --> 42:03.023 these inner planets of, I don't know, 42:03.017 --> 42:06.767 10^(-7) to 10^(-5) of the Sun. 42:06.769 --> 42:10.219 And they're in basically circular orbits. 42:10.220 --> 42:13.660 42:13.659 --> 42:19.099 Then you've got the asteroids, which are irregular, 42:19.103 --> 42:26.183 very small rocks in--mostly in orbits between Mars and Jupiter. 42:26.179 --> 42:30.289 And out beyond the asteroids, you've got the outer planets, 42:30.293 --> 42:33.133 sometimes called the "Jovian" planets, 42:33.130 --> 42:37.420 because Jupiter is the prime example of these things. 42:37.420 --> 42:39.690 And these are quite different from the inner planets. 42:39.690 --> 42:43.200 They've got a lot of gas and ice, both gas, 42:43.201 --> 42:47.551 which I'm defining here to be hydrogen and helium, 42:47.550 --> 42:51.440 and ice, which I'm defining to be, as I said, 42:51.439 --> 42:55.769 water, ammonia, methane, and similar compounds. 42:55.769 --> 43:03.549 And these are more massive, 10^(-4) to 10^(-3) of the Sun. 43:03.550 --> 43:09.280 They have rings, many moons, also circular 43:09.281 --> 43:10.541 orbits. 43:10.540 --> 43:15.980 43:15.980 --> 43:19.680 And all these orbits, both these planets and these 43:19.677 --> 43:22.467 planets are basically in one plane. 43:22.469 --> 43:27.229 Out beyond the Jovian planets are these trans-Neptunian 43:27.231 --> 43:30.231 objects, or Kuiper Belt objects. 43:30.230 --> 43:34.070 43:34.070 --> 43:37.630 This is Pluto, etcetera. 43:37.630 --> 43:40.990 We don't really know what they are yet, but it seems like 43:40.985 --> 43:44.515 they're probably going to be rocky, just to judge from their 43:44.521 --> 43:45.601 size and mass. 43:45.600 --> 43:47.460 So we think they're rocky. 43:47.460 --> 43:51.860 They're a low mass, less than 10^(-7) of the mass 43:51.858 --> 43:53.048 of the Sun. 43:53.050 --> 44:00.660 And they are in elliptical and inclined orbits, 44:00.662 --> 44:03.312 way out there. 44:03.309 --> 44:06.579 And then, finally, in the outer region, 44:06.575 --> 44:10.785 in this so-called Oort cloud, you have the comets, 44:10.786 --> 44:13.446 which are little snowballs. 44:13.449 --> 44:17.309 Balls of ice, again, ice in this generic 44:17.308 --> 44:18.098 sense. 44:18.099 --> 44:21.859 So, here are the six categories that I would claim exist in the 44:21.860 --> 44:22.770 Solar System. 44:22.769 --> 44:26.049 And here's my problem with the whole Pluto debate. 44:26.050 --> 44:29.130 The Pluto debate was basically about whether these guys are 44:29.125 --> 44:30.605 going to count as planets. 44:30.610 --> 44:33.730 But the thing is, "planets" is already a bad 44:33.734 --> 44:38.534 description, because it contains two quite different categories; 44:38.530 --> 44:41.780 namely, these inner terrestrial planets, and the outer Jovian 44:41.778 --> 44:42.318 planets. 44:42.320 --> 44:46.360 So, it seems to me that arguing whether category five should be 44:46.359 --> 44:49.359 part of some category that already contains two 44:49.356 --> 44:53.266 fundamentally different kinds of objects is kind of a strange 44:53.265 --> 44:55.085 argument to be having. 44:55.090 --> 44:58.200 Either we should split these two things off from each other, 44:58.204 --> 45:00.744 or, if we're going to join these two kinds of the 45:00.738 --> 45:02.858 categories, fine, bring in anything you 45:02.855 --> 45:03.145 like. 45:03.150 --> 45:04.470 I don't care, add the asteroids, 45:04.465 --> 45:04.715 too. 45:04.719 --> 45:07.039 And, in fact, in the original proposal, 45:07.043 --> 45:09.553 one of the asteroids qualified as well. 45:09.550 --> 45:14.500 And so, it doesn't seem to me that this controversy was really 45:14.497 --> 45:19.607 paying justice to an appropriate classification of the things in 45:19.606 --> 45:21.306 the Solar System. 45:21.310 --> 45:23.280 Okay, so now. 45:23.280 --> 45:27.870 Having classified it, the next step is 45:27.870 --> 45:29.980 interpretation. 45:29.980 --> 45:33.220 And, I think I will leave that--let's see, 45:33.218 --> 45:35.428 how are we doing for time? 45:35.429 --> 45:37.389 Yes, I'll leave that one for next time. 45:37.389 --> 45:41.009 So the question is going to be: now that you have these six 45:41.014 --> 45:43.704 kinds of objects, what is the story you tell 45:43.702 --> 45:46.142 about how the Solar System evolved? 45:46.139 --> 45:50.069 And what, in turn, does that tell you about what 45:50.068 --> 45:54.998 other kinds of planets you should see around other stars?