1 00:00:01,200 --> 00:00:05,859 Our solar system's an exciting place. one of the most exciting developments in 2 00:00:05,859 --> 00:00:10,174 the past few decades has been the realization that our solar system is not 3 00:00:10,174 --> 00:00:12,762 unique. There are solar systems all over the 4 00:00:12,762 --> 00:00:15,926 place and we're starting to learn something about them. 5 00:00:15,926 --> 00:00:20,356 And it seems very fitting to end this week of solar system science with some 6 00:00:20,356 --> 00:00:24,900 discussion of what it is that we found out there, the topic that goes under the 7 00:00:24,900 --> 00:00:27,777 name extrasolar planets, or exoplanets for short. 8 00:00:27,777 --> 00:00:30,753 And so, the fact that planets should exist around 9 00:00:30,753 --> 00:00:35,410 other stars and that the sun is not unique in having planets is an old idea. 10 00:00:35,410 --> 00:00:40,496 the fact that we saw protoplanetary disks around stars suggests that many stars 11 00:00:40,496 --> 00:00:44,714 will have planets. and people have been looking for planets 12 00:00:44,714 --> 00:00:50,092 around other stars for a long time. The first successful discovery was in 13 00:00:50,092 --> 00:00:55,542 1988, so only 27 years ago. by now at last count, there are 853 14 00:00:55,542 --> 00:01:01,062 confirmed planets orbiting 672 different stars, so we have whole solar systems 15 00:01:01,062 --> 00:01:05,097 that we have found and the field is growing very fast. 16 00:01:05,097 --> 00:01:11,325 I think we're just starting to learn what these extra solar systems have to teach 17 00:01:11,325 --> 00:01:14,297 us. finding planets is a tough business. 18 00:01:14,297 --> 00:01:19,293 what you're looking for basically is a chunk of rock or gas that is not hot and 19 00:01:19,293 --> 00:01:25,821 is not glowing and is very near to something, a star, that is hot and is 20 00:01:25,821 --> 00:01:32,655 glowing, so it is a difficult problem trying to actually see a planet at great 21 00:01:32,655 --> 00:01:36,218 distance. we see the light of the star, but seeing 22 00:01:36,218 --> 00:01:40,330 the reflected light off a planet is a difficult proposition. 23 00:01:40,330 --> 00:01:47,174 despite this, it has been applied and all 32 planets and 38 systems have been 24 00:01:47,174 --> 00:01:51,196 actually imaged. In other words were found in actual 25 00:01:51,196 --> 00:01:56,593 images of a star, the way this is done is the same way we will learn next week. 26 00:01:56,593 --> 00:02:01,791 technology developed to image the outer regions of the star, of the Sun, the 27 00:02:01,791 --> 00:02:07,320 corona when basically superimposes a disc that blocks out the star light in the 28 00:02:07,320 --> 00:02:10,962 one's telescope and then looks for the dim lights around it. 29 00:02:10,962 --> 00:02:15,777 there are future missions that are designed to specifically image extra 30 00:02:15,777 --> 00:02:20,221 solar planets and we might be able to increase the rate of discovery. 31 00:02:20,221 --> 00:02:24,357 Anyone can learn a lot about a planet if one can actually image it. 32 00:02:24,357 --> 00:02:29,048 But most of the 900 planets we've seen have not been discovered that way. 33 00:02:29,048 --> 00:02:32,998 In fact, most of the time, all you can see is the light of a star. 34 00:02:32,998 --> 00:02:37,690 So, how does the light of a star tell you that there is a planet orbiting it? 35 00:02:37,690 --> 00:02:42,880 The following few simulations might help understand that. 36 00:02:42,880 --> 00:02:48,155 One key to understanding how a star can tell us that a planet is orbiting it is 37 00:02:48,155 --> 00:02:53,429 to remember that if a planet is orbiting the star, then the star is also orbiting 38 00:02:53,429 --> 00:02:56,554 the planet. In fact, they are both orbiting their 39 00:02:56,554 --> 00:03:00,721 common center of gravity as this animation here will show us. 40 00:03:00,721 --> 00:03:05,343 We have here a planet and a star. I have just, for the purposes of 41 00:03:05,343 --> 00:03:08,599 discussion, given them a mass ratio of one to four. 42 00:03:08,599 --> 00:03:13,742 And, what we see is that they each orbit I've assumed a circular orbit and they 43 00:03:13,742 --> 00:03:21,415 each orbit in a circle about the relative common center of mass, which is here this 44 00:03:21,415 --> 00:03:25,356 green X. And what we see is that, the radii of the 45 00:03:25,356 --> 00:03:30,193 two circles are of course distances from the center of mass. 46 00:03:30,193 --> 00:03:36,571 In other words if the larger star here is four times as massive as the smaller as 47 00:03:36,571 --> 00:03:42,223 the smaller planet, then the radius of the orbit in which the star orbits is a 48 00:03:42,223 --> 00:03:45,630 fourth the radius of the orbit in which the 49 00:03:45,630 --> 00:03:50,390 planet orbits. In other words, if this, orbital radius 50 00:03:50,390 --> 00:03:53,883 is our and this orbital radius is our planet, 51 00:03:53,883 --> 00:03:59,611 then, since they're orbiting their center of mass, these satisfy mass of the planet 52 00:03:59,611 --> 00:04:05,340 times the radius of the planet's orbit is equal to the mass of the star times the 53 00:04:05,340 --> 00:04:09,951 radius of the star's orbit. And of course, the distance between the 54 00:04:09,951 --> 00:04:14,981 planet and the star, since they're on opposite sides, are the distance is 55 00:04:14,981 --> 00:04:17,915 simply rs plus rp, the sum of the two radii. 56 00:04:17,915 --> 00:04:20,920 And this distance is constant at all times, 57 00:04:20,920 --> 00:04:25,551 of course, most planets don't orbit in circular orbits but as usual we will make 58 00:04:25,551 --> 00:04:29,313 that simplifying assumption, it'll help us make calculations more 59 00:04:29,313 --> 00:04:33,221 straightforward. Of course, since typically, a star is far 60 00:04:33,221 --> 00:04:38,216 more massive than the planets that orbit it, the star will move a far small, in a 61 00:04:38,216 --> 00:04:43,420 far smaller circle than well the planets and detecting the motion of a star might 62 00:04:43,420 --> 00:04:46,196 be difficult because the motion is very small. 63 00:04:46,196 --> 00:04:51,145 To give us a sense for, of how this works we'll try to detect, imagine detecting 64 00:04:51,145 --> 00:04:55,912 from a distance, a solar, the planets of a solar system we understand, namely our 65 00:04:55,912 --> 00:04:59,111 Solar System. And indeed, because earth orbits the sun, 66 00:04:59,111 --> 00:05:03,697 the sun is also orbiting the earth, the center of mass of earth and sun lies 67 00:05:03,697 --> 00:05:07,258 well deep within the sun. And it would be very difficult to 68 00:05:07,258 --> 00:05:11,916 actually note the motion of the sun. In fact the only planet whose common 69 00:05:11,916 --> 00:05:16,196 center of mass with the sun lies outside just outside the surface of the sun is 70 00:05:16,196 --> 00:05:18,765 Jupiter. And in this animation, we see that if we 71 00:05:18,765 --> 00:05:23,045 take into account only the gravitational effects of Jupiter, then we observe the 72 00:05:23,045 --> 00:05:28,192 sun moving, around in a circle around a point just outside the surface of the 73 00:05:28,192 --> 00:05:31,064 sun. Saturn's influence is the next most 74 00:05:31,064 --> 00:05:35,934 remarkable and because we now have two periodic motions, the sun's motion is a 75 00:05:35,934 --> 00:05:40,742 little bit more complicated and no longer moves in a circle. So by doing some 76 00:05:40,742 --> 00:05:45,799 interesting analysis, we could, in fact, disentangle perhaps, the two motions due 77 00:05:45,799 --> 00:05:50,008 to the sun, to Saturn and Jupiter if we can observe this motion. 78 00:05:50,008 --> 00:05:55,320 So, we need to understand how far the sun is moving and try to see what 79 00:05:55,320 --> 00:05:58,810 limits this places on our ability to detect planets. 80 00:05:58,810 --> 00:06:03,831 So now, we, we, we have an idea. Maybe we can observe that a planet is 81 00:06:03,831 --> 00:06:07,130 orbiting a star by watching the star wobble. 82 00:06:07,130 --> 00:06:10,009 We need to understand how much the star will wobble. 83 00:06:10,009 --> 00:06:13,996 We expect it not to be very much, but because it is a periodic motion, we 84 00:06:13,996 --> 00:06:18,758 might be able to actually distinguish it and realize that what is going on is that 85 00:06:18,758 --> 00:06:22,690 it, there's a planet orbiting it. And we said, that if we assume circular 86 00:06:22,690 --> 00:06:27,120 orbits, there's a distance r between the star and the planet, then we can write r 87 00:06:27,120 --> 00:06:34,085 as R is the radius of the circle in which the star moves plus the radius of the 88 00:06:34,085 --> 00:06:40,439 circle in which the planet moves and the two radii are determined by the center of 89 00:06:40,439 --> 00:06:44,649 mass relation. And plugging one relation into the other, 90 00:06:44,649 --> 00:06:50,543 I see that the total distance is Rs + Rp, but Rp is Rs * Ms / Mp. 91 00:06:50,543 --> 00:06:55,290 We write it this way and now, I can solve this equation for Rs, 92 00:06:55,290 --> 00:07:05,540 Rs = to R / 1 + Ms / Mp. And then, I can pick a common denominator 93 00:07:05,540 --> 00:07:10,693 on the bottom to write Mp + Ms and Mp comes here. 94 00:07:10,693 --> 00:07:16,540 Mp + Ms is nothing but the total mass, which I'll call M. 95 00:07:16,540 --> 00:07:21,684 And this is not surprising, their amount by which the star moves, is related to 96 00:07:21,684 --> 00:07:27,234 the distance between the two by the ratio between the planet's mass and the total 97 00:07:27,234 --> 00:07:30,957 mass, the larger the planet the more the star will move. 98 00:07:30,957 --> 00:07:36,090 Not too surprising, lets see what that tells us for example about an idea, 99 00:07:36,090 --> 00:07:39,733 let's try to detect Jupiter. How far would the sun move? 100 00:07:39,733 --> 00:07:42,980 Well, plugging in this result we just figured out. 101 00:07:42,980 --> 00:07:48,088 I find that the radius of the circle in which the sun moves due to the 102 00:07:48,088 --> 00:07:53,844 gravitational effect of Jupiter is the ratio of the mass of Jupiter to the mass 103 00:07:53,844 --> 00:07:59,672 of the sun times the radius of Jupiter's orbit, which I've put in here and that's 104 00:07:59,672 --> 00:08:03,702 indeed right outside the sun. This is 740,000 kilometers. 105 00:08:03,702 --> 00:08:07,515 The sun itself is about 696,000 kilometers in radius. 106 00:08:07,515 --> 00:08:13,564 This point lies right outside the sun. Detecting the wobble of a star due to the 107 00:08:13,564 --> 00:08:17,708 effects of a planet that orbits it is going to be difficult. 108 00:08:17,708 --> 00:08:22,887 In fact, of all of the planets that we know, one was detected by this method, 109 00:08:22,887 --> 00:08:24,890 this is called astrometry. So, 110 00:08:24,890 --> 00:08:30,502 finding the motion discerning the motion of the star is not going to be our most 111 00:08:30,502 --> 00:08:33,273 successful strategy. What else can we do? 112 00:08:33,273 --> 00:08:37,840 Let's go back to the simulation and see a different approach. 113 00:08:37,840 --> 00:08:41,674 The secret, it turns out, is that while it is very difficult to 114 00:08:41,674 --> 00:08:46,745 measure the position of a star with the precision necessary to detect a planet 115 00:08:46,745 --> 00:08:50,147 surrounding it, we may be able to detect its wobble by 116 00:08:50,147 --> 00:08:53,482 measuring its velocity. How can you measure velocity without 117 00:08:53,482 --> 00:08:56,067 measuring position? That's an interesting question. 118 00:08:56,067 --> 00:08:59,789 Well, there is a sense in which we can measure velocity directly without 119 00:08:59,789 --> 00:09:02,374 measuring position and this is the Doppler effect. 120 00:09:02,374 --> 00:09:06,509 Remember that the spectrum of a star will contain absorption lines for whatever 121 00:09:06,509 --> 00:09:10,283 gases are in the star's atmosphere, as we saw for the spectrum of the sun, 122 00:09:10,283 --> 00:09:14,909 we can identify a particular absorption line and find that it is shifted in 123 00:09:14,909 --> 00:09:18,989 frequency or in wavelength from its natural wavelength. 124 00:09:18,989 --> 00:09:24,849 We can use the Doppler formula. I remind you that lambda is lambda 0 * 1 125 00:09:24,849 --> 00:09:31,006 - v / c, where c is the speed of light and v is the speed with which something 126 00:09:31,006 --> 00:09:34,345 is moving. Since spectral lines are very, very 127 00:09:34,345 --> 00:09:37,980 narrowly known we can measure rather small 128 00:09:37,980 --> 00:09:40,386 velocities compared to the speed of light. 129 00:09:40,386 --> 00:09:42,792 In fact, down to a few meters per a second. 130 00:09:42,792 --> 00:09:48,617 So one part in 10^8 we can make pretty accurate velocity measurements provided 131 00:09:48,617 --> 00:09:52,912 there is a Doppler effect. Remember, the Doppler effect only obtains 132 00:09:52,912 --> 00:09:57,656 if the object to the source of light is moving towards us or away from us. 133 00:09:57,656 --> 00:10:02,336 So in the position in, that, this animation currently depicts, of course, 134 00:10:02,336 --> 00:10:07,016 we have no such luck, because as this planet orbits the star and the star 135 00:10:07,016 --> 00:10:10,030 orbits the planet, there is no motion at all 136 00:10:10,030 --> 00:10:16,790 no change in the star's distance from us so there, that there is absolutely no 137 00:10:16,790 --> 00:10:22,974 radial velocity component. The star or the planet neither approach us nor 138 00:10:22,974 --> 00:10:26,503 recede. On the other hand, if I tilt and I 139 00:10:26,503 --> 00:10:30,837 imagine that the system happens to be oriented in this direction, 140 00:10:30,837 --> 00:10:35,239 then what I observe is that now the planet, and therefore the star, 141 00:10:35,239 --> 00:10:40,640 periodically approaches earth and recedes from earth and maybe we can detect the 142 00:10:40,640 --> 00:10:45,641 variations in wavelength of some absorption line by measuring the velocity 143 00:10:45,641 --> 00:10:50,242 with which the star is moving. And, the way we do this is precisely by 144 00:10:50,242 --> 00:10:52,710 doing spectroscopy. Let's take a look. 145 00:10:54,220 --> 00:10:55,894 This animation sort of shows you the set up. 146 00:10:55,894 --> 00:11:00,457 We have here a planet orbiting the star, therefore the star orbiting the planet, 147 00:11:00,457 --> 00:11:04,672 albeit in a much smaller circle. We cannot see the planet, but we can see 148 00:11:04,672 --> 00:11:07,387 the star and things here are perfectly aligned. 149 00:11:07,387 --> 00:11:11,814 The motion of the system is in such a plain that the entire velocity of this 150 00:11:11,814 --> 00:11:17,263 vector of the star will be visible to us. There is no motion out of the plain of 151 00:11:17,263 --> 00:11:22,100 this image, which would be invisible to us and we observe some absorption lines 152 00:11:22,100 --> 00:11:26,141 in the spectrum of the star. And when the star is receding from us, 153 00:11:26,141 --> 00:11:30,794 the absorption lines are shifted to the red, when the star is approaching us, 154 00:11:30,794 --> 00:11:34,896 they are shifted to the blue. And in, we see this periodic shift in the 155 00:11:34,896 --> 00:11:37,485 Doppler shift in the absorption lines and by 156 00:11:37,485 --> 00:11:41,730 detecting the periodicity, we can measure the period with which the planets orbits 157 00:11:41,730 --> 00:11:45,510 the star and perhaps learn some more. Of course, the Doppler shift here is 158 00:11:45,510 --> 00:11:48,720 greatly accelerated. Let's do a little calculation to see what 159 00:11:48,720 --> 00:11:51,270 kind of velocities we'd be trying to detect. 160 00:11:51,270 --> 00:11:56,572 So perhaps, the way that we can detect the fact that a planet is orbiting a 161 00:11:56,572 --> 00:12:01,898 star, hence the star orbiting a planet, is to hope that the orientation of the 162 00:12:01,898 --> 00:12:07,708 orbit is such that at least some fraction of the star's motion is radial towards us 163 00:12:07,708 --> 00:12:12,619 or away from us and we can use Doppler shifts to detect that velocity. 164 00:12:12,619 --> 00:12:16,146 Let's figure out how fast the star would be moving. 165 00:12:16,146 --> 00:12:20,020 so we have the system. how fast is the star moving? 166 00:12:20,020 --> 00:12:24,170 Well, the star moves around the circle of radius Rs, as we said, 167 00:12:24,170 --> 00:12:30,081 and it moves with a velocity speed Vs, whose velocity is continually changing. 168 00:12:30,081 --> 00:12:33,037 So, as we know, the acceleration, from our 169 00:12:33,037 --> 00:12:38,801 second week of class, the acceleration with which the star accelerates towards 170 00:12:38,801 --> 00:12:46,264 the center is Vs^2 / Rs and if I multiply this by the mass of the star this is the 171 00:12:46,264 --> 00:12:52,028 force acting on the planet, on the star to accelerate it and of course this is 172 00:12:52,028 --> 00:13:01,582 Newton's gravitational force. In other words, this is G * Mp * Ms / by the 173 00:13:01,582 --> 00:13:05,611 square of the distance between them. So let's put that in. 174 00:13:05,611 --> 00:13:09,568 Here's our equation. what do we do with this equation? 175 00:13:09,568 --> 00:13:14,445 Well, of course. we first realize that the Ms completely 176 00:13:14,445 --> 00:13:18,120 cancels from here, this is characteristic of gravity. 177 00:13:18,120 --> 00:13:24,082 now what I want to do is figure out v. I want to know how fast the star moved. 178 00:13:24,082 --> 00:13:30,171 so from here, I have that vs^2 is given by G times the mass of the planet times 179 00:13:30,171 --> 00:13:36,030 the radius of the circle in which the star moves divided by R^2. 180 00:13:36,030 --> 00:13:43,010 But it would help to remember that we computed that the radius of the circle in 181 00:13:43,010 --> 00:13:48,595 which the star moves is related to R by the mass of the planbet divided by the 182 00:13:49,904 --> 00:13:54,179 total mass. And so, using that in here, I find that 183 00:13:54,179 --> 00:14:10,280 this is G * Mp / R^2 * R * Mp / total M. And what I find is that this is given by 184 00:14:10,280 --> 00:14:22,260 well, I can cancel one of the Rs here and I find that vs^2 is g * 185 00:14:24,500 --> 00:14:31,221 Mp^2 / M * R, which is almost nice, except I will find 186 00:14:31,221 --> 00:14:39,308 it more convenient to write G * M / R, this is a quantity with dimensions of 187 00:14:39,308 --> 00:14:45,049 velocity squares, * ratio of Mp to the total M^2. 188 00:14:45,049 --> 00:14:50,152 All I've done here is multiply and divide by the mass the total mass. 189 00:14:50,152 --> 00:14:55,539 And so, this is my expression for vs and when I plug it in, I see that the 190 00:14:55,539 --> 00:15:00,996 following things are true for M is usually typically just the mass of the 191 00:15:00,996 --> 00:15:06,312 star, the mass of the planet is small. Anyway what we see is that massive 192 00:15:06,312 --> 00:15:10,140 planets will cause more motion. This is certainly true, 193 00:15:10,140 --> 00:15:16,871 but, unlike the case in the when we were trying to do astrometry, in the case of 194 00:15:16,871 --> 00:15:21,940 radial velocity measurements, the closer a planet is to its star, 195 00:15:21,940 --> 00:15:28,656 the smaller R, the more radial velocity the star will acquire. 196 00:15:28,656 --> 00:15:34,529 So this method will be useful in discovering planets that are massive and 197 00:15:34,529 --> 00:15:38,841 orbit close in. We can solve this equation now for vs by 198 00:15:38,841 --> 00:15:42,626 taking the square root and this is the expression we get. 199 00:15:42,626 --> 00:15:47,806 Again, the largest values of velocity will arise from heavy planets that orbit 200 00:15:47,806 --> 00:15:51,525 near their stars. We can plug in the numbers for Jupiter. 201 00:15:51,525 --> 00:15:56,306 If you look at this quantity in the square root, it's simply the orbital 202 00:15:56,306 --> 00:15:59,295 velocity of Jupiter to good approximation. 203 00:15:59,295 --> 00:16:04,474 And so, plugging in the numbers we see, that due to Jupiter orbiting it, the sun 204 00:16:04,474 --> 00:16:09,024 moves at 12.5 meters per second. This is a small velocity relative to the 205 00:16:09,024 --> 00:16:13,163 speed of light, but wavelengths can be measured with extreme precision. 206 00:16:13,163 --> 00:16:17,125 We would have very little trouble if orbits were aligned correctly, 207 00:16:17,125 --> 00:16:21,797 identifying the motion of the sun by ten meters per second, and in fact, this is 208 00:16:21,797 --> 00:16:25,582 one of the most productive ways of detecting extrasolar planets. 209 00:16:25,582 --> 00:16:30,667 a total of almost 500 planets in almost 400 solar systems have been detected by 210 00:16:30,667 --> 00:16:33,151 what we call radial velocity measurements. 211 00:16:33,151 --> 00:16:36,185 And remember, this is only possible for a systems so 212 00:16:36,185 --> 00:16:39,491 aligned that part at least of the motion is radial. 213 00:16:39,491 --> 00:16:44,205 If the system happens to lie so that the motion of the planet in the star is 214 00:16:44,205 --> 00:16:48,368 perpendicular to our line of site, there just is no radial component, 215 00:16:48,368 --> 00:16:52,044 there is no Doppler shift. We're out of luck trying to find it. 216 00:16:52,044 --> 00:16:57,520 despite this, this has been one of the most productive ways of measuring it. 217 00:16:57,520 --> 00:17:01,844 But if the system is aligned properly, we can do even better. 218 00:17:01,844 --> 00:17:05,658 Let's check this out. One great advantage of the radial 219 00:17:05,658 --> 00:17:11,479 velocity measurement detection is that if we have some way of estimating mass of 220 00:17:11,479 --> 00:17:16,506 the star, independently, we'll see that we have such methods, then, it gives us 221 00:17:16,506 --> 00:17:19,945 information about the mass of the orbiting planet, 222 00:17:19,945 --> 00:17:22,878 ass well as the distance at which it orbits. 223 00:17:22,878 --> 00:17:27,916 And so, we can extract the beginnings of more data about what kind of planet we 224 00:17:27,916 --> 00:17:31,678 see, and what kind of orbit. And there some circumstances, if the 225 00:17:31,678 --> 00:17:34,611 alignment is just right, we can do even better. 226 00:17:34,611 --> 00:17:39,840 if the alignment is such that the planet in fact transits in front of the star, 227 00:17:39,840 --> 00:17:42,905 then the planet will partially eclipse its star. 228 00:17:42,905 --> 00:17:48,017 And the way we will observe this is that the star light appears to dim when the 229 00:17:48,017 --> 00:17:52,529 planet is transiting and we call this measurement measuring the light curve. 230 00:17:52,529 --> 00:17:56,851 The light curve is the dependence of the brightness of the star on time. 231 00:17:56,851 --> 00:18:01,288 And, what we will see is a periodic change as the brightness of the star dips 232 00:18:01,288 --> 00:18:04,285 periodically, when the planet transits it, we can use 233 00:18:04,285 --> 00:18:08,434 this to compute the period. We can use this to study from the details 234 00:18:08,434 --> 00:18:12,353 of the shape of the light curve. You can learn many things about the 235 00:18:12,353 --> 00:18:16,617 planet and such as its size, its mass. If we're lucky, we might even see the 236 00:18:16,617 --> 00:18:20,965 planet's atmosphere beginning the eclipse process, and see the star through the 237 00:18:20,965 --> 00:18:24,702 planet's atmosphere, and attempt to do spectroscopy, and understand the 238 00:18:24,702 --> 00:18:28,764 chemistry of this star's atmosphere. So in principle, the transit method is a 239 00:18:28,764 --> 00:18:32,068 very constructive method, which gives us a lot of information. 240 00:18:32,068 --> 00:18:35,155 Some 300 planets and over 200 systems have been detected. 241 00:18:35,155 --> 00:18:39,488 There's a dedicated space telescope, the Kepler Space Telescope, which is doing a 242 00:18:39,488 --> 00:18:44,402 survey of the sky and already has over 2,000 candidate planets and over a 1,000 243 00:18:44,402 --> 00:18:48,810 candidate systems. But the transit system the transit method 244 00:18:48,810 --> 00:18:53,953 is not considered a complete detection until verified by some other means, so 245 00:18:53,953 --> 00:18:58,903 these are undergoing verification. The sample of planetary systems that we 246 00:18:58,903 --> 00:19:03,019 have outside the solar system is certainly growing very quickly and we 247 00:19:03,019 --> 00:19:06,694 should have a lot of information. Ideally, when detects the same planet 248 00:19:06,694 --> 00:19:10,841 both using radio velocity and transit method and then one can extract lots and 249 00:19:10,841 --> 00:19:14,201 lots of information. We will talk more about transits when we 250 00:19:14,201 --> 00:19:18,453 talk about binary stars and light curves. There are a few other methods that have 251 00:19:18,453 --> 00:19:21,708 been used. The lights of a star can be intensified 252 00:19:21,708 --> 00:19:25,225 by the place, presence of the planet, and some sixteen planets have been 253 00:19:25,225 --> 00:19:28,787 observed this ti-, this way. If we have one transiting planet, we can 254 00:19:28,787 --> 00:19:33,141 study its, orbit and find protobations to its orbit, and describe them to another 255 00:19:33,141 --> 00:19:37,440 planet, and learn about the existence of the other planet in the same system. 256 00:19:37,440 --> 00:19:41,905 Some planets have been detected this way. In all, we're compiling the collection of 257 00:19:41,905 --> 00:19:44,220 solar systems and it helps us to see, well, 258 00:19:44,220 --> 00:19:48,662 how do they stand to our understanding? How does this match our understanding of 259 00:19:48,662 --> 00:19:51,827 our solar system? We developed sort of a good theory that 260 00:19:51,827 --> 00:19:54,437 describes the one solar system in the universe, 261 00:19:54,437 --> 00:19:58,768 but now that we suddenly have thousands of them, you can ask does this theory 262 00:19:58,768 --> 00:20:00,490 work? And so, what have we found? 263 00:20:00,490 --> 00:20:05,599 well this, as this image clearly shows, we haven't looked very far. 264 00:20:05,599 --> 00:20:09,309 We've looked to about 300 light years around the solar system, 265 00:20:09,309 --> 00:20:13,857 that's a tiny fraction of the galaxy. We have all these thousands of planets, 266 00:20:13,857 --> 00:20:18,619 depending on how exactly you make the estimate, and as I said, this is a young 267 00:20:18,619 --> 00:20:22,500 science between one and 40% of sunlight stars have planets. 268 00:20:22,500 --> 00:20:28,452 Either number means that in a galaxy with a few hundred billion stars, there are 269 00:20:28,452 --> 00:20:33,009 lots and lots and lots of planets and planetary systems all over the place. 270 00:20:33,009 --> 00:20:37,505 no, the solar system is not unique. Now, the methods that we've used, both 271 00:20:37,505 --> 00:20:42,305 the, the most constructive ones, both the transit method and the radial velocity 272 00:20:42,305 --> 00:20:47,347 measurement method are most sensitive to what we call hot Jupiters planets that 273 00:20:47,347 --> 00:20:50,264 are very massive and orbit very near their star. 274 00:20:50,264 --> 00:20:53,787 Remember, we saw that with the radial velocity measurement. 275 00:20:53,787 --> 00:20:58,405 It's also true that it's more easy to arrange an alignment if the planet is 276 00:20:58,405 --> 00:21:01,190 close or methods are sensitive to hot Jupiters. 277 00:21:01,190 --> 00:21:05,978 Maybe that's not, it's not surprising that mostly we find, at least initially, 278 00:21:05,978 --> 00:21:10,766 lots of planets with masses similar to or larger than Jupiter's orbiting at 279 00:21:10,766 --> 00:21:13,050 Mercury-sized orbits. Now, 280 00:21:13,050 --> 00:21:17,287 well yes, there is a sample bias going on here, clearly a selection bias. 281 00:21:17,287 --> 00:21:21,941 if one had a detector that was only sensitive to unicorns and one found no 282 00:21:21,941 --> 00:21:26,805 coyotes, this would not be a surprise. But if one had a, a detector sensitive 283 00:21:26,805 --> 00:21:31,468 only to unicorns then found unicorns, that would still be a discovery. 284 00:21:31,468 --> 00:21:36,584 Discovering all these gas giant planets orbiting within a half an astronomical 285 00:21:36,584 --> 00:21:41,636 unit of a sunlight star should still be jarring, because, remember we explained 286 00:21:41,636 --> 00:21:46,687 that great length, Y, it is impossible for such planets to form so close to the 287 00:21:46,687 --> 00:21:50,250 star, because the solar nebula is too warm and 288 00:21:50,250 --> 00:21:55,396 volatile materials are not solid. What we learn from the fact that they 289 00:21:55,396 --> 00:21:58,895 exist is presumably they did not form, did not position, 290 00:21:58,895 --> 00:22:02,861 they formed farther out where the nebula was colder, and migrated in. 291 00:22:02,861 --> 00:22:07,235 So the kind of migration that the Nice model proposes for the solar system 292 00:22:07,235 --> 00:22:09,743 apparently is occurring all over the place. 293 00:22:09,743 --> 00:22:14,175 And this indeed, seems to be a ubiquitous feature of the solar systems we're 294 00:22:14,175 --> 00:22:18,782 finding the kind of sedate stability that we see in our solar system is not 295 00:22:18,782 --> 00:22:22,223 actually the standard. There's lots of strongly interacting, 296 00:22:22,223 --> 00:22:26,796 very resonant orbits, instabilities in that sense, at least, the orbits we 297 00:22:26,796 --> 00:22:31,579 are find, we find in our solar system are a little bit uncommon. 298 00:22:31,579 --> 00:22:36,292 What are the planets like? Well again most of the planets we find 299 00:22:36,292 --> 00:22:39,966 are very massive and lots of them are very closed in. 300 00:22:39,966 --> 00:22:45,857 If you look at this logarithmic plot right in the center of the plot over down 301 00:22:45,857 --> 00:22:49,875 here is our Earth with and then the horizontal axis is the 302 00:22:49,875 --> 00:22:53,372 distance from the star, the vertical axis is the mass of the 303 00:22:53,372 --> 00:22:55,529 planet. Both of these are logarithmic. 304 00:22:55,529 --> 00:23:00,017 So most of the planets that have been found are much more massive than earth. 305 00:23:00,017 --> 00:23:02,757 In fact, much more massive than Jupiter up here. 306 00:23:02,757 --> 00:23:07,245 And many of them orbit very close in, these are these hot Jupiters that, we 307 00:23:07,245 --> 00:23:10,393 spoke of, but we're also finding planets that orbit 308 00:23:10,393 --> 00:23:14,997 out at far larger distances from their stars and we've even found some planets 309 00:23:14,997 --> 00:23:18,611 that are on the order of magnitude of earth's mass or less. 310 00:23:18,611 --> 00:23:23,747 And so if you take the selection bias into account, the prediction is that the 311 00:23:23,747 --> 00:23:28,922 fact this data suggests that there are earth-like planets in much larger numbers 312 00:23:28,922 --> 00:23:33,841 than there are Jupiters or gas giants. Because, the fact that we're detecting 313 00:23:33,841 --> 00:23:36,588 them at all means they are far more common. 314 00:23:36,588 --> 00:23:41,060 we also find some super duper Jupiters, some gas giants with masses, 315 00:23:41,060 --> 00:23:46,491 15, 20 times the mass of Jupiter. Much larger than one thought planetary 316 00:23:46,491 --> 00:23:50,836 systems could create. And some interesting, strange cases like 317 00:23:50,836 --> 00:23:54,570 a planet called Kepler-16b that orbits a binary system. 318 00:23:54,570 --> 00:23:59,933 So two stars orbiting each other with a planet in an almost perfectly circular 319 00:23:59,933 --> 00:24:03,057 orbit around the center of mass of the system. 320 00:24:03,057 --> 00:24:09,001 this had been thought to be impossible to arrange in a stable orbit and Kepler-16 321 00:24:09,001 --> 00:24:11,995 shows us that nature manages to find a way. 322 00:24:11,995 --> 00:24:17,495 So, the field of extrasolar planets is despite the fact that it's been a few 323 00:24:17,495 --> 00:24:20,906 decades since we first discovered the first ones. 324 00:24:20,906 --> 00:24:26,685 This is really a field in its infancy, we're just starting to learn what we can 325 00:24:26,685 --> 00:24:31,419 learn from these exciting systems. And well, I anticipate a lot of news, 326 00:24:31,419 --> 00:24:37,337 both in terms of discovery and in terms of insight into how solar systems work in 327 00:24:37,337 --> 00:24:40,535 the coming decades. Let's sum up a busy week. 328 00:24:40,535 --> 00:24:44,684 Well, we found a lot to learn, right, in our own solar neighborhood. 329 00:24:44,684 --> 00:24:49,790 A lot of it, both in our solar system and certainly beyond is just being learned. 330 00:24:49,790 --> 00:24:53,732 I hope you're starting to develop an appreciation for the fact that we tried 331 00:24:53,732 --> 00:24:57,674 to understand some of the fundamental physics, because whatever we can, we can 332 00:24:57,674 --> 00:25:00,234 apply, we can actually understand what is going on, 333 00:25:00,234 --> 00:25:04,129 we can make calculations, that tell us what to expect and match them to 334 00:25:04,129 --> 00:25:06,965 experiment. I think understanding the physics, 335 00:25:06,965 --> 00:25:10,805 roughly of the greenhouse effect might be a good example of this. 336 00:25:10,805 --> 00:25:15,709 And, as I said exoplanets are probably going to eventually completely change the 337 00:25:15,709 --> 00:25:18,544 way we think of solar systems and so stay tuned. 338 00:25:18,544 --> 00:25:23,802 we've not covered many of the interesting topics in the understanding of our solar 339 00:25:23,802 --> 00:25:26,638 system. I want you to think of this week more as 340 00:25:26,638 --> 00:25:29,200 a hunting license. You now have 341 00:25:29,200 --> 00:25:34,942 the tools to go and read about the moons of Jupiter and why it is bizarre that 342 00:25:34,942 --> 00:25:40,483 Neptune has a moon that orbits completely outside the ecliptic, what that probably 343 00:25:40,483 --> 00:25:43,591 tells you about Triton and so on and so forth. 344 00:25:43,591 --> 00:25:48,591 And so I think you are equipped with at least the tools to go and acquire 345 00:25:48,591 --> 00:25:51,699 information. next week, we aim for the stars.