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← 6. Microlensing, Astrometry and Other Methods

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Showing Revision 1 created 07/08/2012 by Amara Bot.

  1. Professor Charles
    Bailyn: You'll recall that
  2. where we got to last time is
    that there are lots of Hot
  3. Jupiters;
  4. lots of Hot Jupiters.
    And the alternative theories
  5. that were presented to explain
    the evidence for Hot
  6. Jupiters--the evidence for Hot
    Jupiters comes in this form of
  7. these velocity curves,
    where you plot the radial
  8. velocity versus time of the
    star.
  9. And the star goes back and
    forth, and you infer from that
  10. that there must be a planet
    going around it,
  11. which is pulling the star back
    and forth.
  12. And some alternative
    explanations were proposed to
  13. explain these kinds of data.
    Those alternative explanations
  14. didn't seem to work very well,
    and so you have to kind of take
  15. the Hot Jupiters seriously.
    Before I go on,
  16. let me mention that,
    by now, Hot Jupiters are not
  17. the only kind of planets that
    have been seen this way.
  18. There are also planets in--now
    known to have much longer
  19. orbits, up to a few years.
    These are harder to see for two
  20. reasons.
    First of all,
  21. it takes you a few years to see
    them.
  22. And second of all,
    when the orbits get longer,
  23. the velocities that they induce
    go down, because things in long
  24. orbits go slowly.
    But, nevertheless,
  25. now we've been able to see a
    bunch of things.
  26. Oddly, in many cases,
    they turn out to have highly
  27. elliptical orbits,
    in some cases.
  28. Not the Hot Jupiters,
    not the ones that are close in.
  29. Those all are more or less
    circular.
  30. But some of these longer ones
    have highly elliptical orbits.
  31. That's also very weird in terms
    of our theories of planetary
  32. formation because one of the
    things that our theories were
  33. designed to explain was the fact
    that all the major planets in
  34. our own Solar System have orbits
    that are close to circular.
  35. It does a very good job of
    explaining that and then,
  36. naturally enough,
    has trouble explaining the ones
  37. we see that are in highly
    elliptical orbits--highly
  38. elliptical--yes,
    I didn't--highly elliptical
  39. orbits, in some cases.
    And just so you know,
  40. the way you recognize a highly
    elliptical orbit is it's not
  41. sinusoidal anymore.
    It's got some other shape.
  42. So, these come in:
    periodic but non-sinusoidal
  43. velocity curves.
  44. So, all this is very amusing
    and you pile up all these very
  45. strange kinds of planets,
    or what you think are planets.
  46. But there's still sort of the
    nagging question of whether
  47. these radial velocity curves
    might be explainable in some
  48. other way.
    And it would be awfully nice to
  49. have evidence of some other kind
    for the existence of these
  50. planets.
    This is, you know,
  51. what happens in science.
    You find some kind of fairly
  52. strong evidence for something,
    but it would be way stronger if
  53. you found two different kinds of
    evidence.
  54. Different--collected in
    different ways or reflecting
  55. different aspects of what you're
    observing that both point in the
  56. same direction.
    And, at a certain point along
  57. the way, about six years ago or
    so, a new kind of evidence for
  58. the existence of these planets
    was discovered.
  59. And the first such case,
    which is now a very famous star
  60. for this reason,
    was something called HD209458;
  61. that's the name of the star.
    And this was a Hot Jupiter
  62. system.
    So, this is the name of the
  63. star;
    Hot Jupiter was discovered in
  64. the usual way by Doppler
    measurements,
  65. by radial velocity
    measurements.
  66. And then, something else was
    discovered.
  67. So, let me show what they found
    in this system.
  68. All right, so let me explain
    what's plotted here.
  69. This is the brightness of this
    system, not the radial velocity
  70. this time--the brightness
    against time.
  71. And so, here it is on the 9th
    of September,
  72. 1999.
    And somebody is measuring the
  73. brightness of this thing.
    And it's set up in units so
  74. that the average brightness is
    1.
  75. And you can see,
    these measurements have a
  76. little bit of error in them.
    They scatter around.
  77. And then, at a certain point
    there's a dip in the brightness
  78. of the star for a few hours;
    notice this is in days.
  79. This is a tenth of a day,
    so 2.4 hours,
  80. something like that.
    So, for a few hours the
  81. brightness of this thing dips
    down to about 98% of its--of the
  82. brightness it has,
    so 2--it loses 2% of its
  83. brightness for a little while
    and then it comes back up.
  84. And then, interestingly enough,
    seven nights later on the night
  85. of the 16th of September,
    it did the same thing.
  86. Now, seven nights is an
    important number for this
  87. particular system,
    because it turns out that the
  88. orbital period of its Hot
    Jupiter,
  89. which was already known at the
    time, is three and a half days.
  90. So, this is exactly two orbits
    later.
  91. The exact same dip occurred.
    You couldn't see it one orbit
  92. later because three and a half
    days later it's daytime,
  93. and one of the features of
    astronomy is that you do it at
  94. night.
    And so you have to wait two
  95. day--two orbits to see the same
    thing again.
  96. So, every orbit--there appears
    to be this little dip.
  97. And what is happening is that
    this is a system that's almost
  98. exactly edge-on.
    And what's happening is that
  99. the planet is passing in front
    of the star, and once per orbit
  100. the planet gets in the way of
    the star.
  101. The planet's this little thing,
    the star's a big thing.
  102. And a small amount of the
    star's light is obscured by the
  103. planet.
    This is called a transit.
  104. So, let me just write that down.
    And then, transits are
  105. discovered--yeah,
    okay, we'll come back to the
  106. overheads in a minute because I
    want to show you more plots.
  107. And it's the planet getting in
    the way of the starlight.
  108. And what this tells you,
    also, is how big the planet is,
  109. because you've got a little
    disc in front of a big disc.
  110. And the amount of light that's
    obscured--this 2%--tells you
  111. what the ratio of the area
    projected by the planet,
  112. compared to the area projected
    by the entire star,
  113. is.
    So, this gives you an
  114. additional piece of information.
    You already know the mass of
  115. the thing because you've
    measured the radial velocity.
  116. Now, we also know its size.
    So, this was very interesting.
  117. And it is, as I was saying just
    a second ago,
  118. an entirely different kind of
    confirmation of the existence of
  119. this planet.
    So, there's no pulsation going
  120. on here, because some--you not
    only are getting the changes in
  121. velocity of the planet you're
    seeing--of the star,
  122. you're also seeing the light of
    the star diminish when the
  123. planet passes in front of it.
    And then, so,
  124. they decided this was
    interesting, and so they did
  125. this same experiment,
    except with this Hubble Space
  126. Telescope instead of with some
    ground-based telescope.
  127. This is what you see with
    Hubble, and you can see why
  128. Hubble is a better telescope
    than the ground-based stuff.
  129. This is exactly the same plot.
    This is the average brightness
  130. of the system outside of the
    transit, so this is 1.000.
  131. This is .985 so it's dropped
    down 1.5% in brightness.
  132. This is in days and in--this is
    a tenth of a day here,
  133. each of these big ticks is .05
    of a day.
  134. And here is the star going
    along perfectly normally.
  135. And you get better measurements
    because you're not distorted by
  136. the atmosphere.
    And then, all of a sudden,
  137. the thing drops and it
    gets--has this very particular
  138. shape to the light curve,
    to the curve of brightness with
  139. time.
    This shape is perfectly
  140. explained by the hypothesis that
    you're passing an opaque disc
  141. across the surface of the star.
    As the disc touches the star it
  142. starts to--and gradually more
    and more of the disc is over the
  143. star.
    You get this steep drop here.
  144. And then, this rounded part
    down on the bottom comes about
  145. because when you look at a star,
    the center of the star,
  146. the central portion of the star
    looks a little bit brighter than
  147. the edges because,
    if you think about looking at a
  148. sphere, if you look at the
    center, you're looking right at
  149. it.
    If you're looking at the side,
  150. you're sort of grazing the edge
    and it turns out there is
  151. something called "limb
    darkening,"
  152. which comes about because of
    this.
  153. So, stars, if you look at the
    Sun for example,
  154. the edges of the Sun look a
    little fainter than the middle.
  155. And so, what happens is,
    as this disc passes across the
  156. face of the star,
    it obscures successively
  157. brighter parts of the star until
    you get down to here.
  158. Then, it obscures gradually
    fainter parts of the star until
  159. it gets to the edge.
    Now, the disc is--as it goes up
  160. here, is passing beyond the edge
    of the star.
  161. So, this is exactly the shape
    of a light curve you would
  162. predict if you passed an opaque
    disc in front of a star.
  163. And it really works out
    remarkably well.
  164. In fact, there's a line
    underneath these points,
  165. you can see it off here and off
    here, which is a prediction.
  166. And you can't see the line
    because it's under the point so
  167. nicely.
    So, this works out incredibly
  168. well.
    This is almost for sure an
  169. opaque disc, namely the planet,
    passing in front of the star.
  170. So, transits are
    discovered--the dip in light due
  171. to planet passing across the
    star.
  172. Now, this does not happen in
    every Hot Jupiter,
  173. because you have to be lined up
    pretty,
  174. pretty perfectly in order for
    the planet to pass in front of
  175. the star.
    That is to say if--here's you,
  176. here's an observer,
    here's the star.
  177. And, if the planet--if the
    orbital plane of the planet
  178. looks like this,
    then when the planet's in front
  179. of the star, it'll be below it.
    So, if you think about it,
  180. imagine a planet that's
    orbiting like this.
  181. Here's its star.
    And it never passes in front of
  182. the star, because when it's in
    front of the star,
  183. it's below the star.
    Whereas, if it--if it's exactly
  184. edge-on, then once per orbit,
    it will pass in front of the
  185. star.
    But, you have to have a very
  186. precise alignment for that to
    take place--so,
  187. requires a precisely edge-on
    alignment.
  188. But, if you have such an
    alignment, you know how these
  189. dips ought to line up with the
    radial velocity curve.
  190. So, now what you see is,
    you measure the brightness of
  191. the star versus time,
    and you see these dips.
  192. Here's a dip.
    Then one orbit later,
  193. you get another dip,
    and so forth.
  194. If you then,
    at the same time,
  195. measure the radial
    velocity--and again,
  196. this is the radial velocity of
    the star--then you know how this
  197. has to take place.
    Because here you are.
  198. You're looking at it.
    And the moment where the dip
  199. takes place, the planet's got to
    be exactly in front of the star.
  200. That means the planet is at
    this point in its orbit,
  201. going that way,
    for example,
  202. and the star is in this part of
    its orbit, going that way--which
  203. means this--the radial velocity
    at the moment the dip occurs has
  204. to be zero.
    Because for them to be lined
  205. up, one in front of the other,
    they've both got to be going
  206. sideways.
    And shortly before that,
  207. the star was moving away from
    you, because they're moving into
  208. position, like this.
    Shortly afterwards,
  209. the star is coming towards you.
    And, remember that positive
  210. radial velocities are coming
    toward--or going away from you,
  211. negative velocities are coming
    towards you.
  212. So it used to be--it used to be
    going away from you,
  213. positive.
    It's now at zero at the point
  214. of the dip.
    And then it's going to be
  215. coming towards you,
    so it has to be that these dips
  216. take place at that point in the
    curve.
  217. And so, here again,
    it's got to be like this.
  218. And so, the way this has to
    work out is something like this.
  219. So, you can predict what the
    radial velocity has to be at the
  220. particular moment where the
    transit occurs.
  221. And so this works.
    This works for HD--what is it?
  222. 209458.
    So this works out.
  223. So, now you've got a lot of
    evidence that this really is a
  224. planet.
    Not only is it moving the star
  225. back and forth the way you
    expect a planet to do,
  226. it's also passing in front of
    that star at exactly the moment
  227. you expect it to pass in front
    of the star,
  228. and the shape of the dip that
    it creates is exactly what you
  229. would expect if you pass an
    opaque disc in front of a star.
  230. So now, you've got a whole
    bunch of different kinds of
  231. evidence.
    You can ask,
  232. "Does this prove it's a
    planet?"
  233. And then you get into this
    problem with scientific proof.
  234. What does it mean to prove
    something in science?
  235. You could probably figure out
    some clever way that it's
  236. actually a pulsating star,
    which has a star spot on it
  237. that somehow precisely mimics
    exactly the way a planet would
  238. behave.
    Impossible that this
  239. would--well not--I should be
    careful with that word.
  240. Highly improbably that this
    would actually occur in real
  241. life, so I think,
    you know--people have the
  242. feeling that science is truth,
    somehow, and that,
  243. you know, that you can prove
    things in science,
  244. kind of,
    to a mathematical certainty.
  245. That actually isn't the case.
    The legal standard,
  246. you know, which you're familiar
    from cop shows,
  247. of beyond a reasonable doubt,
    applies in science as well.
  248. And that's actually a more
    appropriate standard to use for
  249. anything, which,
    in the law or in the natural
  250. world,
    in which you try and reason by
  251. induction.
    And so I would say that this,
  252. plus this, plus the shape of
    the--of the spectral lines,
  253. which we talked about last
    time, plus the shape of these
  254. dips--that's proof way beyond a
    reasonable doubt that there's a
  255. planet around this particular
    star.
  256. And that really ended the
    conversation.
  257. Yes?
    Student: Is it always
  258. directly edge-on or could there
    be a case in which it's
  259. [inaudible]
    Professor Charles
  260. Bailyn: Right.
    Student: --sort of like
  261. a little bit below or a little
    bit [inaudible]
  262. Professor Charles
    Bailyn: So,
  263. is it always edge-on?
    Well, orbits don't change,
  264. so for this object it's always
    edge-on.
  265. For most objects,
    it is not edge-on,
  266. and so, in fact,
    what had happened was,
  267. by the time they discovered
    this one, there were a couple of
  268. dozen of Hot Jupiters known.
    None of the rest of them do
  269. this, because in all the rest of
    them, the orbit isn't aligned
  270. properly,
    and so the planet never goes in
  271. front of the star.
    And so, in the vast majority of
  272. cases, you don't get this.
    And so, they only discovered
  273. this after they'd found so many
    of them, that one of them turned
  274. out to be aligned properly,
    you know, just by chance.
  275. Student: Is there--it
    possible for an orbit to
  276. not--never pass in front of the
    center of the star?
  277. Professor Charles
    Bailyn: No.
  278. What it does is it passes--it's
    got to do this,
  279. it's got to have some moment
    where the planet is as far
  280. towards you as it can be,
    and then it turns around and
  281. comes back.
    But for most orbits,
  282. at that point,
    the planet will either be below
  283. or above the star from your line
    of sight.
  284. So, imagine this orbit as the
    planet goes--here's a star.
  285. Planet's going around the star.
    It reaches its furthest forward
  286. point, but it's well below the
    star from your line of sight.
  287. So, it depends on the angle of
    the orbital plane.
  288. Does that make sense?
    Student:
  289. It, it doesn't have to pass
    through the center of the star,
  290. it could pass [inaudible]
    Professor Charles
  291. Bailyn: Oh,
    oh -- Student:
  292. [inaudible.]
    Professor Charles
  293. Bailyn: Yes,
    sorry, exactly right.
  294. It doesn't have to pass through
    the center of the star.
  295. It can graze the tip at the
    bottom of the star a little bit.
  296. And in fact,
    in this object,
  297. you can tell how close to the
    center it goes by the exact
  298. shape of the curvature down at
    the bottom of that light curve.
  299. Because, you know what the
    expected distribution of
  300. brightness across the face of
    the star is.
  301. You also know something about
    the radius of the star.
  302. This is supposed to be a
    solar-type star,
  303. presumably has a radius of the
    Sun.
  304. If it passes across,
    sort of grazes the bottom,
  305. then the dip will be shorter,
    because it will pass through
  306. faster.
    And so, you can figure out
  307. exactly what the trajectory
    across the thing is.
  308. Most of the time it misses.
    And in this case,
  309. you can figure out that the
    angle of the inclination,
  310. so-called,
    which is 90 degrees if it's
  311. exactly edge-on,
    is 80--I think 88 point
  312. something,
    I don't remember the answer.
  313. Because, it doesn't go exactly
    across the center,
  314. it goes, sort of,
    sort of halfway down.
  315. So yeah, that actually gives
    you additional information.
  316. How long the dip is and what
    the exact shape of the thing,
  317. tells you which part of the
    star it passes across.
  318. Yes good, thanks Bethany.
    Other questions about this?
  319. Yeah?
    Student: Have we ever
  320. discovered a planet through
    transit without having
  321. discovered it previously?
    Professor Charles
  322. Bailyn: Right,
    have we discovered a planet
  323. through transit without having
    discovered the previous radial
  324. velocity curves?
    Yes, and we'll get to that.
  325. Because, let me just say this:
    as soon as this was discovered,
  326. people got way excited,
    because it's quite difficult to
  327. make these kinds of precise
    radial velocity measurements.
  328. You have to have very
    specialized equipment.
  329. Only a few people in the world
    can do it.
  330. There's this bunch of people in
    California who can do it.
  331. There's this bunch of people in
    Switzerland who can do it.
  332. Us – normal astronomers,
    don't have that kind of
  333. expertise and equipment,
    but measuring a change in
  334. brightness of a star by 2%,
    that's easy.
  335. I can do it up on Science Hill
    [an area of the Yale campus
  336. where the Astronomy Department
    is located]
  337. in our little observatory up
    there.
  338. In fact, I have done it for
    this star.
  339. You can--if the timing were
    better, this star is up in the
  340. summertime, unfortunately.
    If it were up in the fall we
  341. would have this be an exercise
    in Astro 155 [a Yale Astronomy
  342. class],
    because it's entirely
  343. straightforward to go and
    measure a 2% difference in
  344. brightness in a star.
    Anybody, you know,
  345. with 2,000 bucks' worth of
    equipment, you can go out into
  346. your backyard and do this
    experiment.
  347. This is a big deal.
    So, as soon as that was
  348. possible, everybody got fired
    up, you know,
  349. "I'm going to play this game
    myself."
  350. And we all laid very elaborate
    plans for going out and finding
  351. zillions of planets.
    And the problem is that it has
  352. to be exactly aligned edge-on
    for this to even occur.
  353. And so, most stars with
    planets--this doesn't happen.
  354. And so, you would have to
    observe many,
  355. many, many stars in order to
    see this one time.
  356. So you have to--while it's easy
    to do for any one particular
  357. object, it's hard to discover
    them this way because you have
  358. to do it in bulk.
    You have to look at many stars
  359. at once.
    So then, people thought,
  360. well, great,
    we know how to do this.
  361. We'll look at many stars at
    once.
  362. We'll take pictures of star
    clusters, of--where there are
  363. lots and lots of stars.
    And we'll just keep taking
  364. pictures of these star clusters
    over and over again.
  365. You'll look at 30,000 stars at
    a time, and if one of them has a
  366. dip, we'll find that dip.
    And so, the way we'll deal with
  367. the fact that this doesn't
    always occur is by looking at
  368. star clusters.
    And so, they tried this,
  369. again, with the space
    telescope, because it works
  370. better at doing this kind of
    thing.
  371. The problem with star clusters
    is that the stars are really
  372. close to each other,
    so from the ground it just
  373. looks like a mush.
    In this case,
  374. you have to see the stars
    separately.
  375. So, they took a particular star
    cluster--also the advantage of
  376. doing it from space is no
    daytime, and so,
  377. you can observe continuously.
    So, they took eight consecutive
  378. days of Hubble Space Telescope
    time and did nothing but look at
  379. a star cluster,
    a cluster called--famous
  380. cluster called 47 Tuc,
    37 stars.
  381. They took repeated images of
    this star cluster,
  382. and they figured,
    we ought to see some number of
  383. planets by the transit method,
    first, in this cluster.
  384. So just, here's what the
    cluster looks like.
  385. So this, on the left,
    is a ground-based picture of
  386. this cluster,
    and you can see what the
  387. problem with observing this
    stuff from the ground is.
  388. If you look down in the middle,
    it's all mushed together.
  389. You wouldn't be able to pick
    out individual stars.
  390. But then, this box here,
    blown up to the right-hand side
  391. of the picture--this is what
    that little box looks like from
  392. the Hubble Space Telescope.
    And now, you have the
  393. resolution to observe each of
    these stars, or many of them
  394. individually.
    And so they took eight nights
  395. of Hubble Space Telescope
    time--that's a lot of time on
  396. the space telescope.
    The space telescope costs about
  397. a $1,000,000 a day to run.
    And so, this is 8,000,000
  398. bucks' worth of observing time.
    Okay, so what did they expect
  399. to see?
  400. So, finding planets directly
    from transits.
  401. And so, they observe a cluster
    of stars--30,000 stars at a
  402. time.
    So, what do you expect?
  403. Well, it turns out,
    from the radial velocity
  404. studies, they had gone and
    looked at a lot of,
  405. sort of, Sun-like stars.
    So, 30,000 more or less
  406. Sun-like stars,
    I should specify,
  407. from the radial velocity
    measurements from the Doppler
  408. measurements.
  409. They got answers like
    approximately 1 out of 10
  410. stars--this is a rough estimate
    – have Hot Jupiters.
  411. And then, how precisely aligned
    do they have to be?
  412. Approximately 1 out of 100 Hot
    Jupiters is aligned properly to
  413. get a transit.
  414. And so, now,
    you have a prediction.
  415. You look at 30,000 stars.
    A tenth of them have planets,
  416. that's 3,000 planets.
    One hundredth of those will be
  417. aligned up in such a way that
    you can see a transit.
  418. That's thirty transits.
    Predict-- thirty transits.
  419. And it was that kind of
    calculation--this is just a
  420. rough estimate,
    but they had done this much
  421. more precisely.
    It was that kind of estimate
  422. that persuaded the people at the
    Space Telescope Institute to
  423. allocate all these many hours of
    space telescope time to this
  424. project.
    The result was zero transits,
  425. none.
    Not seen.
  426. And this was a little
    distressing, except it didn't
  427. take people very long to figure
    out that they should have known
  428. this in advance.
    And this is--you know,
  429. you get a result and then of
    course people--you get an
  430. unexpected result,
    and then people start writing
  431. papers which say,
    well, of course that's what you
  432. should have gotten.
    Any fool could have predicted
  433. that you would not see any
    transits in a star cluster,
  434. except none of the fools did.
    Let's see, so--but,
  435. in retrospect,
    it was clear that this wasn't
  436. going to give you the thing you
    thought because of two factors.
  437. First, in--why?
    So, first factor:
  438. in clusters,
    the stars are really close
  439. together.
    Stars--they sometimes collide
  440. or more often have near
    collisions.
  441. And when a star comes cruising
    into your planetary system,
  442. the star has a lot of gravity.
    It's going to completely wreck
  443. the orbits of the planets
    because you're going to have the
  444. gravity of a second star.
    And it turns out that what this
  445. does is it liberates the
    planets.
  446. And so, it will disrupt
    planetary orbits.
  447. So, you kind of don't expect
    there to be any planet;
  448. you expect there to be planets
    in the cluster,
  449. but they'll be free floating
    planets sort of wandering around
  450. the cluster,
    because they will have been
  451. detached from their parent star
    by incoming stars--by close
  452. encounters with other stars in
    the cluster.
  453. Put it this way.
    The nearest star to the Sun,
  454. beside the Sun,
    Alpha Cen, is about a parsec
  455. away.
    In a cubic parsec,
  456. at the center of one of these
    clusters, there are a million
  457. stars.
    And so, there are a million
  458. stars packed into the space
    where only one star exists in
  459. our corner of the galaxy.
    So, they run around--you know,
  460. if you were in a cluster,
    if you were in a planet on a
  461. cluster,
    and you were looking up at the
  462. sky, the constellations would
    change from year to year,
  463. because the stars are so nearby
    that their motions would be
  464. readily apparent.
    And once every few hundred
  465. million years,
    a star would come cruising into
  466. your Solar System and detach all
    of the planets from the star.
  467. And so, you don't expect there
    to be any planets.
  468. You also don't expect there to
    be any planets for an entirely
  469. different reason,
    which is, one of the things
  470. that had been discovered as they
    were piling up all these Hot
  471. Jupiters found by the Doppler
    Shift method,
  472. is that stars are more likely
    to have planets--have
  473. planets--if they have high
    amounts of heavy elements--heavy
  474. elements.
    Now, let me explain that.
  475. Most stars--stars are mostly
    hydrogen and helium.
  476. Astronomers do chemistry in a
    very peculiar way.
  477. We have--we consider there are
    three kinds of things in the
  478. universe.
    There's hydrogen,
  479. there's helium,
    and there's metal.
  480. Chemists--everything else is a
    metal.
  481. If don't care if it's oxygen,
    carbon, whatever.
  482. The chemists get really
    uncomfortable with this.
  483. But, you know,
    it's like the supposed
  484. primitive tribes.
    I'm not sure these--this
  485. actually exists,
    but the linguists say there are
  486. tribes in the world where they
    count one, two and many.
  487. Well, this is how astrono--I
    don't know if such tribes exist
  488. except for the astronomers,
    who really do.
  489. And we count,
    one, two, and many.
  490. And we call any chemical
    element heavier than helium a
  491. metal.
    So, we have this concept called
  492. metallicity, which is defined to
    be the fraction of something of
  493. elements heavier than hydrogen
    and helium,
  494. which are the first two in the
    Periodic Table,
  495. as you probably know.
    So, everything else is a metal.
  496. The metallicity of the
    Sun--Sun's metallicity--is about
  497. 2%.
    And the metallicity of the
  498. Solar System is therefore about
    2%, because the Sun's got all
  499. the mass.
    And, high metallicity stars,
  500. by which I mean,
    metallicities greater than the
  501. Sun--greater than solar--are
    more likely to have planets.
  502. This makes perfect sense.
    This is the first thing in a
  503. little while that's made any
    sense--because,
  504. how do you make a planet?
    Well, planets aren't made out
  505. of hydrogen and helium.
    Planets are made out of the
  506. other stuff.
    We're made out of silicon and
  507. iron, and the--Jupiter's got a
    lot of ice, and so all these
  508. kinds of heavy elements--you
    don't make--you can't make a
  509. planet if all you've got is
    hydrogen and helium because
  510. there's nothing solid to have it
    form.
  511. And the only way you get to
    keep your hydrogen and helium,
  512. the 98% of the stuff that's
    hydrogen and helium,
  513. is if you already have a big
    core of heavier--either rocks,
  514. or ice, or something else--if
    you already have some metallic,
  515. in the astronomical sense, core.
    And so this makes perfect sense.
  516. If you don't have any metals,
    you can't be forming planets.
  517. And star clusters of the kind
    that Hubble observe are known,
  518. and this particular one is,
    to consist of low metallicity
  519. stars.
  520. In the case of 47 Tuc,
    the one we looked at,
  521. it's--the metallicity of the
    thing is, I think,
  522. about a fifth that of the Sun.
    So, it's got very substantially
  523. fewer heavy elements.
    So, no planets.
  524. So, in two different ways this
    was a mistake,
  525. right?
    But that was only realized
  526. afterwards.
    And, it's unfortunate that it
  527. was a mistake in two different
    ways.
  528. Because if it was a mistake in
    only one different way you would
  529. have learned something from
    this,
  530. because you would have
    confirmed the idea that a
  531. star--close stellar encounters
    strip planets off of
  532. stars--except maybe it's only
    because of the low metallicity.
  533. Or, you would have confirmed
    the idea that low metallicity
  534. stars can't have planets;
    except in this case,
  535. maybe it's just because of the
    stars and the planets have been
  536. stripped away.
    So it's kind of unfortunate
  537. that there were two excellent
    explanations different from each
  538. other about why you should have
    predicted this result before you
  539. obtained it.
    However, having done that,
  540. it was clear what the next
    experiment had to be:
  541. that you wanted to go out and
    try and measure these transits
  542. in some region that's a little
    less dense,
  543. and a whole lot more metal rich.
    So, next experiment,
  544. which was done last summer,
    didn't look at a star cluster.
  545. It looked at the center--a
    region close to the center of
  546. the galaxy.
  547. So, this is lots of stars,
    but significantly less dense
  548. than what you would get in the
    center of a cluster.
  549. Fortunately,
    they had put up a bigger camera
  550. into the space telescope since
    then, so you could cover more of
  551. the sky, which was useful.
    This is the camera;
  552. by the way, you may have read,
    over the weekend,
  553. that Hubble's had a little bit
    of a problem.
  554. The electronics in this wide
    field camera have shorted out.
  555. And so, Hubble's kind of in
    trouble for--until they get the
  556. next visit, because its major
    camera is now blind.
  557. And this happened,
    I should say,
  558. 12 hours after the deadline for
    submitting proposals.
  559. So, there had been almost 1,000
    proposals submitted from around
  560. the world, of which,
    maybe only a fifth would ever
  561. get done.
    People in our department and
  562. basically every department in
    the whole--astronomy department
  563. in the whole world were going
    nuts trying to prepare their
  564. detailed proposals for using the
    thing.
  565. They submitted them,
    and a few hours later it broke.
  566. And so, then there was this
    nice little email from the
  567. people at the Space Telescope
    Institute saying,
  568. "uhh, you might want to
    reconsider, and we've created a
  569. new deadline three weeks from
    now so that you can revise and
  570. extend."
    There are other instruments
  571. that still work but the cameras
    aren't as good.
  572. And so, now,
    everybody's scratching their
  573. head and thinking,
    can I actually do this,
  574. and if I can,
    what's the good strategy?
  575. Do I want to try and do it now
    when the competition's going to
  576. be much less because the
    instruments are lousy?
  577. Or, do I want to wait until
    after September 2008,
  578. when they're scheduled to go up
    replace the camera and repair
  579. the thing,
    and try and do it with the good
  580. instruments then,
    when everybody else in the
  581. world is going to want to do
    their experiment?
  582. And so, we're all grappling
    with this at the moment.
  583. However, last summer this was
    working like crazy.
  584. And they did another one of
    these ten consecutive days of
  585. observations,
    but this time of the center of
  586. the galaxy,
    which has lots of stars--but
  587. less dense than in the center of
    the cluster, and also has the
  588. advantage that many of these
    stars,
  589. most of them,
    are high metallicity stars.
  590. So, hopefully,
    this will work,
  591. right?
    Because you've eliminated both
  592. of the excellent reasons why it
    didn't work the last time.
  593. So, here is the field of view
    that they were looking at.
  594. This is a tiny piece of the
    center of the galaxy.
  595. Lots and lots and lots of
    stars, which is good,
  596. because you want to see them.
    And these little circles here,
  597. which are numbered from 1 to
    16, I believe,
  598. are circles around little
    Sun-like stars.
  599. You can see the stars in the
    middle of those circles;
  600. you probably have a hard time
    seeing.
  601. Let's turn the lights down.
    This is kind of a nice picture.
  602. This is what the center of the
    galaxy looks like to the Hubble
  603. Space Telescope.
    And these bright things--the
  604. colors are sort of quasi-real.
    These things are red giant
  605. stars and solar type stars.
    The center of the galaxy is
  606. quite far away--are really quite
    faint, you can barely see it.
  607. See in the middle of this
    circle there's a little star
  608. there, that's a Sun-like star in
    the middle of the galaxy.
  609. And these circles are these
    stars in which they found--in
  610. which they found transits.
    So, in this case, it worked.
  611. So, it's true that one or the
    other or both of those reasons
  612. they had why it didn't work in
    the star cluster,
  613. were in fact the reason it
    didn't work in the star
  614. clusters.
    Because here's a situation
  615. where those problems don't
    exist.
  616. And now they found sixteen of
    these things,
  617. which is pretty close to the
    amount you would have expected.
  618. And so now it is--these are
    planets, which have been
  619. discovered first by the transit
    method.
  620. I should say,
    there are a couple of others
  621. that have been discovered.
    First by the transit method,
  622. from people at the ground just
    going out and looking at random
  623. stars.
    And in some of those cases,
  624. they have been done the
    opposite thing,
  625. and gone back and found the
    radial velocity measurements
  626. after having discovered the
    tran--the planet by transits.
  627. You can't do radial velocity on
    these guys because they're too
  628. faint, and it's just not going
    to work out.
  629. Okay, how're we doing?
    All right.
  630. Now, a couple of consequences
    of this.
  631. If you have both radial
    velocity measurements,
  632. which tell you the mass of the
    planet,
  633. and transits,
    which tell you the radius of
  634. the planet, then you know
    something very important.
  635. You know the density.
    Density--then,
  636. you get the density.
    Density is usually written down
  637. with the Greek letter ρ,
    for some reason,
  638. which I don't understand,
    but it's always done that way,
  639. so we'll do it too.
    And density is defined as mass
  640. of something divided by its
    volume.
  641. And so, for a spherical object,
    the volume--if you go back to
  642. some geometry book,
    you can look this up.
  643. 4/3π times the radius cubed,
    and that's the density.
  644. The density,
    if you--the density of water is
  645. about--is 1 gram per cubic
    centimeter.
  646. Now, of course,
    that's a lousy set of units.
  647. We do things in kilograms per
    cubic meter.
  648. That's, as it turns out,
    1,000 kilograms per cubic
  649. meter.
    So, if you picture a cubic
  650. meter, that's a pretty huge
    aquarium.
  651. It's actually pretty heavy.
    And, let's see, is this right?
  652. 10^(2), 10^(6) times a
    gram--yeah, that's right.
  653. And, in fact,
    interestingly enough,
  654. this is the definition of a
    gram.
  655. This is where they came up with
    that metric unit--is to make it
  656. work out so that the density of
    water is one--is exactly 1 gram
  657. per cubic centimeter.
    You have to specify the
  658. temperature, too,
    because--but,
  659. so, that's a typical density of
    water, which is on ice,
  660. right?
    Rocks have higher density--much
  661. higher density.
    That's why a handful of iron is
  662. heavier than an equivalent sized
    handful of snow,
  663. or something like that.
    So, if you know the density,
  664. you can tell something about
    the composition of the planet.
  665. And it turns out that the Hot
    Jupiters, in the few cases we
  666. now have--it's not just one
    case, it's several cases--Hot
  667. Jupiters have low density.
    They really are ice balls.
  668. They're not made out of rock.
    And so, an idea you might have
  669. had about where the Hot Jupiters
    come from is,
  670. well, in these high metallicity
    stars, there's just a whole
  671. bunch more rock,
    and the Earth-like planets kind
  672. of get big.
    But that isn't what happened.
  673. These things really are made
    out of ice.
  674. And now you have a problem,
    because you know how far away
  675. they are from the star.
    And the temperature of these
  676. things--the surface temperature
    of these things is,
  677. like, 1,000 degrees.
    This is not a good place to put
  678. a snowball, all right?
    You put a snowball in some
  679. place which is 1,000 degrees--we
    won't specify the place--and
  680. something bad is going to happen
    to it.
  681. Now, the--and so,
    therefore, how could these
  682. planets have formed?
    And so the idea--the current
  683. thinking on this is an idea
    called migration.
  684. And the idea behind migration
    is that you make Jupiters in the
  685. Outer Solar System,
    where they belong.
  686. And then, through some
    mechanism, which people have
  687. ideas about, but which
    aren't--isn't at the moment
  688. really specified--these things,
    after they are formed,
  689. they migrate into the Inner
    Solar System.
  690. And the reason this is a useful
    thing to do is that a big thing
  691. melts slowly.
    The reason for that is that the
  692. surface area to volume ratio is
    low.
  693. Volume goes up as the cube of
    the radius--surface area,
  694. goes up--as the square.
    If you shine a light on
  695. something, the amount of energy
    it picks up depends on how big
  696. it is, not on how massive it is.
    And so, changes of temperature
  697. in interacting with your
    environment are less,
  698. the bigger you are.
    This is why,
  699. by the way, arctic animals tend
    to be big: because they're
  700. trying to survive in an
    environment that's much colder
  701. than they are.
    And so, they generate energy by
  702. their volume and they lose it
    out their skin,
  703. by their surface area.
    So, you want to have a lot of
  704. volume compared to your surface
    area.
  705. Big things melt slowly.
    So, you can't have ice
  706. planetesimals in the Inner Solar
    System.
  707. But you can have something the
    size of Jupiter.
  708. And it'll melt,
    but it'll take longer than the
  709. age of the Solar System,
    or even the age of the
  710. universe, to do so.
    So this is the idea.
  711. And of course,
    the key point here is this
  712. migration.
    And there are ideas about why
  713. this should happen,
    but it isn't wholly clear.
  714. Now, one thing,
    a consequence of this idea,
  715. is that there are no
    terrestrial planets.
  716. No terrestrial planets,
    because as this Jupiter is
  717. moving inward,
    if there's an Earth in the way,
  718. it's going to either knock it
    out of its orbit completely,
  719. or the Earth will run into
    Jupiter and become incorporated
  720. into that planet.
    So, if you've got a
  721. Jupiter-size system in--near
    where your Earth-like planet is
  722. orbiting,
    that Earth-like planet's in big
  723. trouble for exactly the same
    reason the planets are in
  724. trouble,
    in general, when another star
  725. comes through,
    because you've got something
  726. much more massive coming by,
    screwing up your orbit.
  727. So, there are no terrestrial
    planets.
  728. Their orbits are disrupted by
    the migrating Jupiter.
  729. Now, in fact,
    I have to qualify this a
  730. little.
    There was a paper that showed
  731. up in the literature,
    a few months back,
  732. with some clever way of
    preserving the terrestrial
  733. planets as the Jupiter came by.
    I have to say,
  734. I don't fully appreciate the
    force of this argument.
  735. It is perhaps true--because I
    haven't read the paper carefully
  736. enough, but I feel obliged to
    mention that somebody,
  737. at least, has an idea about how
    you can avoid it.
  738. But, in sort of straightforward
    terms, you don't expect there to
  739. be terrestrial planets in a
    situation where a Jupiter has
  740. migrated from the Outer Solar
    System to the Inner Solar
  741. System.
    Questions?
  742. How are we doing?
    Yes?
  743. Student: I was just
    wondering what caused the Hot
  744. Jupiters to migrate?
    Professor Charles
  745. Bailyn: What causes the Hot
    Jupiters to migrate?
  746. This is a subject of some
    discussion among the experts.
  747. The idea is that when the
    Jupiter forms,
  748. remember it forms out of a disc
    of gas?
  749. There might still be a
    substantial residual gas and
  750. dust disc.
    And as the Jupiter plows
  751. through it, the friction with
    that disc slows it down.
  752. And if you slow down the orbit
    of a planet, it'll gradually
  753. fall inwards.
    That's one idea.
  754. There are some--there are other
    ideas.
  755. But it isn't clear.
    And if it were clear,
  756. you would have predicted it to
    happen.
  757. And indeed, there's a whole
    problem with the migration
  758. theory.
    So, problem with migration is,
  759. sometimes it doesn't work.
    Because, Jupiter in our Solar
  760. System is where Jupiter is.
    It didn't migrate.
  761. It has to work--needs--so,
    the problem with the migration
  762. system is, it needs to work,
    but not always.
  763. It also can't work too well,
    because if it works too well,
  764. then the Jupiter falls right
    into the Sun,
  765. right?
    So, it has to work most of the
  766. time, but not all the time,
    but not too well.
  767. This is a complicated balancing
    act.
  768. And, whatever theory you
    propose, whether it's this
  769. friction with a dust disc,
    or anything else,
  770. you've got a little problem
    with this whole concept.
  771. Because you have to tune this
    mechanism up so that it takes
  772. most Jupiters,
    moves them into the Inner Solar
  773. System,
    and then stops working at just
  774. the right point to leave you
    with a whole bunch of Hot
  775. Jupiters,
    most but not all of the time.
  776. All right, that's as much as we
    know about it.
  777. That's--you know,
    you got to come back three
  778. years from now and see if we've
    straightened this out,
  779. because that's where we are at
    the moment.
  780. All right, more later.