So now let's leave our solar system and look at planets outside of solar system. This has been a booming industry and one of the most active areas of astronomy over the last decade or so, because technology finally got there that we can discover such planets. And as of yesterday, I looked and there are about 1,800 confirmed Extrasolar planets and there are 1,100 planetary systems. So around some stars, we found more than one planet. There could be more but we just haven't seen them yet. And there's another 3,000 possible planets that people still haven't checked So that's actually pretty good. And the reason why we would like to study them in addition to our future science fiction interests, like exploring those planets directly, or settling on them. Is that by learning about other planetary systems, we'll get to understand our own a little better. Turns out that our solar system is somewhat unusual in some of its properties, but that could still be a selection effect. So there are four ways in which we can look for planets around other stars. The first obviously would be well, let's take a picture and take a look. The problem with that is that the scattered light of the parent star, say due to seeing in Earth's atmosphere, or even scattered light in the optics, is so much brighter than the reflected starlight from planet itself, about a billion times for a typical case, that this is almost impossible. However, if you go into the infrared, then the thermal emission from the planet itself comes in. So planet shines its own energy and that makes the contrast much more favorable. So a lot of imaging is now done for extra-solar planets combines these two things, somehow suppressing scattering and turbulence. And also looking at infrared. I'll show you pictures. The first way in which we actually found any Extrasolar planet is the Doppler shift technique, radial velocity technique. The star and planet actually orbit a common center of the mass and then there is a little bit of velocity there for the star itself in its radial velocity. And if you have very precise toposcopy, you can measure that. Then you can look for eclipses. If you happen to be in the ecliptic plane of that planetary system, then a splice transit to disc, you're going to have teeny tiny eclipse. By looking at it with very precise photometry, you can find planets. And this is by far the most productive method from Kepler satellite but also from the [INAUDIBLE] measurements in some cases. In somewhat unusual ways through gravitational microlensing it'll show you diagram how that works. But that is relatively rare, and also imaging is so hard that the bulk of the planets we have found is two in the middle, the radial velocity method and the eclipses. Let me show you some pictures of this. So on the top right is illustration why direct imaging is a problem, that if you took a picture and your planet is completely lost in the glare of the star. If you took the same picture in infrared, well, the star is not as bright because it's the [INAUDIBLE] side of the spectrum, so star is intrinsically dimmer in those wavelengths. And the planet starts to shine its own thermal radiation, so the contrast improves. Now, this is still idealized. So even with the Hubble space telescope there is no Earth's atmosphere in turbulence, they have to apply coronography, which is a instrument whereby inside your telescope you put some occulting disk or something right in the focal plane to block the parent star. And then you just look for other things that would otherwise be lost in the glare. Now there are a lot of faint stars and galaxies in the sky. So in order to actually claim that any one of the faint ones that you've seen is a planet, you have to monitor, yes? Stars move. They have their own velocities, their own planets will be coming along, and planets also going in orbits around them. So if you take a picture a couple years apart, at least, and you see that one of those things has moved according to what could be a Keplerian orbit, then it's a pretty solid case. And here's the real case from Fomalhaut, and now there is a whole bunch of these. You may remember pectopicotious the first prototsteller disc, also first protoplanetary disc which was seen. The big blue circle in the middle is artifact coronography obscuring the central portion. Now, you'll see the infrared emission from side on protoplanetary disk, but there is also this one little source that was seen right close to the star and that is one of the first formed planets in this system. Then there is this one called HR 8799 that's designation of a star. This is image from Keck telescope, also using adoptive optics and so on. So now this is done. Not exactly an industrial scale, but we've seen enough planets directly and everything kind of fits together with other measurements, so that's good. What about the radial velocities? So, When two mass points are in gravitational interaction, they will actually orbit around common center of mass. And the product of mass and velocity is the constant. So if a planet moves at some velocity, then the ratio of the planet's mass to the star's mass gives you the velocity of the star. Now of course this will depend on inclination. Ideally you'll be looking right at the orbital plane, but if you're not then there is the inclination angles and an extra variable has to be solved. And so if you do this for Earth and Sun, you find out that to find Earth around Sun somewhere else, you have to measure velocity of that star with something with per second precision. That's way beyond our state of the art given today. However, if you take a really big fat planet, like 10 times the mass of Jupiter, and to make life easier, you put it closer in. Say in orbit of Mercury. Then you have a vastly higher effect. Because it's closer, the velocity is gonna be higher, right? Kepler's Law, and because it's more massive, again. The velocity will be higher in order to balance kinetic and potential energy. So for a hypothetical fat Jupiter like that, it'll be some hundreds of meters per second. And in fact, first planets that have been found outside solar system were of this nature. They were very massive planets, very close in. And this is entirely a selection effect that's the only one we can find. Also, you have to monitor the thing for a long time, for at least one or two periods. And in our solar system, Jupiter goes around the Sun every 11 years, so you better monitor it over 11 years period. So today we can actually do this with a meter per second precision that's sort of state of the art. Incidentally, you can do the same thing in astrometry. If you had a really precise measurements of positions of stars, then even if you look face down on the orbit you can see star make wiggle like motion. Now that has never been seen yet, but with forthcoming instrumentic missions and plans that might be possible. Actually, the first planets outside our solar system were found with the version of this method, and it was pulsar. Pulsar timing is a very precise measurement. These are neutral stars with extremely high moments of inertia and they spin at extremely well defined frequency. So, as you are counting pulses coming from pulsar with atomic plaques and so on. If the pulsar is moving away from you, then there will be some delay coming towards you, it will be piling up. So it's like a Doppler shift but with pulsars, not wavelength to flight. And in fact, in '92 this was seen, around a pulsar with this designation by two people in people call them the rocks around the clock. Where did these planets come from? They may have been leftovers from parent stars exploded and made pulsar, but they can be also freshly condensed planets. Because supernova ejects materials, some of the material cools, it retains, it may make second generation for a planetary disk and you make planets all over again. So this has been somewhat neglected, but it's actually, I thought, a result that was well worth emphasizing. So most of the planets nowadays were found through the transits method. And most of them with Kepler Mission, which was launched in 2009 and kind of ended last year. Although still using telescope for limited type of observations because it's lost its pointing capabilities. So the idea here was very simple. You have a telescope with a lot of CCDs imaging the same piece of sky, again and again and again. And that piece of sky's close to the galactic plane Signus, why do this from space? Well, because you can achieve precision in photometry from space that you could never achieve on the ground. Because of the stellars scintillation and seeing the limit on ground based photometry is really better than one part in thousand, but in space you can easily do one part in a million with proper detector. The person behind this was Bill Barookie, and he deserves lot of credit for making this happen. European space agency also had similar satellite called. But Kepler by far produced many more. So how this works is kind of very simple. Planet comes in front of the stellar disc, you see a slight eclipse, goes away. But flat bottom type eclipse. In our Solar System, we see that with Mercury and Venus. And so every how many years there is transit of Venus and transit of Mercury. You can see disk of Venus occulting part of Sun. Now for Earth like planet in front of Sun like star, a radius is 100 times smaller than [INAUDIBLE] Sun. Square of that is the area scaling so you in 10,000. This is a very, very shallow eclipse. All right. And so better measure with like one to million precision, and that's exactly what Kepler does. And I don't know if you can read the numbers on the Y axis, but those measurements really are of the order of part in million. And the final method is gravitational microlensing. We'll probably come back to gravitational lensing at some point later in the class. But, the idea here is that if you put any massive object in front of some background source, say a star, general theory of relativity tells you that gravitational field will bend light rays. This is how it was proven in the first place, with solar eclipse. And essentially, the mass in front of your source acts as a lens. And it magnifies the background source making it brighter. So if you have moving lens, say a star that crosses the line of sight towards the background star, you're going to see it brighten and dim out. And this is called gravitational microlensing and it's now done on industrial scales, you know, seen thousands of times. Now what happens if you have a planet orbiting the lensing star? Then this will be like a little extra mask component to provide a little extra magnification if it is just the right angle. And so we're going to see an extra spike due to the planetary lensing on top of the stellar micro lens. And that too has now been seen a number of times, although it's very hard to catch. You have to monitor large numbers of stars. And then we have to quickly make sure you don't miss one of these.