Welcome to our first lesson on exoplanets. One of the most exciting and active areas of astronomy today. Now what is an exoplanet? An exoplanet is a planet that orbits a different star, not our own Sun. We know about the planets orbiting our own Sun, Jupiter, Saturn, Earth and so on. What about planets orbiting other stars? There are billions upon trillions upon trillions of other stars out there, do they have planets orbiting them? Are they like our own? These are sort of questions we're going to address through this course. Now why is this hard? The basic problem is that our own solar system seems pretty big to us. We're trying to send our furthest humans ever been to the moon, not many of the planets. But in the scheme of the galaxy, our own solar system is very small, exoplanets are very far away. Let's try an analogy for that. To get a sense of how big the solar system is and how much empty space there is in it, let's have a scale model. As our scale, imagine the Earth is the size, the size of the small. After that scale, the moon would be the size of a tennis ball, about 10 meters away. Jupiter would be the size of a car on that hill. Pluto would be about twice as far away as this mountain. That's a solar system to get to the nearest other solar system, alpha-scented, and you have to go 25 times around the world. So on this scale, the faster space trough the human race ever come up with a traveling at about the speed of a garden snail. So it's going to take a long time to get anywhere. 25 times around the world. So we're not going in any foreseeable future to be actually sending space probes to let alone visiting any other solar system. So if we can't go there, what can we do? I suppose we could try looking at quite a telescope there, like coming from us at the speed of light, and actually see what's going on. So we can take the biggest telescopes in the world and point them at a nearby star. So they don't just look for them, right? Well it sounds simple, and if you do the calculation, it turns out that in principle you can actually see a planet that's bright enough to pick that up with even a medium-sized telescope, let alone one of the biggest in the world. The problem is not the planets are too faint. The problem is they're very near the star they're orbiting as viewed from here, and that star they're orbiting is much, much, much brighter. So let's say if we consider it a dark alleyway at night and someone standing there with a candle, or you're 10-20 metres away, would you be able to see that? Probably. But let's say that person is sitting in front of a car and they suddenly turn the full beam headlights on. Can you now see it? Well let's find out. What have you done to her? So the difference of brightness between the candle and full beam headlights is about a factor of a thousand if you do the figures. The difference of brightness between a planet and the star is about a billion, so it's actually someone standing with a candle with an atom bomb going off behind them, and that's actually about the right ratio. That sounds hopeless. So we're kind of stuck, and we can't go there, and we can't see them. So there's some other ideas that we can maybe get out of how to find these things. So one idea is the idea of reflex motion, because as a planet orbits a sun, the sun of course orbits the center of mass, both objects orbit the center of mass, and so the star doesn't make a big motion, but it does make a motion that is potentially measurable. Yeah, this simulation is close to the exaggeration, the mobile of the star, I mean I think like this big, but the basic idea is that the star is leading it back a little bit against the ground to the planet, and so we'll be doing a very small circle. So principle if you see a star that's doing the loops, even though you can't see the planet, you might be able to deduce it's there. Yeah. So that's one technique. The next idea is the transit. The idea is that if you're lucky the planet might actually go in front of the disc of the star, a scene from here, in which case once every orbit you won't be able to see the actual disc of the star, even the nearest star as they're just a dot or a fastball or pixel, but what might happen is as it goes in front, the brightness will decrease very slightly, it might be blocking you 1% or a 10% of the star's surface. So here we go. So a very small fraction of the light from the star is not reaching us, and so you might see periodic dips in brightness that tells you a planet is there. We know this happens because just in 2012 we had Venus go in front of the sun, and it blocked out one part in 10,000 of the sun's light, which isn't going to be easy to measure, but maybe it's not impossible. A third approach is gravitational lensing. Now this one's kind of tricky. So we know that gravity can bend light, and so imagine that you have an object between us and another star. If that planet gets exactly lined up with the star behind it, it will act as a lens, a gravitational lens, and magnify the background star a little bit. Now this is a little confusing because planets maybe they're on their own, but they're normally around other stars. And so you're going to have to figure out whether or not we can use this if a planet's going around, for example, another star. But in principle, this can work for objects that are a long, long ways away, and we really don't care how bright they are. We only care that they have gravity. And the final approach is the brute force threat imaging. We have said it's really hard to see these things because the star is a billion star here is a billion times brighter than the planet. But that's sort of challenge at astronomers, engineers, relish. Maybe there is some way to have such a really good telescope that despite this overwhelming dazzling light from the star, you can pick out the billion times fainting lights on the planet. It's going to be hard, but maybe it's possible. There are some tricks of quantum mechanics we might be able to play to get light in the waves, the line up, and get rid of them. So those are the four techniques we're going to talk about. Now, I should warn you, if we were talking about our own solar system, then we could be very descriptive, of course, because they look Jupiter's like this. It's got these properties, Mars's like that, so hot and dry, and so on. The X-air planets, we can't do that, basically because our data are so bad. What we're going to have to do is combine understanding of the techniques we used to study these things with the results we get. At the moment, we only have a few clues. It's like being in a dark and warehouse, we've shown the two narrow beams of the light. And we've only got a very small fraction of knowing what's really going on, and without knowing exactly which directions you're showing the light in, you can't extrapolate from what little we know to what's really out there. But this is one of the things that makes astronomy so exciting, as it really is frontier, where you are trying to piece together what's happening at a fundamental level from just the beginnings of the information, but it's a great place to learn new things.