Combined Transcripts: Exoplanets Course V0.0_Intro [BRIAN] Hello and welcome to the ANU edX course on exoplanets. My name is Brian Schmidt, and I'm an astronomer here at the Australian National University. I'm normally thought of someone who studies cosmology, the universe on its largest scales, but I also work in the field of exoplanets through a program known as the HAT South telescope network, where we're out searching for planets going around nearby stars. [PAUL] And I'm Paul Francis, I'm the other lecturer in this course. My interest in exoplanets comes from the study of comets, which occur both in our solar system and in other solar systems. but I also do work on giant black holes and the origin of the universe. [ BRIAN] So exoplanets and the study of the exoplanets is arguably one of the most exciting topics in astronomy today, it has come from nowhere to being a science that is literally lighting up humanity and discovery. [PAUL] 15 years ago we did not know about any planets around stars other than our own. Now there are over a thousand, and not a week goes by without some new and wierd discovery being in the papers about some strange planet or something else going on out there. [BRIAN] and in this course we're going to try to bring you up to date with everything that's happening but it's going to be a challenge because things are changing so quickly. [PAUL] This is the second of four courses that together make up the Australian National University's first year astronomy unit. The first course, introduction, is on the greatest unsolved mysteries of the universe, you can do that course online through edX as well. If you haven't done that course, don't worry, we will repeat the important bits of it here. [BRIAN] Now this course is at a level which is a little bit more than your average documentary. We really need you to have some understanding of math and science and physics at the high-school level. If you are unsure, have a look at the first homework set for, problem set for this course and you'll get a sense because that's the level that's throughout the course. [PAUL] If you can do that, you'll be fine to do the whole course, it doesn't get any harder. Let me show you how the course works. Most of the course material can be found here in the coursework tab. each week a new section will be released in each section the two crucial parts are the lesson and the homework you should do the lesson first this is our equivalent of a lecture and it consists of a whole bunch of videos interspersed with questions Down here at the bottom of the video are the controls. They will differ depending on your browser. But you can go full screen, change the speed you play things at, turn the captions on and off and generally play around with them. also each week you will need to do the homework if you want to get a certificate for this course and this will consist of a bunch of numerical or formula type questions and maybe the occasional multiple choice question. In addition there are a number of things to help you. There are reference notes. This gives you the key facts from the videos so that when you're coming back to look up something you don't have to go to go through all the videos again There are links to papers, papers referred to in the talks, there are practice questions which will give you practice at solving the same sort of questions you are going to need for the homework. The practice questions are not worth marks, they are just for practice, and in some cases there will also be web-cast worked examples. And there is a mystery. Week by week this will build up. And then at the end of the course, in the exam wee will test you on it. also crucial is the discussion here you can pose questions to us and answer questions and generally interact with the other members of the class [BRIAN] So I think that's all you need to know to start this course. [PAUL] If there's anything still unclear to you, check the reference notes out in this section, or put a post on the bulletin board, the discussion board, and we will let you, answer your question. [BRIAN] So let's start with, looking at one place where we really expected not to find a planet. V1.1 Welcome to our first lesson on 'exoplanets', one of the most exciting and active areas of astronomy today. What is an 'exoplanet'? An exoplanet is a planet that orbits a different star, not our own sun. We know about the planets orbiting around our sun like Jupiter, Saturn, Earth and so on. What about planets orbiting others stars? There are billions upon trillions upon trillions of other stars out there. Do they have planets orbiting them? Are they like like our own? These are the sort of questions we are going to address through this course. Now, why is this hard? Well, the basic problem is that our own solar system seems pretty big to us, when the furthest a human being has ever been is to the moon and not any of the planets, but in the scheme of the galaxy, our own solar system is very small and exoplanets are very far away. Let's try an analogy of 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 of this ball. To that scale the moon would be the size of a tennis ball, about 10 metres 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 the solar system. To get to the nearest other solar system, Alpha Centauri, you have to go 25 times around the world. So, on this scale, the fastest spacecraft that the human race have ever come up with are travelling 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, to any other solar system. So, if we can't go there, what can we do? I suppose we could try looking. We'll point our telescopes there, to light 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 and 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 actually bright enough that it can be picked up with even a medium- sized telescope, let alone one of the biggest in the world. The problem is not that the planets are too faint, the problem is that they're very near the star that they're orbiting, as viewed from here, and that star they're orbiting is much, much, much brighter. So, let's say, if you consider, you're in a dark alleyway at night, and someone is standing there with a candle about 10 to 20 metres away. Would you be able to see that? Probably, yes. 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? The difference in brightness between a candle and full-beam headlights is about a factor of a thousand if you do the figures. The difference in brightness between a planet and a star is about a billion, so it's actually a lot like someone is standing with a candle with an atom bomb going off behind them. That's actually about the right ratio. That sounds hopeless. So, we're kind of stuck. 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. One idea is the idea of 'Reflex Motion', because as a planet orbits a sun, the sun, of course, orbits the centre of mass. Both objects orbit the centre of mass and so the star doesn't make a big motion, but it does make a motion that is potentially measurable. Well, this simulation is grossly exaggerated, the wobble of a star is nothing like this big, but the basic idea is that this star is leaning back a little bit against the gravity of the planet and so we'll be doing a very small circle. So, in principle, if you see a star doing loopty-loops, even though you can't see the planet, you might be able to deduce that it's there. Yes, sounds like a good idea. 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 a disc of a star as seen from here, in which case, once every orbit, you will, well you won't actually be able to see the actual disc of a star, even the nearer stars are just a dot or are far smaller than a pixel, but what might happen is that as it goes in front, the brightness would decrease very slightly, it might be blocking a mere one percent or ten percent of a stars surface.