 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about extrasolar planets or planets that exist outside of our solar system. We've known about planets themselves, of course, since ancient times, and ancient astronomers were able to watch several planets within our solar system as they wandered through the stars. But planets outside our solar system are something much more recent. So, let's get started here, and we want to start looking at first detection. How can we find these planets outside our solar system? As early as the mid-1980s, no planets were known outside of our solar system. We only knew of those in our solar system. However, we were getting some very good hints. We were seeing things like dusty disks of material that gave us the ideas that maybe planetary formation was ongoing in these systems. So, we were getting ideas as to the fact that solar systems might be forming elsewhere, but we didn't know of any planets that existed outside of our own solar system until the late 1980s. Today, we know thousands of exoplanets, thousands of planets outside our solar system, which includes large planets much larger than Jupiter, and includes planets down to and including the size similar to that of the Earth. Now, how can we detect these planets? And there are several methods that are used, including gravitational effects as one example, and what we call the transit method or eclipses that occur. Let's first look at the radial velocity effects. Essentially, what happens is when we talk about a planet orbiting a star, it's not absolutely correct. The planet does orbit the planet and the star both orbit around a common center of mass. So, the center of mass here is always going to be a lot closer to the much more massive star, but the star does orbit as well. And that means that when we see the light from the star, when it's coming towards us, the Doppler effect says that it will be blue-shifted, and the light will be shifted towards the blue, and a little while later, it will be red-shifted. So, while we cannot see the planet itself, we can see the effect that it has on the star, in that it is causing the star to wobble a little bit and move and give it these various shifts. So, we can use the Doppler effect and measure the spectrum of the star and see how it is moving. And what that tells us, if we look at an example here, is that the planet's will, the star will have a velocity, whether it's coming towards us or away from us. So, when we see the different radial velocity here, we can see that it's negative. Here's where the star is coming towards us, and here is where it is moving away from us when the radial velocity is positive. And when we see just a single planet here, it's very simple. We can see that the velocity is changing, and it changes in a very distinct pattern. It allows us to determine then the period of revolution of the planet, how long does it take it to orbit around once, and the amount of the shift, the amount of the velocity can help us determine the mass of the planet. So, we can learn some very important properties of the planet by looking at this at these radial velocity curves. Now, what I don't show here is they can get much more complicated if you have multiple planets involved, because sometimes they're tugging together, and sometimes they're pulling away, and you won't get something as simple as the example I'm showing here. So, when we find some with multiple planets, it can be a little more complex. The other method that we use is the transit, we call the transit method, and that works when cases where the planet passes in front of the star, causing its light to dim. So, right here at position one, the planet is separate from the star. At position two, it is just starting to block out part of the star's light, and at position three, it is blocking out the star's light, making it a little bit fainter than it otherwise would be. So, if we measure right here, the star is a little bit fainter. That can be only a fraction of a percent, depending on the size of the star and the brightness of the star and the size of the planet. It can matter a lot or a little, depending exactly on the conditions there. But we can just, all we have to do is measure the brightness of the star and look for these kind of dips in the light that then occur at a regular basis. So, if we were looking for our Earth passing a star or a planet like our Earth passing in front of a distant star, we would see a very small dip once every year when it passed in front. So, it could take a long time to find certain planets like this. Now, when we measure some of these, we find things like this. And what we see is, you know, in some cases there's a lot of noise, very hard to find. Can you see the pattern here? If you imagine that you could not see the line that's drawn in there, can you really see for sure that there is a dip? Whereas some where it's much stronger, we can see very distinct dropping down and rising up of the light. So, depending on how much noise there is, how bright the star is, and how much, how big the planet is relative to the star, we can measure those, measure these. But then when we see the dip occurring and then we can see it again and again, we can then get confidence that there is a planet there that is blocking out some of the star light on a regular basis. And that allows us to determine some of the properties of the planet. In fact, how long it takes it to eclipse it can tell us something about the size, how quick this drops down, can tell us something about the size of the planet. So, we can learn some of these properties of the planet just by looking at these eclipses. Now, what do we find? When we look at these, what types of planets are we finding? Well, we find, first of all, we want to say that the detection methods are biased. That doesn't mean that they're wrong, it just means that right now it's much easier to find a large planet and it's much easier to find planets that are orbiting closer to their stars. And why is that? Well, a large planet is going to eclipse the star more and cause a bigger drop in its light, making it easier to find. A large planet is going to tug on its star more and make it easier to detect as well through the Doppler effect. So, it's easier to find large planets, it's also easier to find those that are closer to their stars. As I said, Earth would make only one eclipse per year for a distant astronomer. If we were trying to look for Jupiter, yes, it would eclipse more, but it would only be once every 12 years. So, you would have to wait, well, you'd see one eclipse, you'd have to wait 12 years for it to occur again, and then another 12 years, say, to get a third one to really confirm your findings. So, it could take decades to be able to find a planet like Jupiter. So, that's the example here that we really, in just the little time we're looking, we would not have found a planet with an orbit and size like Jupiter. It's also likely that there are smaller planets that exist, but we just can't detect them yet. They don't tug on their stars enough, they don't pull, they don't block enough of the star's light to be able to be detected. So, we find different types of planets that we see here that are unusual, and if we look at some of those, we find that there are different types, some are similar to solar system planets, very rocky planets, with orbital periods, you know, comparable to Mercury at least, if not a little bit larger. So, we do see some of those planets here that would be very similar. The frontier here, these are the ones we can't detect yet. These are the ones where our technology is just not there to be able to detect these. But we do see some very interesting planets, we see some what we call the hot Jupiters. These are very massive planets, Jupiter sized planets, that are very close to their stars. And if you look at the orbital periods here, we're talking about one to 10 days. Mercury takes 88 days to orbit once. So, these planets are Jupiter sized planets, but orbit closer to their stars than Mercury does in our own solar system. So, it's very interesting to consider how these might have formed. So, hot Jupiters, how can we get such a planet forming so close to the star? We also see other things like planets like Uranus and Neptune, Ocean Worlds or the ice giants. We do get the cold gas giants, but those again, when we get out to this edge, we're getting to the frontier of what we are able to detect. And that's simply because it takes a long time to find that wobble, to find that eclipse, and to be able to see enough of them to confirm. So, something like Jupiter that takes 12 years to orbit, we'd have to watch it for decades to really be confident that we could detect it out here. And at 12 years, that would be pushing well out into this frontier out here. And one of the things this is doing, again, in addition to finding hot Jupiters, planets with very high eccentricities. So, instead of orbiting in circles, or close to circles around the stars, we're finding planets that are actually orbiting with very elliptical orbits. Those aren't considered based on our models of planetary formation. So, really, what we're doing, having to do, is to rethink our models of solar system formation. But what we don't know right now is are we seeing the normal planets? Is our solar system the unusual one for having circular orbits and for having no planets close to its star? Or are we catching the unusual ones because they're easier to detect? Are these hot Jupiters really very rare? And once we can detect solar systems with planets much further away from their stars, will we be finding lots and lots of Jupiters, and will the number of hot Jupiters, while they exist, be a relatively small percentage in the long run? Remember that these are planets that are very large and very close to their stars. These are the easiest ones to detect. When we get out to the edge over here with long period and low mass, the ones over here are really hard to detect. So, easy and hard, maybe we're just detecting the easy planets first. But we do have to rethink how things work. Planetary migration may have played a part in our own solar system, but maybe it plays an even larger part in others, allowing those Jupiters to not only migrate closer, but to get much closer, and in fact to the point where they orbit around their stars with periods of just a couple of days instead of many, many years. So, let's finish up here with our summary. And what we've looked at is that since the late 1980s, we have discovered thousands of planets outside of our solar system. Many have been detected by the two methods I went over here. I talked about the transit and the radial velocity method, looking at eclipses of the star by the planet, and looking at the changes in velocity of the star caused by gravitational tugs of the planets. Some of the interesting things we find are hot Jupiters, very large planets close to their stars, and planets with large eccentricities. And these are really reinforcing scientists to rethink their models of how the planets formed. So, our solar nebula theory may need some modification to be able to explain how solar systems formed. Remember, our solar nebula theory was based on our solar system. We didn't know of any others at the time. Now as we start to see more, we have to get enough statistics to find out whether these are the common solar systems, or whether these are the rare exceptions that just happen to be a lot easier to detect. So that concludes our lecture on extra solar planets. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.