 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to talk about the evidence for exoplanets, or planets outside of our solar system. Just a few decades, none were known, and now we know of thousands of planets outside our own solar system. So let's see how we've learned about these and how we've begun to detect them. So what do we know? First of all, earlier we were able to look at disks. So disks are much larger than the planetary systems, and therefore are easier to detect. And we could see these where material would collect around the star as the star was forming. These, or the dust particles there, would then be heated by that proto-star and give off infrared radiation that could then be detected. Now, because of this, and we've known of this for a long time, that disks seem to be a natural part of star formation. We find them around lots of forming stars. And that meant that we thought planetary systems should be very common. However, actually detecting the planets is a little bit harder. So what about planetary formation? Well, we look at some of these disks, and in very young proto-stars, at one to three million years, the disk extends nearly down to the star itself. In older proto-stars, maybe 10 million years, the inner regions have been cleared of their dust. So in just a few million years, that dust is gone, and that means the planets had to form very fast. The planets cannot take a long time to form. They've got to form quickly in order to account for the fact that we see that it only takes a few million years for the stars to get rid of the dust, which is needed to build those planets. Now, when we look at some of these, and we see some of the debris disks we can see around these in the longer wavelengths, and the planets can help concentrate that dust and give us clumps of material. So we'll get clumps of material in some regions, gaps in others. So here we can see that around the forming proto-star at the center. There are various rings of material, and it has begun to clear out. Some of those have begun to clear out, and others are condensing. So we can see things like this in the star HL-Tory, where the debris disk is still visible, and this is a planetary system in the process of formation. So again, these are on a much larger scale they're easier to see, but we want to be able to find the actual planets themselves, and they have now been detected through a number of methods. And we'll talk about some of these in more detail later. But let's look at each of them very briefly here. There is the astrometric method, and we actually see the star move as it's tugged on by the planet. The radial velocity method, which is a change in the Doppler shift of the stars because the planet is pulling on them. So those are both due to gravity. Transit, we observe the dimming of the light when a planet passes in front of the star. We can have direct imaging. We can actually view the planet. These are very rare, but there are a few cases where we can actually image the planet itself. Gravitational microlensing is a brightening of the star by the planet's gravity. And finally, pulsar timing, which gave us the first detection of exoplanets in 1995 around a pulsar, which is the collapsed core of a neutron star, so it was quite surprising to find planets around this star. Now let's look at each of these briefly in a little more detail. First of all, the astrometric method, and what we have to recall is that the planet does not orbit the star. The planet and star orbit around a common center of mass. So we see that here in the x at the center, or the plus at the center, that is where the planet and star are both orbiting around. Now, of course, the star is much larger, so it moves, but it moves a lot less than the smaller object. The planet's going to move more. The star is going to move less. Now, these are very tiny motions. We're looking at things that are 1,100th of an arc second for Alpha Centauri, which is a very, very nearby star, if a Jupiter-sized planet were orbiting. Now, this one depends on distance. The closer the star is, the easier it is to be able to detect these wobbles in its motion. For a very distant star, it would be very hard. So this does not give us a lot of planets, but has helped us with some of the very nearest ones. Next, we'll look at the radial velocity method, and that is the velocity of the star is going to change. Again, we'll have the orbit around the center of mass. Sometimes that star is moving toward us, sometimes it's moving away, and we'll get a red shift. So we can see those shifts in the velocity, and that velocity will change in a periodic manner. It will change regularly, and we can actually look at one of these graphs, and what we see is here is the velocity as the star is moving. So the star moves, it's coming toward us, it's going away from us, and that gives us its motion, and we can then use this to infer the existence of a planet, not only that, but we can also estimate the mass of the planet if we understand the mass of the star. So we can use this as a way to learn a little more about the system that we are looking at. Now this one is good because it does not depend on the distance, other than that we have to be able to see the star and get enough light from the star to get a spectrum, the distance does not matter. So how about the transit method? So in the transit method, and one thing this requires is that the orientation of the system be correct. We have to be looking at it almost exactly edge on, and we see that here because if it's tilted a little too much, then the planet will pass below the star in this or go above the star and we will not get any clips, just as on Earth we have to have the moon and the sun lined up exactly. But if this is the case that things are lined up right, that's the planet will block some of the star's light. And we can observe the light curve where the brightness dips when the planet passes in front of the star, and that gives us evidence that there is a planet there. From this we can determine things like the mass, the diameter, so we get masses from the orbit much as we did previously. Diameters, how much starlight is blocked? Well that can tell us whether it's a very small planet or a very large planet, and we can get the orbital periods from the time between eclipses. Now when we look at the light curve, how do we learn these things? Well we can learn the orbital period by how often the dips occur. We can't get this from just one period, but if we have multiple ones we can then determine the orbital period. The planet size, how much of the starlight is blocked? And how quickly does the eclipse start and end? As we see this does not dip down immediately, so you could have something where it went almost straight down and stayed down, and that would tell us something very different. Here we're saying it took some time for that planet to pass fully in front of the star. The smaller the planet, the steeper this is going to be, the bigger the planet the longer time it will take for this to dip, dip. The mass, if we know the orbital period in the distance, we can then use Kepler's third law to determine the mass. And we can also learn something about the atmosphere if we can see the spectrum of the starlight through the atmosphere. We can learn something about the composition of the atmosphere of the planet if it has one. Now how do we do this transit method? Well it requires looking at lots of stars and we've had a couple of satellites that have been doing this. Kepler Space Observatory from 2009 to 2018 studied a very small region of the sky and monitored the brightness of the stars. Now on this small region it was able to discover over 2,000 confirmed exoplanets. How small of a region? Well here shows what the region that it was searching. So Kepler was searching in one direction to about 3,000 light years. So it was only looking in that one area and in just that area we found thousands of confirmed exoplanets and several thousand more needing additional study for confirmation. Currently we have TESS, the Transiting Exoplanet Survey Satellite launched in 2018 which is observing far more of the sky has detected hundreds and many thousands of new candidates. However we'll see that it takes time to confirm these. So while it doesn't seem like it's gotten very many yet the more and more are coming as they get confirmed so we will have far more exoplanets known over the coming years. How about some of the other methods? Well we mentioned a few others. We talked about direct imaging where you can actually see the planet. These are very rare and that's difficult because the stars and planets are very close together and the star is overwhelmingly bright. But if we can mask out the star we can actually image the planet. Now it's not going to look like a beautiful image of the planet it's going to be a dot of light because of the distance but there are a few cases where we can actually see that. We also have gravitational microlensing and this occurs when the planet brightens the star so as the star behind comes passes here we get additional brightening because of the gravity of the planet and that can also help us to identify planets. The difficulty here is that you can't repeat the measurements you'd have to wait maybe thousands or millions of years for another star to happen to pass directly behind this system this same system. And then we did have pulsar timing which is the first discovery of planets from variations in the pulses of the pulsars. So let's look a little at what kind of planets have been detected and how this has changed over the last few decades. You'll see that it was very small we didn't actually break a thousand until about 10 years ago as of recording this it's been about 10 years and you'll see that we had some major jumps here these are due to Kepler and a lot of confirmations of Kepler objects and then again here and you'll see that in the green these are the transits so a lot of what we've seen the earlier ones are Kepler and we're starting to get more test ones the red radio velocities have just been regularly increasing total number that we have detected here and you can see that we're now breaking the 5,000 mark of planets as of 2022 but the vast majority of the planets that have been discovered are either through transits or the radio velocity method and we will look a little bit more at the results of what we found of these in a coming lecture so let's go ahead and finish up with our summary and what we have found now is that there are a large number of exoplanets thousands that have now been found our early indications were those dusty disks around stars which gave us the hint that there was material and were planets out there now we found them many are now found by radio velocity and transit observations as well as a few other methods so that concludes this lecture on evidence for exoplanets 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