 Greetings and welcome to the introduction to astronomy. In this lecture we are going to talk about evidence for exoplanets. So when stars are forming, planets are likely forming around them and what is the evidence that we have that other planets and planetary systems exist out in the universe. So let's take a look at some of the earliest things that we could see. It wasn't until the last few decades that we were able to actually detect planets around other stars. However we did have evidence in the form of dusty discs of material. So we could see what we thought were perhaps planetary systems forming. Now the idea is that these discs are much larger than the planetary systems themselves making them easier to detect. So the dust particles are going to be heated up by the protostar at the center and they're going to be radiating infrared radiation. So when we look we see these little dusty areas which are likely the beginnings of planetary systems that are forming. And when we look at star-forming regions the disc seems to be a natural part of star formation meaning that there should be lots of planetary systems out there. How long does this process take? Well, young protostars will live a couple million years and the disc will extend nearly into the star. So we can see some examples here where the star has been blocked out at the center and we can see the dusty disc around it. It's all a matter of the orientation with which we see them as well. So here we're looking at one close to edge on and you can see that the disc kind of runs diagonally across this. Here we're tilted a little bit more looking at it more face on and we can see the disc a little more surrounding the star. Now we do have to block out the star because the star is emitting far more light than the disc itself. And that would mean that if we had the star in there it would be overwhelmingly bright. But we can get an idea with the artist's conception sketches that we see down below as to what these systems might look like. So in some of the older protostars what we begin to see even when we get up to 10 million years old when they're very young the disc extends nearly into the star. In older protostars as they get older the inner regions have lost most of their dust. So these intersections here are beginning to be cleared out by the material. So this must happen very quickly because after about 10 million years that means the dust is gone, the material that could possibly build the planets is gone. So the star formation must have formed within this 10 million year time frame. Now when we look at some of these we can also see discs of debris around the material. We get various clumps of material where dust can be concentrated. So we're going to start seeing things like the rings that we see around some of the planets. We get denser areas and less dense areas within the disc of material. So there are various gaps where there's little material and clumps where there is more material. And we see H. L. Torrey as one example where we can see this debris disc. We can actually see this in the infrared portion of the spectrum. And we see the protostar at the center and then the different rings of material. And very likely within each of these rings we are beginning to see the beginnings of the formation of a planetary system. So what are some of the methods that we can search for actual exoplanets? Here we're seeing the beginnings of their formation and we've known about this for a few decades. It's only been in the last 25 years or so that we've begun to detect extra solar planets, planets outside our solar system. So what are some of the methods that we're going to look at? Some of these we will talk about in a little bit more detail over the course of this lecture. There is the astrometric method where we look at the motions of the star and how the position of the star changes. We can look at the radial velocity method where we look at changes in the Doppler shift of the star. So the velocity of the star changes similar to the astrometric method except here we're looking at position. Here we're looking at velocity. We can look at the transit method where we observe the dimming of the light when a planet passes in front of the star. And a little more rare we can see direct imaging where we actually can view the planet. We can actually see the light from the planet. And a couple of less common methods, gravitational microlensing, when the planet passes directly in front of a distant star, not the one it's orbiting but in front of another star, it'll brighten that star and cause it to temporarily increase and that allows us to detect the planet that way. And finally a last one that I really won't go into any more detail on in is pulsar timing. Variations in the timing of the pulsar can imply that there are planets orbiting it tugging on the pulsar. This was actually the first detection of exoplanets back in the 1990s, the first detection of planets outside our solar system. So let's look at some of these methods and first we want to look at the astrometric method. What happens is that stars will wobble because the planets are pulling on them. In reality we talk about a planet orbiting around a star. So, but in reality they orbit around a common center of mass. So really the planet orbits and the star orbits as well. Just that the star's orbit is often within the star, often that center of mass is actually within the star itself. And the image we see here shows where the center of mass of the solar system is. Sometimes it is outside the sun by a little bit and other times it is actually deep inside the sun. It depends on the relative positioning of the planets at the time. So what we would see with this is that depending on where that center of mass is the sun would be moving more or less and some distant astronomer would be able to see the sun wobbling a little bit in its position. You would be able to see its position change. So while the smaller body moves more the larger body the star still does move and that is something that can be detected. However it is extremely small and difficult to detect. For Alpha Centauri the nearest star to us if we had a Jupiter sized planet orbiting it that would give a shift of 0.01 arc second. Now that's definitely detectable. We can detect even smaller motions than that. But it's extremely small and this is for the nearest stars. If it gets a little further away it's going to be harder and harder to detect these motions and they will be able to be detected with less accuracy. And this also again significantly depends on distance. Because it's going the motions are going to be easier to detect if the star is close to us than if it is further away. So astrometric method is one used. One of the more common ones that we use is what we call the radio velocity method. What happens here again we're looking at the same kind of thing we talked about for the astrometric method. In reality the star is actually orbiting as well. So the star orbits around the center of mass based on the exoplanet's motion. And when the star is moving towards us it is blue shifted which we call give a negative velocity. When it is moving away from us it's a red shift or what we call a positive velocity. So as the star orbits at some point it's moving in the direction of the earth and we will see a blue shift. A little while later we will see a red shift. Now the period of the motion is exactly the same as that of the planet. So if this planet takes one year to orbit so will the star. So we would see a blue shift and six months later we would see a red shift for something that took one year to orbit. So there will be a periodic change in this. Now the more orbits we can get the more accurate the determination. So just seeing one orbit may give us a hint that a planet is there but may not be enough for confirmation. We need to get more and more orbits and that's why it's easier to detect closer planets to their stars. Because they orbit faster and they were able to see this quicker when just a few years we can get several orbits. Whereas a planet like Jupiter which takes 12 years to orbit would take decades. The key about this one is that it does not depend on the distance. All we need to see is the star and all we need to do is to measure the Doppler shift of the star. So all we need to do is to take its spectrum. It does not as long as we can see the star it does not matter. So this does not depend on the distance as long as it's not so far away that we cannot see the star. And it does allow us to estimate the mass of the planet based on the motions of the star itself. Now when we can see this what we get in terms of data that we see if we look at an example of one of these we see that the star will vary in velocity. We will get a negative velocity when it's moving towards us down here and then a positive velocity when it's moving away. So we will see this constant change and we can fit that with a little periodic curve here and then allow that to determine the amount of the shift, the amount of the velocity change will allow you to determine the mass of the planet. So we can actually determine some of the properties of this based on these types of observations. Now one of the more common we used, radial velocity is one but the other common method used is called the transit method. The transit method works only if the orientation of the system is correct, meaning that it has to be almost seen edge on. You can imagine you see it here almost completely edge on and the planet passes in front of the star and will temporarily dim its light. So if we look at what we call the light curve we see the brightness here and when the planet passes in front of the star it dims off a little bit of that light. So a little bit of the light is missing from the star and it looks a little bit dimmer. So when we observe this we will see the planet eclipsing part of the star and we will see that light curve drop down in brightness. And what we can determine here is the mass of the planet from the orbit and the diameters, the size and the orbital periods. So we can get an idea a little bit more about that if we look at this image here is a little more detailed light curve that we might see and we can use that to really be able to determine a lot of this by how often the dips occur if they occur once every year then the orbital period is one year. If they occur every five days then the orbital period is five days. So that tells us how often the planet passes in front of the star. For the Earth, the Earth would pass in front of the sun for an object for a distant astronomy once every year. Jupiter would be every twelve years. So how often it happens depends on the orbital period. The planet size we can learn from how much of the star light is blocked so how much does this dip down during the eclipse and we can also get an idea by how quickly the eclipse starts and ends how quickly does that planet get in front of the star and the very faster it occurs, the steeper this is then the planet is much smaller. If it gets into an eclipse and comes out very quickly then it's a very, very small planet because it doesn't take it long to block the star. A very large planet will take more time to slowly get its body in front of the star. We can determine the mass from the orbital period and the distance so once we determine those then we can get ideas of the mass looking at all of this and we may even be able to determine something about an atmosphere by looking at the star light when it's going through that atmosphere. It will absorb various lines depending on its composition and we'll be able to allow us then to determine a little bit about what the composition of that is. Now the key for the transit method has been the Kepler space observatory. It was launched in 2009 and studies 150,000 stars in a small portion of the sky. It has now confirmed over 2,000 exoplanets so a large percentage of the exoplanets that have been discovered so far have been discovered by this satellite and there are 2,000 more that are potential candidates that need additional study. At this point it's discovered two-thirds of the known exoplanets and all it does is look at these stars over and over again measuring their brightness and watching for those little dips that signify that a planet has passed in front of the star. Now some of the other methods that we can use in addition to these more common ones are direct imaging. So if we block out the star light and look here in the infrared we can actually see the planets so we can see several planets here that would actually be orbiting around this star. The star would be present here but we're not looking at a wavelength where the star would be easily visible but where the planets are. In these cases we are just seeing the light from the planet. Anytime you see a sketch of what an exoplanet looks like it is always an artist's conception. We cannot actually see any of these planets other than the fact that they exist in their light. But this is difficult because the planets are often very close to their star and the star is overwhelmingly bright. So it is very a hard thing to do and we don't often get direct imaging of exoplanets. Gravitational microlensing is another method that is used and that is done here and what happens is that a planet can briefly make a star appear brighter when it passes in front of it. So the planet here can cause the star can cause it to lens and a planet can cause it to lens a little bit more. Now this is a nice way to be able to detect things that are further away from their star because it does not depend on being close. The radial velocity method is a lot easier to do when things are closer to the star because they occur more often. The transits will occur more often for very small orbits. A microlensing can occur even if the orbit is very far away. Even a planet like Jupiter or Saturn or Uranus or Neptune could be detected through this method. However, we are also restricted in that we cannot repeat the measurements. We have to wait for something to happen to pass in front of the distant star so unless the planet passes in front of a second star with the same orientation then we cannot repeat the measurement and confirm it as we can with things like the radial velocity and transit methods. And then again, I mentioned pulsar timing. That was our very first discovery of planets outside of our solar system based on the variation of pulses in a pulsar. It was actually quite surprising because we wouldn't have expected a planetary system to exist around a pulsar. A pulsar is the remnant what is left over when a supernova occurs. So we would think that a supernova would be powerful enough to have destroyed all of the material around it. Any planets would be long since gone. But perhaps there was some way that some other material recondensed and new planets were formed afterward because it was very definitely found and these were the first detections of extrasolar planets. So let's finish up here with our summary and what we have is that we have now found a large number of exoplanets, thousands of them that exist. The early indications were the dusty disks around stars which seemed to show that planetary systems were forming but now we have used primarily radial velocity and transit observations to detect many thousands of planets that we now know that exist outside of our solar system. So just a few decades ago we didn't know of any and now we know of well over 3,000 planets that exist outside of our solar system. So that concludes our lecture on the detection of extrasolar 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.