 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about extra solar planets, planets outside of our own solar system. We've looked in detail at each of the planets in our solar system as well as other bits of debris and now we will look outside and see what how many and what kinds of planets we might find outside of our solar system. So as recently as the mid-1980s there were no planets known outside of our solar system. However we did have hints of dusty disks of material such as the one shown here and a planetary formation so we thought that maybe there were planets, it looked like planets were going to be likely, but we were unable to detect them at the time. Today we know of thousands of exoplanets, thousands of planets outside our own solar system including many that are similar in size to Earth and even what we believe are likely planetary systems such as this one in the process of formation. So rings of material where planets are forming around a young star. Now how do we go about detecting some of these? Well we're going to look at two methods here we will come back in another lesson and talk about some more different methods but these are the two primary ones that have been used to detect most of the planets. We will look at the radial velocity method which is due to gravity. So when planets tug on each other we tend they will when planets tug on stars and stars tug on planets they end up orbiting and the planet ends up orbiting the star. However we tend to think of it as the star sitting still at the center while the planet orbits around it. That's not correct. In reality the planet and star orbit around each other they orbit around a common center of mass. Now because the star is many times larger and more massive than the planet the center of mass is very close to the star or even inside the star depending on the exact situation but it is moving so that star is actually moving so sometimes the star therefore is moving away from us and sometimes it is moving toward us and that means we can detect it with Doppler measurements when the star is coming toward us we will see a blue shift and when the star is coming away from us we will see a red shift and that leads us to a radial velocity curve such as the one shown here where we can see the velocity changing and going up and down so the star is going away and coming towards us and we can measure that out and we can use that to infer the existence of a planet. Now we'd want to watch this over a good long period of time. Here we see that this planet takes several years to orbit. How can we figure out the orbit? Well we can go from peak to peak again and we can then estimate that this is probably around three years that it takes this planet to orbit from mid-2004 to mid-2007 so we could use Doppler measurements looking at the radial velocity of the star. We can also use what we call the transit method which is an eclipse of the star and let's look at that transit method here the transit method or an eclipse occurs when a planet passes in front of the star and this occurs only if everything is lined up just right so that the planet passes in front. We can measure the brightness of the star and watch how that dims over time. When the planet passes in front of the star the planet is not as bright and it's blocking out some of the star's light and therefore the star will get a little bit fainter. We can see that in what we call a light curve such as something like this and we can see how the light dimmed here when the planet was passing in front of the star and then started at one level and went back to that same level. So seeing one dip like this wouldn't necessarily guarantee us a planet but when we can see this over at a regular period say happening each year as the planet passes in front of the star we could then again deduce the period of the planet and based on other structures of the light curve can actually figure out more information about that planet. Now what have we discovered? Well we found lots of exoplanets but one of the problems is that the detection methods are biased. Now that's not a bad thing it's just what they're set up to detect. It is very easy to find large planets. It's easier to find a large planet than a small planet. It's also easier to find planets orbiting close to their star. Well that makes sense because we have to see multiple orbits in order to detect and confirm a planet. So if a planet was like Jupiter say that took 12 years to orbit and we detected today then we'd want to wait 12 years and we'd see the second detection and then we might want to wait another 12 years to see to really confirm that there is a planet there that could take decades to detect a Jupiter-like planet using these methods. So with the methods we've talked about we would not have been able to detect a planet like Jupiter in using these methods. There are also smaller exoplanets that likely exist. Those are on the frontier here. Those are hard to detect. So if you notice it's kind of divided here these planets get very hard to detect when you get to the longer periods and the smaller planets. So what have we found? Well we found some things that are unusual. We have found hot Jupiters. Now these were definitely not expected by astronomers. Those are the ones up here. A hot Jupiter means a big planet that orbits close to its star. Now to get an idea of the orbital periods here this is 10 days right here and this is one day. So these are planets that are orbiting around their star in a week or even less and these are Jupiter-sized planets very close to their star. So these were a big surprise to astronomers that such planets could exist. Now to be fair these are the easiest ones to detect. They're large and they're close to the star. So we may be finding the exceptions here the unusual ones but still the fact that they even exist and it leads us to question how can such a planet form so close to a star? And that leads us to rethinking how solar systems form and we mentioned planetary migration previously but maybe these planets formed elsewhere and migrated in much closer to their stars. So something that's very hard to explain. We also see lots of planets that we do not see anything like in our solar system. Here would be Neptune here is Earth. We have lots of planets in fact the majority of the planets are between Earth and Neptune in terms of size. There are no planets like that in the solar system. The vast majority of planets are of a type that we don't see sometimes called super-Earths if they're a little bit bigger than Earth or mini-Neptunes or a little bit smaller than Neptune. But those types of planets do not exist in our solar system however they are very common in the universe. So our models of solar system formation that we talked about previously are undergoing a transformation and that is because we now have more data. When our models were made we had one solar system to consider. Now we have a thousand solar systems and we can start to get much better statistics to be able to understand these. So let's go ahead and finish up with our summary and what we've looked at here is that since the late 1980s we have detected thousands of planets outside of our solar system. Most of these have been detected by transit or radial velocity methods that we talked about here. We will discuss some other methods in a later lecture and the discovery of some things like hot Jupiters and planets with very large eccentricities are really causing the astronomers to rethink their models of planetary formation. How do planets actually form? So that concludes this 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.