 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about planetary formation, and we looked at this previously, but we are going to look at what the discovery of all these exoplanets has done to change our perspectives on how planetary systems form. So what are other planetary systems like? Well, our early thoughts was that the planetary systems would be like ours. Why? Because that's the only planetary system we had to go by, and we would expect things like circular orbits, and we would expect the small terrestrial planets close to the star, and the massive planets far away from the star. What are we actually finding? New types of planets, planets that are larger than Jupiter. Many planets that are of intermediate size between that of Earth and Neptune, planets that do not exist in our solar system, and hot Jupiters, massive planets that exist very close to their stars. So let's look at these in a chart that we can show here, and we see the exoplanets shown here show size on the y-axis and their orbital period on the x-axis. So we see that rocky planets are very common. In fact, there are a lot more rocky planets than there are other types. Now we can see the rocky planets in yellow here. Lava worlds would be similar to rocky planets except very molten. But when you get up to the giant planets, there are far fewer dots up there, each dot representing a planet. We also see these unusual hot Jupiters. These are up in the corner here, and these are planets that are Jupiter-sized. So Jupiter is this line right here. These are as big as Jupiter, if not bigger. And look at their orbital periods. Here would be an orbital period of about 10 days. Here would be an orbital period of one day. So these things orbit around much faster than things like Mercury, which would be closer to 88 days over here. So they're really massive planets, really close to their stars, and that gives us a good thing to change, something to think about when we're looking at how planetary systems form. We also look at the super-Earths and the mini-Neptunes. These are intermediate-sized planets that do not exist in our solar system. So in our solar system, this is a gap. There are no planets larger than Earth and no planets smaller than Neptune. But look at all the planets that exist there. So why do we not happen to have any of those types of planets in our solar system? Now we can also look at them here by size in a bar graph. Here are the super-Earths and the mini-Neptunes, and that is the vast majority of the planets that we have found so far. In our solar system, we see the range over here and the range over here, and that is it. So it's just very interesting to see that the vast majority of planets that have been discovered so far are not of types that we have in our own solar system. Now part of what we see is a selection effect. So what types of planets can we detect? Radiovelocity and transit methods. These have given us the vast majority of the planets that have been detected. They are very good at detecting large planets that are close to their stars. Large planets will give a stronger radiovelocity shift and will dim more of a planet of a star's surface. So they're easier to detect. They'll also occur more often. We will get the radiovelocity shifts faster and the transit's faster. This introduces a bias toward detecting these types. So maybe we've detected a lot of the hot Jupiters that were in our selection area. And maybe they're not as common as we think because they're just easier to detect. Smaller planets probably exist but really are not yet detectable. Although technist technology increases, we are finding more and more Earth-like planets, but it's still hard to find the smaller ones. Jupiter-like planets at great distances. There aren't a lot of those that we found so far. This problem, we need time. Jupiter around our sun would not be detectable for decades. It would take a long time. It has a 12-year orbit. So if we saw an eclipse today, then we'd have to wait 12 years to see the next one. And then we'd probably want to wait another 12 years to confirm that at least. So we're talking at least several decades, and that's for Jupiter. If things are orbiting even further out, they're going to be really hard to detect by either radiovelocity or transit methods to be able to confirm. So let's look at some of the systems of exoplanets that have been detected. We're going to look at, again, a number of systems with multiple planets. We have the Kepler-90 system that we look at here. The Kepler-90 system does have eight known planets just like our own. However, they all orbit very close to the star. If you'll note here, there are one, two, three, four, five planets here, and then this red circle is number six, and there's two in even closer that are not visible here. So all eight of these planets orbit closer to their system than Earth does in our solar system. So it's interesting to see, again, how close can planets be spaced and the system remain stable? Ours are pretty well spread out. Here we have a lot of planets very close together. We can also look at these a little bit more, and let's take another look at Kepler-90 system. Now, here we're looking at them not to orbital scale but to size scale. These are all big planets. So the largest here is larger than Jupiter, and there's two very large planets. The others here are more of the Uranus or Neptune size, and even the smaller ones are larger than Earth. So it's a very interesting system. Remember that all of these orbit closer to their star than Earth does to our Sun. So we have to start thinking about how that can affect our models of how planets form. We also have the Trappist-1 system, another common one that is talked about, and we have all of those orbits, and if we look at their orbital periods running from one and a half days up to about 19 days. Again, all of those are very close to their stars. Mercury in our solar system is 0.387 AUs. The closest one here, the furthest one is 0.06 AUs. All of these planets are much closer to their star than Mercury. So it's interesting to see how you can form a stable planetary system very close to a star. And several of these are in the habitable zone of that star. Now it's a different type of star than our Sun, but several of these are within the habitable zone of this star, which we can see here, is the green region around it. In our solar system Venus is just off the inner edge. Mars is in the outer region. Earth is right in the habitable zone. Here we have several of these planets that are within the habitable zone of their planet. And that does not mean they have life, but there is the potential for liquid water to be present here. So how does this affect our ideas of how planets form? So how do these change? Well, our solar nebula model does not explain a lot of these things. How does it explain a Jupiter-sized planet forming close to the star? We can't explain that under the condensation sequences. Jupiter-type planets should not form close to the star. So we know that many of them exist, since they do exist they can form, and one thought is perhaps planetary migrations. That planets actually do not form, or have not formed, or are not currently present where they formed. We also detect planets with high eccentricities, not very circular orbits, those orbiting at large angles. So what we don't know yet is our system the unusual one? Are we the odd system? Or are we simply biased towards detecting these other types? Or are these other types really far more common? So here we see several artist's conceptions of a number of these exoplanets that have been discovered. So, what our models show now is that migration may be common, and that even in our system, Uranus and Neptune probably formed closer to Jupiter and Saturn, and then were migrated away. We know that planetary systems are common, very close to stars. Not just the Jupiters, but lots of planets are close to the stars. Why not? What happened to this in our system? One of the problems with our model is that it was based on one system. Ours. And that's not bad. It's simply the only system we had for a long time. Now we know of thousands of systems, and we're starting to get a better picture. It's not clear yet, and it's going to take a lot of studies of these planets to really be able to better understand them. So, we want to talk about one more thing before we finish here, and that is what we mean by habitable planet. A habitable planet means simply that the potential for liquid water to exist on the surface. So, they've got to be similar in size to Earth. If they're a gas giant planet, they're probably not going to have a surface with liquid water. If they're too small, then they're not going to be able to hold an atmosphere and have water. As of this point, we have a dozen or so that have been detected, and more of these are likely. Now, again, this does not mean that they have life. It simply means that they're in the right range from their star to have liquid water on their surface. And of course, we consider liquid water as important for life. And here we see the artist's conception of a planet around Proxima Centauri B, the closest exoplanet to Earth, only four light years away, which is in the habitable zone around what we call Proxima Centauri, which is a part of the Alpha Centauri system and is actually even a little bit closer right now than Alpha Centauri itself. Now, again, that doesn't mean that there's life there. Again, only that there is the potential for life and potential for water. So let's go ahead and finish up with our summary. And what we looked at this time was that exoplanets are found that are very different from the planets we know in our solar system. We talked about hot Jupiters and other distinct types of orbits that are causing astronomers to rethink the models of planetary formation. And we're also beginning to detect habitable planets, so potential for finding something with life on it. So that concludes this lecture on changing perspectives of planetary formation. 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.