 Greetings and welcome to the Introduction to Astronomy. In this video we are going to discuss some of the changing perspectives, how our ideas of planetary formation have changed over the last few years and decades as we've begun to discover more and more planets outside of our own solar system. And if we look at what we thought before, what we see is that our early thoughts were that planetary systems, if they existed out there, they'd be like ours. They would be much like our own planetary system. And that's because we developed our models based on the planetary system that we knew, and until just a few decades ago, the only planetary system that we knew of for sure. And in our solar system everything orbits pretty much in a circular sense, so all the planets orbit circular around the Sun. There are some other objects that have elliptical orbits, but we don't have any planets with extremely elliptical orbits. And we also have some other patterns with small terrestrial planets close to the star and massive Jovian planets far from their star. But what are we finding now as we've begun to discover thousands of planets outside our own solar system? And what we find is that there are new types of planets including many that are larger than Jupiter. There are some that are, if intermediate size, that sort of bridge the gap between Earth as the largest terrestrial planet and Uranus and Neptune as the smallest Jovian planets, but there's still a big gap in between them. So we're starting to find a lot of other types of worlds, things like super-Earths that are Earth-like planets, but two or so times the size of the Earth. We have also found a very puzzling thing we call hot Jupiters. These are very massive planets that orbit close to their stars. So we don't see that in our solar system. All of the massive planets are far away from the Sun. In other systems we find Jupiter mass planets that orbit closer to their stars than Mercury does to our own Sun. So let's take a look at some of the types of exoplanets that we see here. And what we see are we do get rocky planets which are very common and seem to be more common than the gaseous planets. Now that may be an issue of the way we are detecting these as to which ones are easiest and hardest to detect right now. But we do find that rocky planets are extremely common as you can see in the little plot here showing all of the a bunch of different planets that have been discovered. There's a whole bunch of them down here in the rocky planet section. And while we discover many planets, Jupiter size or even larger, the density of the dots is much much lower. So we do not see as many if just counting what we've been able to discover so far we don't see near as many. Now we also find a lot of these hot Jupiters noted here meaning that they have orbital periods. If we look right here anything to the left of this line has an orbital period of 10 days or less. That means these things are whipping around their stars and are very close to them. In our solar system Mercury is the closest planet to the Sun and it has an orbital period of about 88 days. And everything else is a lot further away than that. So everything we see to the left of this line orbits in just a little over a week so their years would be less than a week in some cases even less than one day. So we do see those hot Jupiters which are something to study. We also see super earths, rocky planets that are much larger and many Neptune. So planets like Uranus and Neptune in the ice giants range but a little bit smaller than them. So we're seeing a more of a continuous spectrum of planets in terms of sizes from very tiny planets to very large ones things much smaller than the earth to things much larger than Jupiter. So what are these intermediate size planets that's something that we don't see in our solar system. So we haven't seen those before. So our models are not going to be able to discuss those, discuss those as well. Now if we look at this in another way we can see again the planets here where are the planets found. Well things that are earth sized or less which are right about in here or lower are relatively rare but we do get a lot of rocky planets that fall in this super earth to sub Neptune range. So all of these are relatively small planets compared to what we see in our solar system. In our solar system the planets are about equally divided. We get about half of them that are small and rocky and in fact those types are all here or to the left and we have several that are out here. And if we look at that we're missing this entire range of planets where most of the planets that we're finding seem to exist. So the planets that seem to be the most common in the universe at least based on our measurements to date do not exist within our solar system. We have the very large planets which are rare and the very small planets that we have not detected a lot of yet. Now there may be some problems with this in terms of what we call a selection effect. So let's look at that here for a minute. First of all what types of planets can be detected? Primarily we have detected planets through two methods and that was radial velocity and the transit methods. So these are the two methods that have been used the most in order to detect planets. They are very good at detecting large planets and planets that are close to their stars. So there is going to be a bias towards detecting these types because they are easier to detect. Why are they easier? Because the larger planet is going to make a larger variation in radial velocity. It is going to cause the velocities to be larger and therefore easier to detect. A small planet does not tug as much on its star so its radial velocity shifts will then be less for a small planet. So that is harder to detect. The transit method that we look at tells us that the planet is blocking out part of the star's light so it is going to detect larger planets as well. They are going to cause the star to dim a little more and again make the detection easier. So we are very biased towards detecting large planets and we also have to look at the orbital period. We look at planets that are close to their stars because the orbital period is small. So if the orbital period is one year and we want to observe this four or five times to see a good radial velocity curve and get a good transit, it could then take five years to get this measurement. That is not unreasonable but when we start to get to things like Jupiter which orbit every 12 years and if we want to wait five Jupiter orbits to really confirm that it exists, we are waiting decades in order to be able to do that. So there is a bias that very large, certain planets, by our primary method, certain types of planets are not going to be easily detected. I also mention here smaller planets probably do exist but just are not yet detectable. We are not able to pull them out of the noise in the data to be able to find out that they actually exist yet. We also believe that probably Jupiter like planets at great distances probably do exist. We just need more time to detect these and this is the number I gave here for 12 year orbit of Jupiter. If we want to see it, if we want to detect that first signal and we get that today then we have to wait 12 years to get a complete cycle. Whether we are looking at radial velocity or transits and then we want to wait another 12 years to confirm it and maybe another 12 years to give us some good confidence on it so we can be waiting many decades to confirm a planet like Jupiter in our solar system. A planet like Jupiter that orbits within two weeks around its star, we can get a confirmation of that very quickly. So let's look at some of the systems of exoplanets that we have been able to see. And here we are going to look at one of these. This is the Kepler 90 system. These are all very close to the star. In fact, the Kepler 90 system has eight known planets just like our own solar system. There's also the Trappist 1 system that has seven planets. These may be ones that you've heard about. Several of these are within the habitable zone of their star. Now what we do note is that we have found a lot of systems with multiple planets. Look how many planets we have here. We've got six planets here all within the orbit of the Earth. So we can see all of those very concentrated together including very, very small. And I'm sorry that's six, but there's actually eight of them because two more you can't even see to this scale. And here's those other two very, very close to their star. So we know of eight planets in this system within the orbit of the Earth. So in the solar system we have Mercury, Venus, and Earth. Here we'd have eight planets within that distance. The Trappist system has seven planets, several of which are in what we call the habitable zone of the star. Habitable zone is just the area where liquid water could be present. Does not mean that liquid water necessarily is present, but only that the temperatures are correct so that liquid water may be present on those planets. Now, other one we can look at here. Let's look at these in terms of sizes. And this again is the Kepler-90 system compared to our solar system. And if we look at these, remember that these are all closer to their star than the Earth. So we have planets that are Jupiter, larger than Jupiter, that exist in this system. So while it looks like here we're comparing only sizes in this case, not distances. We are looking specifically at their relative sizes of the planet. So some of these are comparable to Jupiter and Saturn. Some are comparable to Uranus and Neptune. And the other ones are actually larger than our terrestrial planets. In terms of distances, we've got to put this one in here in a little bit closer than the Earth. So this entire system of eight planets is much closer to its star than the Earth. Now, the trappist one system that I mentioned, we can look at that as well. And again, I should have mentioned on the other sheet, these are all artist's conception. So we do not know what these planets actually look like. We are basing it on the sizes that we know, their temperatures, how far they are away from their stars. And that gives us some kind of idea, perhaps as to what they might be like. But we're using just an educated guess to really be able to determine what these look like. We do not know for sure that these are look exactly like this. Now when we look at this, several of these are within the habitable zone of their star. If you note their orbital periods range from about a day and a half to about 19 days. This is a much smaller star. So they're much closer to it. But even though they are that close, they will then be at the right temperature for liquid water to possibly exist. And we can look at the comparison. There are some of these planets that are actually relatively comparable to the Earth. In fact, E here is a little bit smaller than the Earth, a little bit less massive. But about the density of the Earth, F, a little bit larger planet, actually a little bigger than the Earth, although its mass is slightly smaller, is dense, but it's dense and a little bit less dense, so a little more concentration of rocky material and less concentrations of metallic materials. So we can get some ideas of how these compare to our own solar system. But remember, again, these are all very close to their stars. So all of these orbit within 0.06 astronomical units. Mercury is more than a third of an astronomical unit away from our sun. So these would all be much, much closer to their star than Mercury is to ours. And you can get that if you look at some of the statistics here, how much the orbital period is here for Mercury and its distance as compared to the others. But we can get an idea that there are these planets out there that would have surface gravities and masses that are very similar to that of the Earth. So how does this change what we know about planetary formation? And what we see is that, first of all, our solar nebula theory that we used could not explain hot Jupiters. We discussed how planets would form relative to their temperature within the solar system. So the closer to the sun you had more metallic material, you couldn't get ices forming that close. So we know that many exist. They may not be the most common type of planet, but we haven't just detected one oddball. We've detected dozens of them that do exist. So we talk about this maybe in terms of planetary migrations that planets do not end up where they are today. Now we say that for our solar system as well. There were some thoughts that maybe the giant planets actually migrated from a different part of the solar system and moved to their current locations early on. Maybe this happens in other solar systems to a more extreme amount, giving us that some of the things like the hot Jupiters that we see. We also detect planets orbiting at high eccentricities and at large angles. So what we don't know is, is our system unusual? Are having circular orbits and everything in a flat plane unusual? Or are we biased towards detecting certain types of planets? Are we biased towards detecting these unusual planets, things like hot Jupiters, as I've already said. We are biased towards detecting. So are we biased towards detecting things with high eccentricities? Does that make them easier to detect? Things orbiting at large angles, does that make them easier to detect? And as we go from only knowing thousands of planets to knowing tens of thousands and hundreds of thousands, will we get a much better statistical basis for planetary formation? So a little bit more on this here. What our models show right now is that Uranus and Neptune probably formed closer to Jupiter and Saturn. So migration occurred in our solar system and may actually be quite common. We do see that planets are common close to stars. We see a lot of this, not just hot Jupiters, but hot Earths in the lava worlds that form very close to that. Why is our solar system different? Or are we only detecting the unusual solar systems first? And then will we find that there are numbers do not increase as much and we start to find more solar systems like ours. We have to remember that the solar nebula model was based on one system, ours. So we based everything on what we knew about planetary formation. Now we know of thousands of systems and we're getting a better idea. We're able to refine our models using the scientific method to be able to get a better picture, although it is still not clear at this point. Now the one thing, one last thing to look at is what we mean by a habitable planet and you'll hear this often in the news that a planet was discovered in the habitable zone and when we talk about a habitable planet there are a couple conditions that have to exist. It has to have the potential for liquid water on its surface. Why? Because we see liquid water as being a necessity for life. So if it's not at the right temperature for liquid water where water would be vaporized because it's too hot or water would be all solidly frozen because it's too cold we don't count that as a habitable planet. It has to be similar in size to the Earth and it has to be of these we've detected more than a dozen of these and more likely as our technology improves. Even including one of these that is only a few light years away and that is Proxima Centauri B. Proxima Centauri is a small red star that actually orbits the Alpha Centauri system but it does have a planet as well which is actually very close to us only a few light years away and the closest habitable planet yet to be detected. So let's finish up here as we do with our summary and what we found is that many of the exoplanets that we have found so far are very different than the planets we know in our solar system. We find some that are similar but we find lots of different types. Things like hot Jupiters are causing astronomers to rethink their models of planetary formation and we're also getting to the point now where habitable planets are beginning to be detected. So we're able to detect smaller planets that are going to be at the right distances from their star for liquid water to exist. So that concludes our lecture on changing perspectives in 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.