Welcome back everyone. So now we've reviewed quite a bit about exoplanets and th question we really want to ask now is what have they taught us? And taken as a whole, what have we learned from actually going out and seeing exoplanets. This is a really interesting point about science in general. You know, you can conjecture all of, all you want right, you can play arm chair scientist as much as you want. And think about well where life forms and how it forms and what kind of different civilizations are they and you know do they all have. Prosthetic foreheads and you know but until you actually go out and measure things you really you know you, just don't know and then when you do measure things and we do open up a new window on the universe usually what we find is that we're completely wrong you know that in general nature is more clever. Nature is more fecund nature is more creative then we could possible imagine. As someone once said anything that the laws of physics and chemistry allow is going to happen so let's see what sort of surprises were in store for us a when we actually went out and measure exo planets. A so okay so we're just going to get some statistics right now but its important to understand that a these statistics are changing there's new results coming a coming out all the time the statistics maybe not hopefully you know a won't change that much we'll a but the actually numbers would have changed. So as of January 7th, 2014 there have been 1,015 confirmed planets. Found around 758 stars okay. A there's been 174 systems with multiple planets which is great so we're actually seeing solar systems and there have been 3,603 planetary candidates And confirmed planets altogether so that's this larger number is or perhaps particularly from the Keppler a satellite mission where we have lots of candidates and we've gotta go back and and a look at them again to confirm that they are in fact candidates and more being found everyday now you may wonder wait a minute I just said at most I have. 3,000 candidates you know add candidates plus confirmed how beforehand was I talking about billions well its what the great thing about the a the science that is involved in the exoplanet studies is by going out and making these measurements we can infer from them what we think is the. The general rule. So by looking at 3,000, we now have 3,000 planets. And 758 or at least yeah, 758 stars with planet, confirmed stars with confirmed planets. We can generalize from that. We can actually use that statistics to argue up to what is the conditions over the entire sol Solars or the entire galaxy so that's how we end up with those numbers we talked about in the last last lecture of billions of stars. Okay so that's the just general numbers what about the sizes so what we found is that planets so far come in a range of sizes from very big Jupiter its from several times the mass of Jupiter all the way down to half the size of the Earth that's what we've. Found so far. There are lots of super Earths. And by super Earths we mean an Earth a planet that has you know maybe five, six, seven times the mass of the Earth. And it's not really clear yet what a planet like that would look like. Would it be a rocky planet? Or would the pressure on the body from all from all of that mass actually force it you know force a different kinds of internal structure some people have talked about some of these as being sort of like water worlds even you know mostly liquid a but its not clear its really unclear right now. So a most of the planets that have been found are sort of in the range of masses of Neptune but that's also partly because their easy to find so there's going to be lots more surprises as we get better and better looks a at these systems. Now let's talk about the surprises let's talk about the things that we didn't understand if you grew up say in the 1970's like I did. What you were taught was there was a solar system, which was ours, and it had the terrestrial planets on the inside, and the gas giant on, on the outside. It was all very tidy and orderly. Is that what other planetary systems look like? The answer is no. In fact, what we have found in an abundance of what are called hot planets, and what that means is that their orbital radius is less than, a tenth of a, an astrological unit. Or about a third of Mercury's orbit. We found lots of planets [UNKNOWN]. A lot of different sizes that are orbiting, you know, almost like the outside parts of the atmosphere of their star. They're very, very, going to have to be very, very hot. For example, 51 peg, a sunlight star with a planet that has a period is a of just four days. It takes just four days to go. That's its year, is it takes only about four point two three days to complete. So, [UNKNOWN] And the reason why we found so many of these, is they were the easiest to find. With the radial velocity method, the closer you are, the closer the planet is to the star, the more it's going to make the star wobble, in its orbit. In their mutual, dance together. So, the first ones we actually discovered were Jupiter's. We, we'd, the, the, the first [UNKNOWN] we discovered were Jupiter-like planets in very close orbits, or so called hot Jupiter's. Now, the crazy thing about this is there's no way that a Jupiter could form, you know, where Mercury is. Is just from our understanding of the discs out of which planets form and we talked about this the Jupiter's have to form past the snow line so the Jupiter's must form fairly far out in the a a disc that forms planets around the star and then migrate inward something has to happen to allow. The a Jupiter to spiral inward so that it ends up parked on this very hot orbit and we think we understand some of the processes that go into the migration its probably the interaction between the planet and the disc the gaseous disc could also be interactions with other planets gravitational scattering as we call it a planets that almost crash into each other but one way or the other Jupiter's migrate inward. The other surprise that we've seen is that if you look at the solar system, almost all the planets are going around in fairly circular orbits. Most of them have orbits that don't, the inner part, the closest approach and the farest apporach from the sun are not that different. We know that for example, comets have very elliptical orbits, where they'll come in in very cigar shaped orbits, they'll come very close to the star, and then zoom back out and spend a long time out in the depths To space so a we thought we sort of thought that oh all planets will have these very circular orbits but what we found is that there are many planets on very a very eccentric orbits very cigar shaped orbits and this must happen because of interactions between planets when when a solar system forms and there is lots of planets in there. It may be that there are periods where the planets almost collide. And every time a planet, two planets come close to each other, they scatter each other. And if they were on circular orbits they now are, are left on very highly elliptical orbits. So that tells us that solar systems especially early on must be pretty violent places. There's a lot of interactions. So we have for example Mars has an eccentricity of 0.06 which is very close to a circle. And that's one of it's a, for the solar system is some, somewhat of a high eccentricity. But what we've seen in the exosolar planets is that there are 80 cents, 80% of planets that have eccentricities Greater than .1 greater than .1 remember Mars is .06 so its clear that there is a lot of activity going on out in the solar in these exo solar planets so the main result from this is that we are the weirdos right the solar system is not like all these other systems that we're finding. The other systems have very much different a have a very. Different architecture then what we see in our own solar system so we're we're weird we're strange and what that means for life and for civilizations we'll have to see as we go on and as we learn more so a we could expect however that there's going to be a lot more. Variety as we get better and better views of these solar systems we're still at very much at the beginning but but there's been some real real surprises from what we might have though 30, 40 years ago. Okay. [BLANK_AUDIO]
