Welcome back everyone. So now that we've learned about how we find planets and remember that it is just a remarkable achievement a of our science that we actually have the capac the capacity to have proof of the existence of planets orbiting other stars. That question was around for two thousand years. No one knew the answer. And its only been since 1996 that now we can say definitively that there are other planets orbiting other stars. But of course what we are really interested in are stars are planets that can support life. So, we're interested in, is the question of which parts of a solar system, of any solar system, are conducive for life as we know it. Of course, there could be crazy kinds of life that we have no idea about, but the question we want to ask is, are understanding, given our understanding of life, which parts of a solar system can harbor life and that's, the topic of the habitable zone. And this goes to the, Drake equation, the question of how many plan, the fraction of stars that have planets in their habitable zone. Okay, so what is the habitable zone? So, based on our current understanding of life, we believe that the only way to allow life to form is to have liquid water. You need to have liquid water on the surface of your planet in order to allow all the chemistry that that goes along with life to occur. Now again this, there could be things that we don't know about. What could we say, right? So, so our best understanding of the life on, of our, of everything we understand about how life formed on Earth is that water is really necessary. Alright, so we start with the need for liquid water on the surface of the planet. And then it's very easy to calculate how far away a, you know, here's your star and here's your planet, how far away from the star does the planet have to be in order to, for liquid water to exist on the surface. Because if you're very close up against the star in your orbit, the, the, the star light will be so strong the the energy that will be absorbed. Will be strong enough that the temperature of the planet will be so high that if you dropped some water there it would just turn into water vapor, would boil away. And of course if your planet was very far away. If you put some liquid water it would very quickly freeze. So there's gotta be a band of orbits that where things are just right, for in the, in such that the temperature on the surface of the planet would allow water to exist between the being frozen or being boiled away. So that's what gives us, that, that band of orbits is what we call the Habitable Zone. So this is going to depend upon the start that you have. Because as, as we saw, as we will see next in our next set of lectures. That some stars have very high temperatures. Massive stars in particular, stars that are ten times the mass of the sun or more tend to have very high temperatures and so that means you're going to have to move your your habitable zone further outward, ok? So either you couldn't, placing the earth at the one astronomical unit that at the same distance from the star that it is as it orbits the sun around a very massive star would not work because the, the water would all boil away. But there's other problems with massive stars as well. Massive stars have they have very short lifetimes. They will go through their lifetimes in less than, in around, around a million or ten million years. And that seems to be a little bit too short to allow life to evolve all the way up to what we see today. Perhaps you could get life started on a planet like that, but once the star reached the end of its life and blew up, as a supernova, as we'll see, it may be difficult to imagine how life could be, could form there. So, we have to keep in mind the fact that different kinds of stars have different lifetimes have, different conditions on them. Some stars have a lot of magnetic activity going on and that leads to explosions off the surface of this, off the star continuously. And that may make it difficult, for planets to support life. So all of these things have to be factored in when we talk about the habitable zone. We start with a definition that's based on liquid water on the surface. But then people have begun to become more, sophisticated in their understanding of that question. But to first order, we think that the, number of planets in the habitable zone is of order one. At which is pretty remarkable. So, let's just talk about this. How common are planets in the habitable zone based on the fact that we can go out and measure now. You know, we can measure exoplanets. We can go out and find where the planets are and we seem to be able to tell things about the kind of planet that, that's there. So as of June of 20 13, 64 percent, it was estimated that 64 percent of stars have planets in the habitable zone. Which is a remarkable, remarkable conclusion. So that means you can pick any star in the sky and the odds are about 50/50 that you're going to, you're going to have a planet around that star in the right place for, too have life forms. So there's a 50 50 chance essentially that any star in the sky has a planet just in the right place. So for example the Kepler 62 system we call it Kepler 62 because the Kepler Satellite, which was a transiting a satellite designed to find planets via the transiting method. This was one of at least number 62 in its catalog of potential exoplanets. And when they went back and looked in more detail, yes indeed it not only had had one exoplanet, it had two exoplanets in the habitable zone. So there we are, we see a system that has potentially you know, a multiple planets in its system that might have life. Now of course its not just important to have planets in the habitable zone if possible you'd like to have earth like planets. Now of course a, a gaseous planet in the habitable zone its moons might be a good place to think about life and anybody who's watched enough star wars movies knows that, you know, moons are great places to put, rebel bases. but, the question really is, we know, we'd like to know about Earth-like planets. So the question is how common are Earth-like planets? Well, as of, January 2014, scientific report, stated that there about 22%. The estimate is about 22% of sun like stars have an Earth sized planet. An Earth like planet. You know, around, the right sort of size in it's habitable zone. So that means that the nearest Earth sized planet to us, orbiting a star like the sun, is about should be, you know, on, you know, average based proba, you're using probability arguments about 12 light years away. That's not very far. I mean it would take a long time to get there using our current technology but 12 light years is relatively, it's our neighborhood in some sense, speaking in a galactic sense. So it's certainly a star that can be seen with the naked eye. So you know, in our neighborhood, there should be a earth like planet, orbiting a sun like star in the habitable zone. And perhaps right now there are slimy creatures you know running around on that planet so its really its a remarkable thing. So well with the end what you end up with are potentially billions, billions of worlds orbiting other stars that may have the conditions appropriate to life. And when I say that a many of you may have seen the Carl Sagan program Contact where he would talk about billions of stars and planets. When Carl Sagan did that program back in the 70's he had no idea how many planets there were, he was just guessing he was hoping. The difference between now and then that was in the 1970's and this is now you know the second decade of the, the, a the 2000's is that we know, we absolutely know the answer to this question. So with that let's go on.
