 Now, we've been talking about finding transits of earth-sized planets, but we're particularly interested in finding earth-sized planets in the habitable zone of sun-like stars. Let's find out what that means. Okay, who wants to be the sun? I do. All right. Okay. I need you to hold this, I need you to stand right about there, perfect. Imagine that we're all outside, it's a cool night, and all we have is this campfire over here to keep us warm. Her whole upper body is the campfire. Okay? It's not just this, but her whole upper body is the campfire. Okay, I would like you to stand in front of that campfire where you would be comfortable on this cool night. Okay? Yeah. Good. Okay. Now, he's representing the position of earth around our sun, all right? He's at a comfortable distance. Is there liquid water on earth? Yes. Lots of it, right? Yes. Okay. Now, I would like you to step up and be right here, real close, real close. Now, you don't have a jacket on, you're standing right next to the fire. Are you going to be comfortable there? No. No, you're going to be too hot, huh? Yeah. Okay. She represents the position of mercury around the sun. Now, let's see. Let's take you and come around here, and I'm going to take your jacket off. You can just hold it. And I want you to stand right back over there, a little bit far, perfect, perfect. Right there. Now, are you going to be comfortable at this distance? No. What's happening? It's too cold. It's too far away. Yeah, too cold, too far away. So one of the things that determines habitability of a planet is how far away it is from a sun, okay, from a star. So earth is in the habitable zone now. Why do you suppose I had him take his jacket off, and I picked her because she's not wearing a jacket? We're going to use jackets to represent atmosphere, all right? So why don't I want them to wear jackets? No atmosphere, little or no atmosphere on mercury and Mars, all right? Mercury has a temperature of about 800 degrees in the daytime. Is there going to be liquid water on mercury? No. No. Too hot. Now Mars is way far away, and at night it has a temperature of what, yeah, 125 degrees below zero. Is there going to be liquid water on him? No. And then in the afternoon it warms all the way up to four degrees below zero. Any liquid water during the day on Mars? No. No, too cold. It's always going to be frozen if there's any water on Mars. So the one thing was the distance, and the other thing we're going to talk about now is atmosphere, little, almost no atmosphere, very little atmosphere, but earth has a nice comfortable atmosphere keeping it warm at the right distance from our sun. But not all stars are like our sun. When a cloud of gas and dust collapses to form stars, all those stars in there are not all going to be the same size or mass. They're going to be all different sizes, small ones, middle-sized ones, and big ones. Okay, planets, oh wait, I forgot something. We're missing a planet over here. Oh my goodness, what's missing between mercury and earth? Venus. Venus. Nice. Will you be Venus for me? Okay, you want to come over here and stand right there between mercury and earth. Excellent. Now you're wearing a nice jacket, and I'm going to give you this jacket, too, okay? And now imagine that I'm throwing a big old blanket over her, too. Is she going to be comfortable? No. No, what's happening there? Too hot. Too hot. You bet. Venus has a very dense atmosphere, and it always stays about 880 degrees Fahrenheit. Is there any liquid water on her surface? No. No way. Okay, so atmosphere makes a big difference. Now let's let all the planets go back to the other side here. You can take your atmosphere off. But like we were saying earlier, when a cloud of gas and dust collapses to form new stars, those stars come in different sizes, different masses, small stars, medium stars, large stars. So we have the sun here. Let's move the sun up just a little bit. Now who would like to be a little red star? Would you be the little red star? Thank you. Now I would like you to stand right over here, right there, a little bit farther over. Perfect, perfect. Now she represents a cool red star, little star. Here's our sun-like star, yellow-white stars. And let's see, I need two people now for this. You will? Okay. And, all right, thank you. I'm going to give you these, if you could stand over here together, real nice and close here. Now she's got a candle, she's campfire, and this is a big old bonfire. What kind of star does this represent? A really big and hot one? Really big hot star, you bet. So their whole bodies are going to be fire. Remember her upper body is fire, and all she's got is this little tiny candle. Would you, I'm going to give you a little planet, and would you stand in front of the red star and place your planet where it would be comfortable in front of that little red star? Pretty close, huh? Yeah, if you could stand to the side so everyone can see where you're standing. Perfect. All right. Now, would you be earth again and stand comfortably in front of our sun-like star? Okay. You want to stand in front of the bonfire? Okay, where would you stand to be comfortable in front of that big old hot bonfire? Way back, right? Each of you are standing in the habitable zone in front of your star. Is the habitable zone the same distance from each of these stars? No. No, it changes quite radically, doesn't it? Yes. Yes, good. And this right here is what the Kepler mission is trying to find, earth-sized planets in the habitable zone of sun-like stars. And they're also trying to characterize, the other point of the Kepler mission is to characterize all the different kinds of planetary systems there are in our neighborhood of the Milky Way galaxy. Okay?