Hi, I'm David Spergel, and I'd like to welcome you to Princeton University's Coursera course, Imagining Other Earths. I'm a professor here at Princeton University where I'm chair of the astronomy department. By training, I'm a theoretical cosmologist, and a lot of my early work is involved in microwave background, the leftover heat from the Big Bang. I was one of the scientists deeply involved with The Wilkinson Microwave Anisotropy Probe or WMAP. We made a detailed map of the microwave background and use that to determine a lot about the history of the universe. We measured its age, 13.7 billion years, and determine its composition, atoms, dark matter, dark energy. In the last decade or so, I've grown increasingly interested in extra solar planets. About a decade ago, I was involved in designing chronograph. That's something that let us see dim things around something bright, and now deeply involved in a NASA project called WFIRST. This is a telescope that will do a number of things, including study dark energy, dark matter. And with the chronograph, provide the first detailed images and characterization of planets, like those in our solar system, around the nearby stars. It will hopefully be an important step towards imaging and detecting Earth-like planets around other stars. That's where I'm spending a lot of my research time right now. The course that I'll be giving, and I hope you'll take, is based on a freshman seminar called Imagining Other Earths, that I offered to Princeton for three years. When I taught that class, I typically had 12 students in the class. I think I have about 12,000 of you signed up to take this Coursera course. So this will be a new experience for me. It's one I'm really excited about and looking forward to. I've enjoyed putting together these lectures. We've had the opportunity in doing this to talk to some of the leaders in the field, and I hope that you'll enjoy this adventure. Let me tell you a little more detail about how the course is going to work. The goal of the course is multi-fold. I want to introduce you to a lot of physics, astronomy, chemistry, geology, biology. And let you see how we can apply ideas from all these different areas of science to thinking about what life could be like on other planets, how it might evolve, whether there are other planets. Talk about the properties of stars and planets, atmospheres. I hope I had fun introducing you to all these different concepts. The structure of the course is spaced around the series of 24 lectures. Each lecture will take about 45 minutes and be divided into three parts. The first part will be a roughly 15 minutes. After that, you'll answer a short question interactively. If you can't answer the question, just guess something, and we'll help you with it. Then another 15 minutes segment, another question, and then a final 15 minutes segment. A number of the lectures will be supplemented by interviews with leading scientist working on relevant topics. And my colleagues have been very generous with their time, and we'll be able to talk with people like Sean Solomon, when we talk about Mercury. He's leading NASA's exploration of Mercury. Scientists like Debrah Fischer and John Johnson who are at the forefront of the search for extra-solar planets. Scientists like Lisa Kaltenegger, theorists thinking about the possibilities of habitable planets and what their properties might be. And we'll conclude with a conversation with Freeman Dyson, one of the great thinkers of our time. A leading theoretical physicist who at 90 has remained incredibly active and has many insightful ideas on what are the other earths and other places that might host life might be like. A very important part of this course will be the forums. This course should be much more than just listening to these lectures. I strongly encourage you to become part of a community, engage in discussions, meet other students in the course through the forums. Together with Simone, one of the graduate student working with me here at Princeton, we're going to be monitoring the forums. We're going to try to help answer some of the questions that come up in the forums and provide additional comments. I'm also going to be setting up a Twitter account and hope to communicate with you that way and use that to make the course more interactive. Say, a little bit about the assignments in the class, in addition to answering these short questions during the lectures, there'll be three major assignments. The first two assignments will involve you are looking into in reading about a proposed mission to explore either our solar system or to learn more about planets around stars beyond our solar system. In each of these cases, what you'll do is you read about the mission. I will guide you to materials, and you'll write a roughly 750 to 1,000 word paper. So roughly four to five pages about one of these topics. And we'll have more details about the assignments as we go into the course, but we'll have those first two assignments. And then, what will be the capstone of the course? And what I hope this will all lead up to is you're writing a paper or recording a YouTube video where you talk about a solar system that you invent. What I hope you'll do in that paper is go into some depth in one aspect of your solar system and its planets. If you're interested in the astronomy, talk about how your solar system has been detected. How astronomers learn more about it. If you're interested in the geology, talk about the properties of your planet, its atmosphere, its composition. If you're interested in the biology, perhaps you'll talk more deeply about and just speculate about how life might evolve. Our function on your planet. Finally, if you're interested in just thinking about what other civilizations might be like, speculate about how that civilization might evolve, and what things would be like on the planet that you've invented. I've enjoyed putting together these lectures. I hope you'll enjoy listening to them and interacting through them with fellow students engaged in the course. I want to thank the university for supporting this, the broadcast center and the people here for helping me with this and helping to make this, I hope, a success. And finally, I want to thank the class of 1972, here at Princeton, who were generous in providing a support to make this possible and enable this Coursera course. Which will hopefully help communicate some of the exciting things going on in this area of science to the broader world. So I'm looking forward to talking to you some more, and we'll see you soon.