Ever since Carl Sagan first predicted that extraterrestrial civilizations must number in the millions, the search for life on other planets has gripped our imagination. Is Earth so rare that advanced life forms like us--or even the simplest biological organisms--are unique to the universe? How to Find a Habitable Planet describes how scientists are testing Sagan's prediction, and demonstrates why Earth may not be so rare after all. James Kasting has worked closely with NASA in its mission to detect habitable worlds outside our solar system, and in this book he introduces readers to the advanced methodologies being used in this extraordinary quest. He addresses the compelling questions that planetary scientists grapple with today: What exactly makes a planet habitable? What are the signatures of life astronomers should look for when they scan the heavens for habitable worlds? In providing answers, Kasting explains why Earth has remained habitable despite a substantial rise in solar luminosity over time, and why our neighbors, Venus and Mars, haven't. If other Earth-sized planets endowed with enough water and carbon are out there, he argues, chances are good that some of those planets sustain life. Kasting describes the efforts under way to find them, and predicts that future discoveries will profoundly alter our view of the universe and our place in it. This book is a must-read for anyone who has ever dreamed of finding other planets like ours--and perhaps even life like ours--in the cosmos. In a new afterword, Kasting presents some recent breakthroughs in the search for exoplanets and discusses the challenges facing space programs in the near future.

Table of Contents:
Preface xi Part I: Introduction 1 Chapter 1: Past Thinking about Earth-Like Planets and Life 3 The Habitable Zone and the Importance of Liquid Water 5 Carl Sagan and the Drake Equation 9 Other Perspectives on Planetary Habitability: Rare Earth and Gaia 11 Part II: Our Habitable Planet Earth 15 Chapter 2: Critical Updates on How Planets Are Built 17 The Conventional Wisdom regarding Planet Formation 18 Where Did Earth's Water Come From? 21 New Models for Planetary Accretion and Delivery of Water 23 Could Earth's Water Have Come from Comets? 25 An Up-to-Date Simulation of Planetary Accretion 28 Chapter 3: Long-Term Climate Stability 32 Solar Evolution Theory 32 Solar Mass Loss? 36 Electromagnetic Radiation and the Greenhouse Effect 37 Planetary Energy Balance 41 The Faint Young Sun Problem 42 Possible Solutions to the Problem 45 The Carbonate- Silicate Cycle and Controls on Atmospheric CO2 49 The CO2-Climate Feedback Loop 53 Chapter 4: More Wrinkles in Earth's Climate History 57 The Phanerozoic Climate Record 58 Precambrian Climate 63 Geologic Evidence for the Rise of Atmospheric O2 65 Cause of the O2 Rise: Cyanobacteria 68 Methane, Methanogens, and the Universal Tree of Life 71 The Archean Methane Greenhouse 75 The Paleoproterozoic Glaciation 77 Chapter 5: Runaway Glaciation and "Snowball Earth" 80 Milankovitch Cycles and the Recent Ice Ages 81 Ice Albedo Feedback and Climatic Instability 86 Evidence for Low- Latitude Glaciation 88 Mechanisms for Explaining Low- Latitude Glaciation 90 Snowball Earth 92 Part III: Limits to Planetary Habitability 97 Chapter 6: Runaway Greenhouses and the Evolution of Venus' Atmosphere 99 The History of Water on Venus 100 The Classical Runaway Greenhouse Effect 103 An Alternative Runaway Greenhouse Model 106 Evolution of Venus'Atmosphere 111 Chapter 7: The Future Evolution of Earth 116 High- CO2 Atmospheres and Temperature Limits for Life 116 Future Solar Evolution and Lifetime of the Biosphere 118 A Geoengineering Solution to Solar Luminosity Increases 121 Chapter 8: The Martian Climate Puzzle 125 Evidence for Liquid Water near Mars'Surface 126 CH4 in Mars'Atmosphere? 130 Evidence That Water Flowed in Mars'Distant Past 131 When Did the Martian Valleys Form? 135 How Warm Was Early Mars? 136 Mechanisms for Warming Early Mars 138 Where Are the Carbonates? 144 Chapter 9: Is the Earth Rare? 147 Planetary Size / Magnetic Fields 147 Ozone and Ultraviolet Radiation 152 Availability of Nitrogen and the Importance of N2 155 Is Plate Tectonics Common? 157 A Planet's Impact Environment 161 Stabilization of Earth's Obliquity by the Moon 164 Chapter 10: Habitable Zones around Stars 171 Historical Attempts to Defi ne the Habitable Zone 171 A More Modern Model for the Habitable Zone around the Sun 176 Hertzsprung- Russell Diagrams and Main Sequence Stars 179 Habitable Zones around Other Stars 181 Problems for Planets Orbiting Early- Type Stars 185 Problems for Planets Orbiting Late- Type Stars 188 Further Extensions of the Habitable Zone Concept 191 The Galactic Habitable Zone 192 Part IV: How to Find Another Earth 195 Chapter 11: Indirect Detection of Planets around Other Stars 197 Barnard's Star 198 The Astrometric Method 199 Pulsar Planets 205 The Doppler Effect 207 The Radial Velocity Method 210 Gravitational Microlensing 216 Chapter 12: Finding and Characterizing Planets by Using Transits 221 Transits of Mercury and Venus 221 Transits of Extrasolar "Hot Jupiters" 222 Space- Based Transit Searches: CoRoT and Kepler 227 Observing Exoplanet Atmospheres during Transits 229 Secondary Transit Spectroscopy 233 Characterizing Earth- Like Planets around M Stars 235 Chapter 13: Direct Detection of Extrasolar Planets 239 What Wavelength Region Should We Choose? 240 Infrared Interferometers: TPF- I and Darwin 245 Searching for Planets at Visible Wavelengths TPF- C 248 The Visible Occulter: TPF- O 253 Nearby Target Stars 254 Chapter 14: The Spectroscopic Search for Life 258 Spectral Resolution 259 The Visible / Near- IR Region: TPF- C or -O 260 The Thermal- IR Region: TPF- I or Darwin 266 Looking for Life on Early Earth- Type Planets 269 Possible False Positives for Life 271 Polarization Measurements: Looking for the Glint of Surface Water 274 The Holy Grail: Simultaneous Detection of O and Reduced Gases 276 Chapter 15: Prospects for the More Distant Future 284 NASA's Life Finder Mission 284 Using the Sun as a Gravitational Lens 287 The Drake Equation Revisited: The Search for Extraterrestrial Intelligence 290 Notes 299 Index 317