James Kasting is Distinguished Professor of Geosciences at Pennsylvania State University. He is a renowned expert in planetary atmospheric evolution and is actively involved in the search by NASA for habitable planets outside our solar system. He is the coauthor of the introductory textbook The Earth System.

This is a very well-written book that serves perfectly as an introduction for the lay reader to astrobiology and the search for life beyond Earth, and it also presents some important and up-to-date material for the specialist researcher. Of particular note for me is Kasting's rebuttal of several aspects of the Rare Earth hypothesis and his discussion of the features that truly are crucial for planetary habitability."--Lewis Dartnell, Times Higher Education" "How to Find a Habitable Planet will fascinate those who care about the evolution of Earth's climate and wonder about how we will eventually detect life--even simple cellular life--elsewhere."--Debra Fischer, Nature "He argues persuasively that Earth's large moon and strong magnetic field are red herrings; neither is a prerequisite for life. The second part of his book is a detailed account of the search for Earth-like exoplanets, and prospects for future success."--New Scientist Kasting's book ... is a readable guide to the many things we have just begun to understand about a solar system."--Tim Radford, The Guardian "Kasting, a key planner for future NASA missions seeking Earth-like exoplanets, possesses a deep understanding of all the multifarious complexities that feed into forming--and finding--living worlds. He writes about these topics, for all their profundity, with remarkable precision and clarity, drawing clear linkages between what we observe through telescopes with what we see right here, in the only biosphere we know. How to Find a Habitable Planet is a canonical guide to the probable future of humanity's search for life elsewhere in the Universe."--SEED Magazine "A 'popular textbook', low on speculation but high on nuts-and-bolts science, it covers everything from the habitable zones around other stars to how to find other Earths. If I had to choose just one of these books, it would be Kasting's."--Marcus Chown, BBC Focus Magazine "Written in a clear and often conversational style--and infused throughout with Kasting's personal optimism regarding the existence of, and our ability to detect, habitable and inhabited worlds beyond our own--this is an informative and worthwhile read for anyone who looks to the stars and wonders if there is anybody out there."--Tori M. Hoehler, Nature Geoscience "As Kasting shows in his technical but readable How to Find a Habitable Planet, we are making rapid progress in discovering planets around distant stars--almost 500 so far."--Clive Cookson, Financial Times "A concise yet thorough scientific explanation of how we can narrow down the search [for habitable planets]... Kasting is a world leader in planetary habitability and works closely with NASA. He gives insight into the latest technology being used to hunt for habitable plants, describes the signatures of life that scientists are looking for and makes his predictions for the future of the field... Thorough and interesting."--Cosmos Magazine "An excellent book on a fascinating topic."--Choice Even if you were not interested in finding intelligence elsewhere in the universe, How to Find a Habitable Planet provides an excellent and accessible account of the formation and history of our own planet, and indeed our solar system."--John Gribbin, Literary Review "In How to Find a Habitable Planet, James Kasting begins by describing various factors that geophysicists, astrobiologists and others have deemed necessary (or at least desirable) for producing planets capable of supporting life... How to Find a Habitable Planet offers an eloquent explanation of why such a mission would still be desirable."--Physics World "[Kasting is] an excellent writer, capable of breaking down complex topics into clear and accessible pieces. That skill makes this first-of-its-kind book not just unique but probably indispensible for students and armchair planetologists."--Charlene Brusso, Ad Astra

• 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

James Kasting is Distinguished Professor of Geosciences at Pennsylvania State University. He is a renowned expert in planetary atmospheric evolution and is actively involved in the search by NASA for habitable planets outside our solar system. He is the coauthor of the introductory textbook The Earth System.

This is a very well-written book that serves perfectly as an introduction for the lay reader to astrobiology and the search for life beyond Earth, and it also presents some important and up-to-date material for the specialist researcher. Of particular note for me is Kasting's rebuttal of several aspects of the Rare Earth hypothesis and his discussion of the features that truly are crucial for planetary habitability."--Lewis Dartnell, Times Higher Education" "How to Find a Habitable Planet will fascinate those who care about the evolution of Earth's climate and wonder about how we will eventually detect life--even simple cellular life--elsewhere."--Debra Fischer, Nature "He argues persuasively that Earth's large moon and strong magnetic field are red herrings; neither is a prerequisite for life. The second part of his book is a detailed account of the search for Earth-like exoplanets, and prospects for future success."--New Scientist Kasting's book ... is a readable guide to the many things we have just begun to understand about a solar system."--Tim Radford, The Guardian "Kasting, a key planner for future NASA missions seeking Earth-like exoplanets, possesses a deep understanding of all the multifarious complexities that feed into forming--and finding--living worlds. He writes about these topics, for all their profundity, with remarkable precision and clarity, drawing clear linkages between what we observe through telescopes with what we see right here, in the only biosphere we know. How to Find a Habitable Planet is a canonical guide to the probable future of humanity's search for life elsewhere in the Universe."--SEED Magazine "A 'popular textbook', low on speculation but high on nuts-and-bolts science, it covers everything from the habitable zones around other stars to how to find other Earths. If I had to choose just one of these books, it would be Kasting's."--Marcus Chown, BBC Focus Magazine "Written in a clear and often conversational style--and infused throughout with Kasting's personal optimism regarding the existence of, and our ability to detect, habitable and inhabited worlds beyond our own--this is an informative and worthwhile read for anyone who looks to the stars and wonders if there is anybody out there."--Tori M. Hoehler, Nature Geoscience "As Kasting shows in his technical but readable How to Find a Habitable Planet, we are making rapid progress in discovering planets around distant stars--almost 500 so far."--Clive Cookson, Financial Times "A concise yet thorough scientific explanation of how we can narrow down the search [for habitable planets]... Kasting is a world leader in planetary habitability and works closely with NASA. He gives insight into the latest technology being used to hunt for habitable plants, describes the signatures of life that scientists are looking for and makes his predictions for the future of the field... Thorough and interesting."--Cosmos Magazine "An excellent book on a fascinating topic."--Choice Even if you were not interested in finding intelligence elsewhere in the universe, How to Find a Habitable Planet provides an excellent and accessible account of the formation and history of our own planet, and indeed our solar system."--John Gribbin, Literary Review "In How to Find a Habitable Planet, James Kasting begins by describing various factors that geophysicists, astrobiologists and others have deemed necessary (or at least desirable) for producing planets capable of supporting life... How to Find a Habitable Planet offers an eloquent explanation of why such a mission would still be desirable."--Physics World "[Kasting is] an excellent writer, capable of breaking down complex topics into clear and accessible pieces. That skill makes this first-of-its-kind book not just unique but probably indispensible for students and armchair planetologists."--Charlene Brusso, Ad Astra

• 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

James Kasting is Distinguished Professor of Geosciences at Pennsylvania State University. He is a renowned expert in planetary atmospheric evolution and is actively involved in the search by NASA for habitable planets outside our solar system. He is the coauthor of the introductory textbook The Earth System.

This is a very well-written book that serves perfectly as an introduction for the lay reader to astrobiology and the search for life beyond Earth, and it also presents some important and up-to-date material for the specialist researcher. Of particular note for me is Kasting's rebuttal of several aspects of the Rare Earth hypothesis and his discussion of the features that truly are crucial for planetary habitability."--Lewis Dartnell, Times Higher Education" "How to Find a Habitable Planet will fascinate those who care about the evolution of Earth's climate and wonder about how we will eventually detect life--even simple cellular life--elsewhere."--Debra Fischer, Nature "He argues persuasively that Earth's large moon and strong magnetic field are red herrings; neither is a prerequisite for life. The second part of his book is a detailed account of the search for Earth-like exoplanets, and prospects for future success."--New Scientist Kasting's book ... is a readable guide to the many things we have just begun to understand about a solar system."--Tim Radford, The Guardian "Kasting, a key planner for future NASA missions seeking Earth-like exoplanets, possesses a deep understanding of all the multifarious complexities that feed into forming--and finding--living worlds. He writes about these topics, for all their profundity, with remarkable precision and clarity, drawing clear linkages between what we observe through telescopes with what we see right here, in the only biosphere we know. How to Find a Habitable Planet is a canonical guide to the probable future of humanity's search for life elsewhere in the Universe."--SEED Magazine "A 'popular textbook', low on speculation but high on nuts-and-bolts science, it covers everything from the habitable zones around other stars to how to find other Earths. If I had to choose just one of these books, it would be Kasting's."--Marcus Chown, BBC Focus Magazine "Written in a clear and often conversational style--and infused throughout with Kasting's personal optimism regarding the existence of, and our ability to detect, habitable and inhabited worlds beyond our own--this is an informative and worthwhile read for anyone who looks to the stars and wonders if there is anybody out there."--Tori M. Hoehler, Nature Geoscience "As Kasting shows in his technical but readable How to Find a Habitable Planet, we are making rapid progress in discovering planets around distant stars--almost 500 so far."--Clive Cookson, Financial Times "A concise yet thorough scientific explanation of how we can narrow down the search [for habitable planets]... Kasting is a world leader in planetary habitability and works closely with NASA. He gives insight into the latest technology being used to hunt for habitable plants, describes the signatures of life that scientists are looking for and makes his predictions for the future of the field... Thorough and interesting."--Cosmos Magazine "An excellent book on a fascinating topic."--Choice Even if you were not interested in finding intelligence elsewhere in the universe, How to Find a Habitable Planet provides an excellent and accessible account of the formation and history of our own planet, and indeed our solar system."--John Gribbin, Literary Review "In How to Find a Habitable Planet, James Kasting begins by describing various factors that geophysicists, astrobiologists and others have deemed necessary (or at least desirable) for producing planets capable of supporting life... How to Find a Habitable Planet offers an eloquent explanation of why such a mission would still be desirable."--Physics World "[Kasting is] an excellent writer, capable of breaking down complex topics into clear and accessible pieces. That skill makes this first-of-its-kind book not just unique but probably indispensible for students and armchair planetologists."--Charlene Brusso, Ad Astra

• 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