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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 How to Find a Habitable Planet 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.
How to Find a Habitable Planet 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.
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."
– 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, 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
"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
"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
"A fascinating read – everything you need to know about habitable worlds."
– Sara Seager, Massachusetts Institute of Technology
"I very much enjoyed reading this book. Kasting writes clearly and explains the issues in an easy-to-follow manner. I learned several new things from this book. I will certainly recommend it to colleagues and students."
– Christopher McKay, NASA Ames Research Center
"We now know quite a bit about planets, and a lot of this knowledge has arrived relatively recently. This book is right up to date. Kasting was directly involved in work that defined the overall technological and scientific goals for upcoming NASA missions to detect potentially habitable worlds beyond our solar system. How to Find a Habitable Planet is a significant contribution to the field."
– Gregory Laughlin, University of California, Santa Cruz
