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Final Thoughts

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Final Thoughts Contents Part I Common Themes 1. The Discovery of Extraterrestrial Worlds........................ 3 Introduction ...................................................................... 3 Radial Velocity: The Pull of Extrasolar Planets .............. 5 Transit: The Shadow of a Planet Cast by Its Star ........... 11 Microlensing: The Ghosts of Hidden Worlds .................. 20 Now You See It, Now You Don’t: Formalhaut B and Beyond ................................................ 24 What Worlds Await Us? ................................................... 27 Conclusions ...................................................................... 38 2. The Formation of Stars and Planets ................................ 39 Introduction ...................................................................... 39 Gravity’s Role in Star and Planet Formation .................. 40 The Effects of Neighboring Stars ..................................... 43 Brown Dwarfs ................................................................... 44 The Planets of Red Dwarfs ............................................... 46 Alternative Routes to Rome ............................................ 47 Energy and Life ................................................................. 50 Planetary Heat .................................................................. 51 Atmosphere, Hydrosphere and Biosphere ....................... 58 Conclusions ...................................................................... 60 3. Stellar Evolution Near the Bottom of the Main Sequence ....................................................... 63 Introduction ...................................................................... 63 Stellar Evolution and the HR Diagram ........................... 64 Spectral Features of M and K Dwarfs .............................. 65 Spectral Features of K-Class Stars ................................... 69 Structure of M- and K-Class Stars ................................... 69 The Stellar Furnace .......................................................... 70 Spectacularly Faint Dwarfs .............................................. 72 xi xii Contents A Primer of Stellar Evolution .......................................... 73 The Life of an “Ordinary” Red Dwarf ............................. 76 The Fate of the Most Massive Red Dwarf Stars .............. 82 The Missing Piece ............................................................ 84 Flings for Mid-Range M-Dwarf Stars ............................... 85 The Evolution of Orange K-Dwarfs ................................. 86 Which Stars Burn Helium? .............................................. 92 The Second Ascension ..................................................... 94 The Fate of the Smallest K-Dwarf Stars .......................... 97 Pulsations .......................................................................... 100 The End of the Lowest Mass K-Dwarfs ........................... 101 Conclusions ...................................................................... 102 4. The Living Planet ............................................................. 105 Introduction ...................................................................... 105 Plate Tectonics: A Primer ................................................ 105 Glorious Granite ............................................................... 109 The Mantle and Oceanic Crust ....................................... 113 Eclogite .............................................................................. 115 Constructing Continents ................................................. 117 Assembling a Continental Jigsaw .................................... 118 The Role of Mantle Plumes in Continent Formation .... 120 Just How Does Plate Tectonics Work? ............................ 123 The Past, Present and Future of Tectonics on Earth and Other Worlds .............................................................. 127 The Organization of Early Plate Tectonics ..................... 131 The Construction of Continents and the Heat-Death of a Planet ......................................................................... 132 Plate Tectonics and the Stability of a Planet’s Biosphere ........................................................................... 134 Conclusions ...................................................................... 135 5. The Carbon Dioxide Connection .................................... 137 Introduction ...................................................................... 137 Carbon Dioxide: The Essential Gas ................................. 137 The Fate of Carbon Dioxide ............................................. 138 Life Under the Thick Lid ................................................. 145 Super-Volcanic Eruptions ................................................. 147 Photosynthetic Miners ..................................................... 150 Conclusions ...................................................................... 151 Contents xiii 6. Stability of Habitable Atmospheres on Red Dwarf Worlds ....................................................... 155 Introduction ...................................................................... 155 Tidal Locking .................................................................... 155 The Atmosphere After Tidal Locking ............................. 157 The Coriolis Effect ........................................................... 161 Atmospheric Modeling ..................................................... 166 The Effect of Topography ................................................. 170 Conclusions ...................................................................... 172 7. The Development and Sustenance of Life ...................... 173 Introduction ...................................................................... 173 Our Alien World: The Hot, Deep Biosphere ................... 174 The Concept of the Stellar Climatic Habitable Zone .... 174 What Is Life? ..................................................................... 175 How Does Life Arise? ....................................................... 178 From Stellar Fluff to Complex Cells ............................... 184 Sense and Sensibility ........................................................ 188 Oxygen: A Contradiction ................................................. 189 Multi-cellularity ............................................................... 193 The Links Between the Biosphere and Geosphere .......... 197 Identifying Living Worlds ................................................. 199 Conclusions ...................................................................... 202 Part II Life Under a Crimson Sun 8. Red Dwarfs, PAR and the Prospects for Photosynthesis .... 207 Introduction ...................................................................... 207 A Primer on Photosynthesis ............................................ 207 Action Spectra and Beyond .............................................. 211 Wavelength and the Products of Photosynthesis ............ 213 Physical Constraints Imposed on Photosynthesis by Red Dwarf Stars ........................................................... 219 The Effect of Planetary Climate ...................................... 223 The Temperature Limits of Life ....................................... 224 The Effect of Star Spots .................................................... 225 Conclusions ...................................................................... 230 9. Gliese 581d: The First Potentially Habitable Water-World Discovered? ................................................. 233 Introduction ...................................................................... 233 Gliese 581g: Now You See It, Now You Don’t ................ 235 Gliese 581d: An Ancient Water World? ........................... 239 xiv Contents Orbital Migration and the Composition of Gliese 581d ................................................................... 243 The Nice Group ................................................................ 246 The Late Heavy Bombardment ........................................ 250 Putting Gliese 581d’s Formation and Early History in Context ......................................................................... 254 The Fate of the Gliese 581 System .................................. 262 Conclusions ...................................................................... 266 10. The Evolution of an Earth-Like World ............................ 267 Introduction ...................................................................... 267 The Development and Fate of Planet Gliese 667Cc ....... 274 Future World ..................................................................... 287 The Geodynamo and the Preservation of the Atmosphere ............................................................ 290 Stellar Evolution and the Ultimate Fate of the System .................................................................... 295 Sustainability of Life on Other Worlds ............................ 298 Conclusions ...................................................................... 299 Final Thoughts .......................................................................... 301 Glossary ..................................................................................... 305 Index .......................................................................................... 321 .
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