Astronomers Universe For further volumes: http://www.springer.com/series/6960
David S. Stevenson Under a Crimson Sun Prospects for Life in a Red Dwarf System
David S. Stevenson Sherwood, UK ISSN 1614-659X ISBN 978-1-4614-8132-4 ISBN 978-1-4614-8133-1 (ebook) DOI 10.1007/978-1-4614-8133-1 Springer New York Heidelberg Dordrecht London Library of Congress Control Number: 2013944655 Springer Science+Business Media New York 2013 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, speci fi cally the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on micro fi lms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied speci fi cally for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher s location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a speci fi c statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Cover illustration: Courtesy of Lynette Cook, 2005 Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)
For my wonderful and long-suffering wife Nikki, and for my children Aleister, Oran, Arabella, Genevieve, and Vincent
Preface Initial ideas regarding the habitability of extrasolar planets have focused on the overall size of the planet and its location within the stellar habitable zone. However, many additional factors exist that affect their potential ability to harbor life. This is particularly true of planets orbiting red dwarf stars, for it is these worlds that will have the most time to evolve within a star s Goldilocks zone. Red dwarf stars live for hundreds of billions or trillions of years, providing a very steady candle with which to light their retinue of worlds. However, red dwarfs throw up a wealth of seemingly contradictory conditions that grossly affect whether a planet will be habitable. Planets orbiting these little crimson suns have a unique set of problems that could, in principle, prevent or reduce their potential to host life. Are the most habitable worlds discovered to date really as they appear? This book follows the ongoing work of researchers studying the many disparate branches of science, including astrophysics, chemistry, geology, and biology. Their aim is to create a more holistic view of habitability, based around a large number of interlinked factors. In essence, the question of habitability cannot be reduced to an answer based solely on the location of a planet within or outside of its star s Goldilocks zone. This book fi rst explores the nature of the stars and then the planets that orbit them. By considering all aspects of planetary existence, a picture then emerges as to how habitable any planet orbiting a red dwarf can be. In the making of this book, there were a number of different areas of science that needed investigation. As such, I would like to offer particular thanks to Greg Laughlin (University of California, Santa Cruz), with whom I had many productive discussions regarding the evolution of the lowest-mass stars. Notably it was clear that inexplicably little if any work had been published on vii
viii Preface the evolution of the orange (K) dwarfs. Aside from one publication in 1993, this area of astrophysics hasn t been visited since. The evolution of these stars was then inferred from work carried out by Greg on red dwarfs, as well as a diverse set of publications on a set of stars called extreme horizontal branch stars. Greg provided some useful clues that validated my assumptions. This is an area of research to which astrophysicists must return. This is all the more true as K-class, orange dwarfs have become targets for searches of habitable worlds. I would also like to offer thanks to the team at www.universetoday.com for their prescient (verging on spooky) ability to publish articles on topics I was writing about at the time. The work of this website s authors often alerted me to very recent research that was either unpublished or had just been so. Finally, the book has many areas that are as yet untested hypotheses. In the coming years, Kepler and other telescopic probes will begin to test these ideas, adding meat to the bones of contention and theory. These are exciting times. Sherwood, UK David S. Stevenson
About the Author David Stevenson studied molecular biology at Glasgow University where he attained a First Class BSc. Honors. He then continues is studies towards a PhD in molecular genetics at Cambridge. Further qualifications in with the Open University include a Distinction in Astronomy and Planetary Science, and separately Geophysics and Geochemistry. His peer-reviewed biological research articles from 1999 to 2003 include a paper on the early development of life, The Origin of Translation, published in the Journal of Theoretical Biology. David s interest in astronomy was encouraged from an early age by his father. This (combined with an interest in explosions!) has led David to research and write about the life and death of stars. After a stint in academia, David became a teacher but continued to write scientific articles for various publications. He has published numerous articles on the Blackwell Plant Sciences website (2002 2007). Turning Out the Lights (an article about red dwarfs) was published in Popular Astronomy in 2003, A Bigger Bang (about Type Ia supernovae) in Sky & Telescope in July 2007 and Supercharged Supernovae (cover article in Sky & Telescope, October 2011). He is currently completing a second book for Springer (Extreme Explosions which is due for publication in September 2013. David lives in Nottingham in the UK with his wife and family. ix
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