RARE EARTH
Why
Complex Life
Is Uncommon in
the Universe
Peter D. Ward Donald Brownlee
Contents
Preface to the Paperback Edition…. x
Preface to the First Edition…. xiii
Introduction: The Astrobiology Revolution and the Rare Earth Hypothesis…. xvii
Dead Zones of the Universe…. xxix
Rare Earth Factors…. xxxi
Chapters
1.Why Life Might Be Widespread in the Universe
2.Habitable Zones of the Universe
3. Building a Habitable Earth
4.Life's First Appearance on Earth
5.How to Build Animals
6.Snowball Earth
7.The Enigma of the Cambrian Explosion
8.Mass Extinctions and the Rare Earth Hypothesis
9.The Surprising Importance of Plate Tectonics
10.The Moon, Jupiter, and Life on Earth
11.Testing the Rare Earth Hypotheses
12.Assessing the Odds
13.Messengers from the Stars
References 289
Index 319
Preface to the Paperback Edition
On November 12, 2002, Dr. John Chamber's of the NASA Ames Research Center gave a seminar to the Astrobiology Group at the University of Washington. The audience of about 100 listened with rapt attention as Chambers described results from a computer study of how planetary systems form. The goal of his research was to answer a deceptively simple question: How often would newly forming planetary systems produce Earth-like planets, given a star the size of our own sun? By "Earthlike" Chambers meant a rocky planet with water on its surface, orbiting within a star's "habitable zone." This not-too-hot and not-too-cold inner region, relatively close to the star, supports the presence of liquid water on a planet surface for hundreds of million of years—the time-span probably necessary for the evolution of life. To answer the question of just how many Earth-like planets might be spawned in such a planetary system, Chambers had spent thousands of hours running highly sophisticated modeling programs through arrays of powerful computers.
The results presented at the meeting were startling. The simulations showed that rocky planets orbiting at the "right" distances from the central star are easily formed, but they can end up with a wide range of water content. The planet-building materials in a habitable zone include dry materials that form locally, as well as water-bearing materials that originate further from the star and have to be scattered inward, mostly in the form of comets. Without water-bearing comet impacts, Earth-wannabes would just stay wannabes— they would never contain any water.
The model showed that the inbound delivery of water worked best in planetary systems where the intermediate planets, in the position of our giants Jupiter and Saturn, were far smaller. In solar systems such as our own, the efficiency of water being conveyed to the surface of an inner, Earth-like planet is relatively small. Yet in systems where the intermediate planets were much smaller—perhaps Uranus or Neptune-sized—water delivery was relatively frequent.
But then another problem arises: in such a system, the rate of waterbearing comet impacts is great; the rate of asteroid impacts, however, is also so great that any evolving life might soon be obliterated. And oddly, it is not only the asteroid impacts, with their fireballs, dust storms, meteor showers, and "nuclear winters," that cause a problem. An excess of water-bearing impacts can amount, in effect, to too much of a good thing: too much water produces planets entirely covered with water, and such an environment is not conducive to the rich evolution seen on our planet. Earth seems to be quite a gem—a rocky planet where not only can liquid water exist for long periods of time (thanks to Earth's distance from the sun as well as its possession of a tectonic "thermostat" that regulates its temperature), but where water can be found as a heathy oceanful—not too little and not too much. Our planet seems to reside in a benign region of the Galaxy, where comet and asteroid bombardment is tolerable and habitable-zone planets can commonly grow to Earth size. Such real estate in our galaxy—perhaps in any galaxy—is prime for life. And rare as well.
We, the authors of Rare Earth, were in the audience that November day. One of us raised his hand and asked the question: What does this finding mean for the number of Earth-like planets there might be—planets with not only water and bacterial life, but with complex multi-cellular life? Chambers scratched his head. Well, he allowed, it would certainly make them rare.
There was one other aspect of the lecture that struck us. Chambers matter-of-factly spoke of the necessity of planets having plate tectonics to be habitable, and of the effect of mass extinctions. We know that plate tectonics provides a method of maintaining some sort of planetary thermostat that keeps planets at a constant temperature for billions of years. We know, too, that mass extinctions can end life on a planet abruptly, at any time, and that the number of mass extinctions might be linked to astronomical factors, such as the position of a planet in its galaxy.
Prior to the publication of the first edition of Rare Earth in January 2000, neither of these concepts had publicly appeared in discussions of planetary habitability. Now they do, as a matter of course, and this has been a great satisfaction to us. Our hypothesis that bacteria-like life might be quite common in the Universe, but complex life quite rare, may or may not be correct. But the fact that we've been able to bring new lines of evidence into the debate, evidence that was once controversial but is now quite mainstream, has been extremely gratifying.
With its initial publication, Rare Earth struck chords among a wide community. Because it took a rather novel position about the frequency of complex life, the discussion spurred by the book often left the realm of scientific discourse, where we'd intended it to take place, and entered the arenas of religion, ethics, and science fiction.
Science has progressed since the publication, yet nothing we have read or discovered in the years since has caused us to change our minds. One of the most remarkable developments has been the continual discovery of new planets orbiting other stars (the count is now over 100). While this shows that planets are common, it also shows how complex and varied planetary systems are, and how difficult it is to make a stable Earth-like planet. Most of the extra-solar planets that have been discovered are giant planets in orbits that preclude the possibility of water-covered Earths with long-term stability.
This edition, then, is changed only in the removal of several egregious and sometimes hilarious typos and errors. We stand by our initial assessment and are proud to see that Rare Earth continues to spawn heated debate even as it makes its way into textbooks as accepted dogma.
Peter D. Ward, Donald Brownlee Seattle, February 2003
Preface to the First Edition
This book was born during a lunchtime conversation at the University of Washington faculty club, and then it simply took off. It was stimulated by a host of discoveries suggesting to us that complex life is less pervasive in the Universe than is now commonly assumed. In our discussions, it became clear that both of us believed such life is not widespread, and we decided to write a book explaining why.
Of course, we cannot prove that the equivalent of our planet's animal life is rare elsewhere in the Universe. Proof is a rarity in science. Our arguments are post hoc in the sense that we have examined Earth history and then tried to arrive at generalizations from what we have seen here. We are clearly bound by what has been called the Weak Anthropic Principle—that we, as observers in the solar system, have a strong bias in identifying habitats or factors leading to our own existence. To put it another way, it is very difficult to do statistics with an N of 1. But in our defense, we have staked out a position rarely articulated but increasingly accepted by many astrobiologists. We have formulated a null hypothesis, as it were, to the clamorous contention of many scientists and media alike that life—barroom-brawling, moral-philosophizing, human-eating, lesson-giving, purple-blooded bug-eyed monsters of high and low intelligence—is out there, or that even simple worm-like animals are commonly out there. Perhaps in spite of all the unnumbered stars, we are the only animals, or at least we number among a select few. What has been called the Principle of Mediocrity—the idea that Earth is but one of a myriad of like worlds harboring advanced life—deserves a counterpoint. Hence our book.
Writing this book has been akin to running a marathon, and we want to acknowledge and thank all those who offered sustaining draughts of information as we followed our winding path. Our greatest debts of gratitude we owe to Jerry Lyons of Copernicus, who invested so much interest in the project, and to our editor, Jonathan Cobb, who fine-tuned the project on scales ranging from basic organization of the book to its numerous split infinitives.
Many scientific colleagues gave much of themselves. Joseph Kirschvink of Cal Tech read the entire manuscript and spent endless hours thrashing through various concepts with us; his knowledge and genius illuminated our murky ideas. Guillermo Gonzalez changed many of our views about planets and habitable zones. Thor Hansen of Western Washington University described to us the concept of "stopping plate tectonics." Colleagues in the Department of Geological Sciences, including Dave Montgomery, Steve Porter, Bruce Nelson, and Eric Cheney, discussed many subjects with us. Many thanks to Victor Kress of the University of Washington for reading and critiquing the plate tectonics chapter. Dr. Robert Paine of the Department of Zoology saved us from making egregious errors about diversity. Numerous astrobiologists took time to discuss aspects of the science with us, including Kevin Zahnlee of NASA Ames, who patiently explained his position—one contrary to almost everything we believed—and in so doing markedly expanded our understanding and horizons. We are grateful to Jim Kasting of Penn State University for long discussions about planets and their formations. Thanks as well to Gustav Ahrrenius from UC Scripps, Woody Sullivan (astronomy) of the University of Washington, and John Baross of the School of Oceanography at the University of Washington. Jack Sepkoski of the University of Chicago generously sent new extinction data, Andy Knoll of Harvard contributed critiques by E-mail, Sam Bowring spent an afternoon sharing his data and his thoughts on the timing of major events in Earth history, Dolf Seilacher talked with us about ediacarans and the first evolution of life, Doug Erwin lent insight into the Permo/Triassic extinction, Jim Valentine and Jere Lipps of Berkeley gave us their insights into the late Precam-brian and animal evolution, David Jablonski described his views on body plan evolution. We are enormously grateful to David Raup, for discussions and archival material about extinction, and to Steve Gould for listening to and critiquing our ideas over a long Italian dinner on a rainy night in Seattle. Thanks to Tom Quinn of UW astronomy for illuminating the rates of obliquity change and to Dave Evans of Cal Tech, with whom we discussed the Pre-cambrian glaciations. Conway Leovy talked to us about atmospheric matters. With Bob Berner of Yale, we discussed matters pertaining to the evolution of the atmosphere through time. Steve Stanley of Johns Hopkins gave us insight into the Permo/Triassic extinction. Walter Alvarez and Allesandro Montanari talked with us about the K/T extinction. Bob Pepin gave us insight into atmospheric effects.
Ross Taylor of the Australian University provided useful information to us, and Geoff Marcy and Chris McKay discussed elements of the text. Doug Lin of U.C. Santa Cruz discussed the fate of planetary systems with "Bad" Jupiters. We are grateful to Al Cameron for use of his lunar formation results.
Peter D. Ward, Donald Brownlee Seattle, August 1999
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PROBABLY ALL OF THE ABOVE NAMES BELIEVE IN BILLIONS OF YEARS TO FORM OUR SUN, EARTH, AND SOLAR SYSTEM. MOST OF THEM IF NOT ALL OF THEM, DO NOT BELIEVE IN GENESIS 1 AS FOUND IN THE HOLY BIBLE.
NEVERTHELESS, MORE AND MORE SCIENTISTS ARE COMING TO SEE THAT A BLUE PLANET EARTH, IS RARE TO FIND IN THE UNIVERSE, SO FAR WE HAVE FOUND NONE. BUT AS SOME MAY ARGUE THE UNIVERSE AND GALAXIES WITHIN THE UNIVERSE, MAY BE IN THE BILLIONS, SO THERE IS MUCH YET TO DISCOVER.
YET THE SCIENTIFIC POINTS BROUGHT OUT IN THIS BOOK, ARE GENUINE FACTS THAT LIFE ON EARTH MUST HAVE THESE SCIENCE FACTS, TO EXIST IN THE FORM THEY EXIST ON THIS BLUE PLANET.
WHILE A LOT OF THIS BOOK IS WRITTEN FOR PhD PEOPLE, THERE IS ENOUGH IN IT THAT THE COMMON PERSON CAN UNDERSTAND.
Keith Hunt
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