Rare Earth:
Why Complex Life is
Uncommon in the Universe
Peter D. Ward
Donald Brown
Page I
Praise for Rare Earth . . .
“ . . . brilliant and courageous . . . likely to cause a revolution in thinking.”
—William J. Broad
The New York Times“Science Times”
“A pleasure for the rational reader . . . what good books are all about . . . ”
—Associated Press
“If Ward and Brownlee are right it could be time to reverse a process that has been
going on since Copernicus.”
—The Times(London)
“Although simple life is probably abundant in the universe, Ward & Brownlee say,
‘complex life—animals and higher plants—is likely to be far more rare than is
commonly assumed’.”
—Scientific American, Editor’s Choice
“ . . . a compelling argument [and] a wet blanket for E.T. enthusiasts . . . ”
—Discover
“Peter Ward and Donald Brownlee offer a powerful argument ....”
—The Economist
“[Rare Earth] has hit the world of astrobiologists like a killer asteroid ....”
—Newsday
“A very good book.”
—Astronomy
The notion that life existed anywhere
in the universe besides Earth was once laughable in the scientific
community. Over the past thirty years or so, the laughter has died away . . . . [Ward and
Brownlee] argue that the recent trend in scientific thought has gone too far . . . .
As radio telescopes sweep the skies and earthbound researchers strain to pick up
anything that might be a signal from extraterrestrial beings, Rare Earthmay
offer an explanation for why we haven’t heard anything yet.”
—CNN.com
“ . . . a sobering and valuable perspective . . . ”
—Science
Page II
“Movies and television give the (optimistic) impression that the cosmos is teeming
with civilizations. But what if it isn’t? . . . Life elsewhere in the universe may never
reach beyond microbes, which, the authors note, could be much more widespread
than originally believed.”
—Sky
& Telescope
“It’s brilliant . . . courageous.... It’s rare in literature and science that a stance goes
so far against the grain.”
—Dr. Geoffrey W. Marcy
Extra-solar planet discoverer
University of California at Berkeley
“It’s a thought that grips most everyone who stares into the unfathomable depths of
a star-speckled night: Is there anybody out there? The odds, say Peter Ward and
Don Brownlee, are probably more remote than you think.”
—The
Seattle Times
“Alien life is more likely to resemble the stuff you scrub off the tiles in your shower
than Klingons, Wookies or Romulans, say Ward and Brownlee.”
—Popular
Mechanics
“Ward and Brownlee have taken an issue that is much in the public domain and
treated it thoughtfully and thoroughly, but with a lightness of touch that draws
the reader on .... Rare Earthis an excellent book for both specialists and non-
specialists.”
—The
Times Higher Education Supplement(UK)
“A provocative, significant, and sweeping new book . . . Rare Earthis a fast-paced,
thought-provoking read that I gobbled like popcorn. It’s one of those rare books that
is at once delightful, informative, and important: an end-of-the-millennium synthesis
of science that tackles the central question of our past, place, and destiny.”
—Northwest Science & Technology
“ . . . well thought out and intriguing . . . ”
—Icarus
“ . . . a startling new hypothesis . . . Highly recommended.”
—Library Journal
“Rare Earth will surely appeal to those who would dare to disagree with icons Carl
Sagan and George Lucas.”
—San
Gabriel Valley Tribune
Page III
“ . . . a timely, entirely readable account.”
—Toronto Globe & Mail
“ . . . a stellar example of clear writing . . . ”
—American Scientist
“ . . . thought-provoking and authoritative . . . ”
—Physics Today
“In this encouraging and superbly written book, the authors present a carefully rea-
soned and scientifically statute examination of the age-old question—‘Are we alone
in the universe?’ Their astonishing conclusion that even simple animal life is most
likely extremely rare in the universe has many profound implications. To the aver-
age person, staring up at a dark night sky, full of distant galaxies, it is simply incon-
ceivable that we are alone. Yet, in spite of our wishful thinking, there just may not
be other Mozarts or Monets.”
—Don Johanson
Director,
Institute of Human Origins
Arizona State
University
“A fabulous book! If we’re to believe what we see in the movies, extraterrestrials
thrive on every world. But this unique book, written by two of the top scientists in
the field, tells a different story. As we know it on Earth, complex life might be very
rare, and very precious. For those of us interested in our cosmic heritage, this book
is a must-read.”
—David Levy
Co-discoverer
of Comet Shoemaker-Levy
“Ward and Brownlee take us on a fascinating journey through the deep history of
our habitable planet and out into space; in the process they weave a compelling ar-
gument that life at the level of an animal should be vastly rarer in the universe than
life at the level of a lowly bacterium.”
—Steven M.
Stanley,
Author of Children
of the Ice Ageand Earth and Life Through Time
The Johns Hopkins
University
“Microbial life is common in the universe, but multicellular animal life is rare. A
controversial thesis, but one that is well-researched and well-defended. A must-read
for anyone who is interested in whether life exists beyond Earth.”
—James Kasting
Pennsylvania
State University
Page IV
Page V
RARE EARTH
Why
Why
Complex Life
Is Uncommon in
the Universe
Is Uncommon in
the Universe
Peter D. Ward
Donald Brownlee
COPERNICUS BOOKS
ANIMPRINT
OFSPRINGER-VERLAG
Page VI
First softcover printing, 2003.
© 2000 Peter D. Ward and Donald Brownlee.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher.
Published in the United States by Copernicus Book, an imprint of Springer-Verlag New York, Inc. A member of BertelsmannSpringer Science+Business Media GmbH
Copernicus Books
37 East 7th Street
New York, NY 10003
www.copernicusbooks.com
Library of Congress Cataloging-in-Publication Data
Ward, Peter D.
Rare earth: why complex life is uncommon in the universe / Peter D.
Ward, Donald Brownlee.
p. cm.
Includes bibliographical references and index.
ISBN 0-387-95289-6 (pbk.: alk. paper)
1. Life on other planets. 2. Exobiology. I. Brownlee, Donald.
II. Title.
QB54.W336 2003
576.839—dc21 2003043437
Manufactured in the United States of America.
Printed on acid-free paper.
987654321
ISBN 0-387-95289-6 SPIN 10838285
Page VII
To the memory of
Gene
Shoemaker
and
Carl
Sagan
Page IX
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
1 Why Life Might Be Widespread in the Universe .... 1
2 Habitable Zones of the Universe ........................... 15
3 Building a Habitable Earth ..................................... 35
4 Life’s First Appearance on Earth ............................. 55
5 How to Build Animals ............................................ 83
6 Snowball Earth ..................................................... 113
7 The Enigma of the Cambrian Explosion .............. 125
8 Mass Extinctions and the Rare Earth Hypothesis .. 157
9 The Surprising Importance of Plate Tectonics .... 191
10 The Moon, Jupiter, and Life on Earth .................. 221
11 Testing the Rare Earth Hypotheses ..................... 243
12 Assessing the Odds .............................................. 257
13 Messengers from the Stars ................................... 277
References ................................................................................................289
Index ......................................................................................................319
Page x
12, Washington.
Page xi
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.
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.
Page xiv
Preface to the
Paperback Edition
12, Washington.
On November of 12, Washington. 2002, Dr. JohnThe
audienceChambers of of about the NASA 100 listened Ames Re-University 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 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.
Page xi
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 inplanetary 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 water-bearing 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.
Page xii
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 Earthstruck 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
Page xiii
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.
Page xiv
We have formulated a
null hypothesis, as it were, to the clamorous contention of many scientists and
media alike that life—barroom-brawling, moralphilosophizing, 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.
Page xv
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 Precambrian 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 Precambrian 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.
of Oceanography at the University of Washington.
Page xv
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 Precambrian 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 Precambrian 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
Seattle, August 1999
Page xvii
Introduction:
The Astrobiology
Revolution and
the Rare Earth
Hypothesis
On any given night, a vast array of extraterrestrial organisms frequent
the television sets and movie screens of the world. From Star
Wars and “Star Trek” to The X-Files, the message is clear: The Universe
is replete with alien life forms that vary widely in body plan, intelligence,
and degree of benevolence. Our society is clearly enamored of the expectation
not only that there is life on other planets, but that incidences of
intelligent life, including other civilizations, occur in large numbers in the Universe.
This bias toward
the existence elsewhere of intelligent life stems partly from wishing (or
perhaps fearing) it to be so and partly from a now-famous publication by
astronomers Frank Drake and Carl Sagan, who devised an estimate (called the
Drake Equation) of the number of advanced civilizations that might be present
in our galaxy. This formula was based on educated guesses about the number of
planets in the galaxy, the percentage of those that might harbor life, and the
percentage of planets on which life not only could exist
Page xviii
The idea of a
million civilizations of intelligent creatures in our galaxy is a breathtaking
concept. But is it credible? The solution to the Drake Equation includes hidden
assumptions that need to be examined. Most important, it assumes that once life
originates on a planet, it evolves toward ever higher complexity, culminating
on many planets in the development of culture. That is certainly what happened
on our Earth. Life originated here about 4 billion years ago and then evolved
from single-celled organisms to multicellular creatures with tissues and
organs, climaxing in animals and higher plants. Is this particular history of
life—one of increasing complexity to an animal grade of evolution—an inevitable
result of evolution, or even a common one? Might it, in fact, be a very rare result?
In this book we
will argue that not only intelligent life, but even the simplest of animal
life, is exceedingly rare in our galaxy and in the Universe. We are not saying
that life is rare—only that animal life is. We believe that life in the form of
microbes or their equivalents is very common in the universe, perhaps more
common than even Drake and Sagan envisioned. However, complex life—animals and
higher plants—is likely to be far more rare than is commonly assumed. We
combine these two predictions of the commonness of simple life and the rarity
of complex life into what we will call the Rare Earth Hypothesis. In the pages
ahead we explain the reasoning behind this hypothesis, show how it may be tested,
and suggest what, if it is accurate, it may mean to our culture.
Page xix
The
search in earnest for extraterrestrial life is only beginning, but we have
already entered a remarkable period of discovery, a time of excitement and
dawning knowledge perhaps not seen since Europeans reached the New World in
their wooden sailing ships. We too are reaching new worlds and are acquiring data at an
astonishing pace. Old ideas are crumbling. New views rise and fall with each
new satellite image or deep-space result. Each novel biological or
paleontological discovery supports or undermines some of the myriad hypotheses
concerning life in the Universe. It is an extraordinary time, and a whole new
science is emerging: astrobiology, whose central focus is the condition of life
in the Universe. The practitioners of this new field are young and old, and
they come from diverse scientific backgrounds. Feverish urgency is readily
apparent on their faces at press conferences, such as those held after the Mars
Pathfinder experiments, the discovery of a Martian meteorite on the ice fields
of Antarctica, and the collection of new images from Jupiter’s moons. In
usually decorous scientific meetings, emotions boil over, reputations are made
or tarnished, and hopes ride a roller coaster, for scientific paradigms are
being advanced and discarded with dizzying speed.
We are witnesses to a
scientific revolution, and as in any revolution there will be winners and
losers—both among ideas and among partisans. It is very much like the early
1950s, when DNA was discovered, or the 1960s, when the concept of plate
tectonics and continental drift was defined. Both of these events prompted
revolutions in science, not only leading to the complete reorganization of
their immediate fields and to adjustments in many related fields, but also spilling
beyond the boundaries of science to make us look at ourselves and our world in
new ways. That will come to pass as well in this newest scientific revolution,
the Astrobiology Revolution of the 1990s and beyond. What makes this
revolution so startling is that it is happening not within a given discipline
of science, such as biology in the 1950s or geology in the 1960s, but as a
convergence of widely different scientific disciplines: astronomy, biology,
paleontology, oceanography, microbiology, geology, and genetics, among others.
Page xx
In one sense,
astrobiology is the field of biology ratcheted up to encompass not just life on
Earth but also life beyond Earth. It forces us to reconsider the life of our
planet as but a single example of how life might work, rather than as the only
example. Astrobiology requires us to break the shackles of conventional
biology; it insists that we consider entire planets as ecological systems. It
requires an understanding of fossil history. It makes us think in terms of
long sweeps of time rather than simply the here and now. Most fundamentally, it
demands an expansion of our scientific vision—in time and space.
Because it involves
such disparate scientific fields, the Astrobiology Revolution is dissolving
many boundaries between disciplines of science. A paleontologist’s discovery of
a new life form from billion-year-old rocks in Africa is of major consequence
to a planetary geologist studying Mars. A submarine probing the bottom of the
sea finds chemicals that affect the calculations of a planetary astronomer. A
microbiologist sequencing a string of genes influences the work of an
oceanographer studying the frozen oceans of Europa (one of Jupiter’s moons) in
the lab of a planetary geologist. The most unlikely alliances are forming,
breaking down the once-formidable academic barriers that have locked science
into rigid domains. New findings from diverse fields are being brought to bear on
the central questions of astrobiology: How common is life in the universe?
Where can it survive? Will it leave a fossil record? How complex is it? There
are bouts of optimism and pessimism; E-mails fly; conferences are hastily
assembled; research programs are rapidly redirected as discoveries mount. The
excitement is visceral, powerful, dizzying, relentless. The practitioners are
captivated by a growing belief: Life is present beyond Earth.
The great surprise
of the Astrobiology Revolution is that it has arisen in part from the ashes of
disappointment and scientific despair. As far back as the 1950s, with the
classic Miller–Urey experiments showing that organic matter could be readily
synthesized in a test tube (thus mimicking early Earth environments),
scientists thought they were on the verge of discovering how life originated.
Soon thereafter, amino acids were discovered in a newly fallen meteorite,
showing that the ingredients of life occurred in space. Radio telescope
observations soon confirmed this, revealing the presence of organic material in
interstellar clouds. It seemed that the building blocks of life permeated the
cosmos. Surely life beyond Earth was a real possibility.
Page xxi
When the Viking I
spacecraft approached Mars in 1976, there was great hope that the first
extraterrestrial life—or at least signs of it—would be found (see Figure I.1).
But Viking did not find life. In fact, it found conditions hostile to organic matter:
extreme cold, toxic soil and lack of water. In many people’s minds, these
findings dashed all hopes that extraterrestrial life would ever be found in the
solar system. This was a crushing blow to the nascent field of astrobiology.
Figure I.1 Percival
Lowell’s 1908 globe of Mars. Some thought that the linear features were
irrigation canals built by Martians.
At about this time
there was another major disappointment: The first serious searches for
“extrasolar” planets all yielded negative results. Although many astronomers
believed that planets were probably common around other stars, this remained only
abstract speculation, for searches using Earth based telescopes gave no
indication that any other planets existed outside our own solar system. By the
early 1980s, little hope remained that real progress in this field would occur,
for there seemed no way that we could ever detect worlds orbiting other stars.
Page xxi
Yet it was also at
this time that a new discovery paved the way for the interdisciplinary methods
now commonly used by astrobiologists. The 1980 announcement that the dinosaurs
were not wiped out by gradual climate
change (as was so long thought) but rather succumbed to the catastrophic
effects of the collision of a large comet with Earth 65 million years ago, was
a watershed event in science. For the first time, astronomers, geologists, and
biologists had reason to talk seriously with one another about a scientific
problem common to all. Investigators from these heretofore separate fields
found themselves at the same table with scientific strangers—all drawn there by
the same question: Could asteroids and comets cause mass extinction? Now, 20
years later, some of these same participants are engaged in a larger quest: to
discover how common life is on planets beyond Earth.
The indication that
there was no life on Mars and the failure to find extrasolar planets had damped
the spirits of those who had begun to think of themselves as astrobiologists.
But the field involves the study of life on Earth as well as in space, and it
was from looking inward—examining this planet— that the sparks of hope were
rekindled. Much of the revitalization of astrobiology came not from
astronomical investigation but from the discovery, in the early 1980s, that
life on Earth occurs in much more hostile environments than was previously
thought. The discovery that some microbes live in searing temperatures and
crushing pressures both deep in the sea and deep beneath the surface of our
planet was an epiphany: If life survives under such conditions here, why not
on—or in—other planets, other bodies of our solar system, or other plants and
moons of far-distant stars?
Just knowing that
life can stand extreme environmental conditions, however, is not enough to
convince us that life is actually there.
Not only must life be able to live in
the harshness of a Mars, Venus, Europa, or Titan; it must also have been able
either to originate there or to
travel there. Unless it can be shown that life can form, as well as live, in extreme environments,
there is little hope that even simple life is widespread in the Universe. Yet
here, too, revolutionary new findings lead to optimism. Recent discoveries by
geneticists have shown that the most primitive forms of life on Earth—those
that we might expect to be close to the first life to have formed on our planet—
are exactly those tolerant life forms that are found in extreme environments.
This suggests to some biologists that life on Earth originated under conditions of great heat, pressure, and lack of
oxygen—just the sorts of conditions found elsewhere in space. These findings
give us hope that life may indeed be widely distributed, even in the harshness
of other planetary systems.
Page xxii
The fossil record
of life on our own planet is also a major source of relevant information. One
of the most telling insights we have gleaned from the fossil record is that
life formed on Earth about as soon as environmental conditions allowed its
survival. Chemical traces in the most ancient rocks on Earth’s surface give
strong evidence that life was present nearly 4 billion years ago. Life thus
arose here almost as soon as it theoretically could. Unless this occurred
utterly by chance, the implication is that nascent life itself forms— is
synthesized from nonliving matter—rather easily. Perhaps life may originate on any planet as soon as temperatures cool
to the point where amino acids and proteins can form and adhere to one another
through stable chemical bonds. Life at this level may not be rare at all.
The skies too have
yielded astounding new clues to the origin and distribution of life in the
Universe. In 1995 astronomers discovered the first extrasolar planets orbiting
stars far from our own. Since then, a host of new planets have been discovered,
and more come to light each year.
For a while, some even thought we had found the first record of extraterrestrial
life. A small meteorite discovered in the frozen icefields of Antarctica
appears to be one of many that originated on Mars, and at least one of
these may be carrying the fossilized remains of bacteria-like organisms of extraterrestrial
origin. The 1996 discovery was a bombshell. The President of
the United States announced the story in the White House, and the event
triggered an avalanche of new effort and resolve to find life beyond Earth. But
evidence—at least from this particular meteorite—is highly controversial.
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All of these discoveries suggest a similar conclusion: Earth may not be
the only place in this galaxy—or even in this solar system—with life. Yet if
other life is indeed present on planets or moons of our solar system, or on fardistant
planets circling other stars in the Universe, what kind of life is it?
What, for example, will be the frequency of complex metazoans, organisms with
multiple cells and integrated organ systems, creatures that have some sort of
behavior—organisms that we call animals? Here too a host of recent discoveries
have given us a new view. Perhaps the most salient insights come, again,
from Earth’s fossil record.
New ways of more accurately dating evolutionary advances recognized
in the Earth’s fossil record, coupled with new discoveries of previously unknown
fossil types, have demonstrated that the emergence of animal life on
this planet took place later in time, and more suddenly, than we had suspected.
These discoveries show that life, at least as seen on Earth, does not
progress toward complexity in a linear fashion but does so in jumps, or as a
series of thresholds. Bacteria did not give rise to animals in a steady progression.
Instead, there were many fits and starts, experiments and failures. Although
life may have formed nearly as soon as it could have, the formation of
animal life was much more recent and protracted. These findings suggest that
complex life is far more difficult to arrive at than evolving life itself and that
it takes a much longer time period to achieve.
It has always been assumed that attaining the evolutionary grade we call
animals would be the final and decisive step: that once this level of evolution
was achieved, a long and continuous progression toward intelligence should
occur. However, another insight of the Astrobiological Revolution has been
that attaining the stage of animal life is one thing, but maintaining that level is
quite something else. New evidence from the geological record has shown
that once it has evolved, complex life is subject to an unending succession of
planetary disasters that create what are known as mass extinction events.
These rare but devastating events can reset the evolutionary timetable and
destroy complex life, while sparing simpler life forms. Such discoveries again
suggest that the conditions hospitable to the evolution and existence of complex
life are far more specific than those that allow life’s formation. On some planets, then, life
might arise and animals eventually evolve—only to be quickly destroyed by a
global catastrophe.
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To test the Rare
Earth Hypothesis—the paradox that life may be nearly everywhere but complex
life almost nowhere—may ultimately require travel to the distant stars. We
cannot yet journey much beyond our own planet, and the vast distances that
separate us from even the nearest stars may prohibit us from ever exploring
planetary systems beyond our own. Perhaps this view is pessimistic, and we will
ultimately find a way to travel much faster (and thus farther), through worm
holes or other unforeseen methods of interstellar travel, enabling us to
explore the Milky Way and perhaps other galaxies as well.
Let’s assume that we
do master interstellar travel of some sort and begin the search for life on
other worlds. What types of worlds will harbor not just life, but complex life
equivalent to the animals of Earth? What sorts of planets or moons should we
look for? Perhaps the best way to search is simply to look for planets that
resemble Earth, which is so rich with life. Do we have to duplicate this planet
exactly to find animal life, though? What is it about our solar system and
planet that has allowed the rise of complex life and nourished it so well?
Addressing this issue in the pages ahead should help us answer the other
questions we have posed.
Rare Planet?
If we cast off our
bonds of subjectivity about Earth and the solar system, and try to view them
from a truly “universal” perspective, we also begin to see aspects of Earth and
its history in a new light. Earth has been orbiting a star with relatively constant
energy output for billions of years. Although life may exist even on the
harshest of planets and moons, animal life—such as that on Earth—not only needs
much more benign conditions but also must have those conditions present and
stable for great lengths of time. Animals as we know them require oxygen. Yet
it took about 2 billion years for enough oxygen to be produced to allow all
animals on Earth. Had our sun’s energy output experienced too much variation
during that long period of development (or even afterward), there would
have been little chance of animal life evolving on this planet. On worlds that
orbit stars with less consistent energy output, the rise of animal life would
be far chancier. It is difficult to conceive of animal life arising on planets
orbiting variable stars, or even on planets orbiting stars in double or triple
stellar systems, because of the increased chances of energy fluxes sterilizing
the nascent life through sudden heat or cold. And even if complex life did
evolve in such planetary systems, it might be difficult for it to survive for
any appreciable time.
Page xxv
Our planet was also
of suitable size, chemical composition, and distance from the sun to enable
life to thrive. An animal-inhabited planet must be a suitable distance from the
star it orbits, for this characteristic governs whether the planet can maintain
water in a liquid state, surely a prerequisite for animal life as we know it.
Most planets are either too close or too far from their respective stars to
allow liquid water to exist on the surface, and although many such planets
might harbor simple life, complex animal life equivalent to that on Earth
cannot long exist without liquid water.
Another factor
clearly implicated in the emergence and maintenance of higher life on Earth is
our relatively low asteroid or comet impact rate. The collision of asteroids
and comets with a planet can cause mass extinctions, as we have noted. What
controls this impact rate? The amount of material left over in a planetary
system after formation of the planets influences it: The more comets and
asteroids there are in planet-crossing orbits, the higher the impact rate and
the greater the chance of mass extinctions due to impact. Yet this may not be
the only factor. The types of planets in a system might also affect the impact
rate and thus play a large and unappreciated role in the evolution and
maintenance of animals. For Earth, there is evidence that the giant planet
Jupiter acted as a “comet and asteroid catcher,” a gravity sink sweeping the
solar system of cosmic garbage that might otherwise collide with Earth. It thus
reduced the rate of mass extinction events and so may be a prime reason why higher
life was able to form on this planet and then maintain itself. How common are Jupiter-sized planets?
In our solar
system, Earth is the only planet (other than Pluto) with a moon of such
appreciable size compared to the planet it orbits, and it is the only planet with
plate tectonics, which causes continental drift. As we will try to show, both
of these attributes may be crucial in the rise and persistence of animal life.
Page xxvi
Perhaps even a
planet’s placement in a particular region of its home galaxy plays a major
role. In the star-packed interiors of galaxies, the frequency of supernovae and
stellar close encounters may be high enough to preclude the long and stable
conditions apparently required for the development of animal life. The outer
regions of galaxies may have too low a percentage of the heavy elements
necessary to build rocky planets and to fuel the radioactive warmth of
planetary interiors. The comet influx rate may even be affected by the nature
of the galaxy we inhabit and by our solar system’s position in that galaxy. Our
sun and its planets move through the Milky Way galaxy, yet our motion is
largely within the plane of the galaxy as a whole, and we undergo little
movement through the spiral arms. Even the mass of a particular galaxy might
affect the odds of complex life evolving, for galactic size correlates with its
metal content. Some galaxies, then, might be far more amenable to life’s origin
and evolution than others. Our star—and our solar system—are anomalous in their
high metal content. Perhaps
our very galaxy is unusual.
Ever since Polish
astronomer Nicholas Copernicus plucked it from the center of the Universe and
put it in orbit around the sun, Earth has been periodically trivialized. We
have gone from the center of the Universe to a small planet orbiting a small,
undistinguished star in an unremarkable region of the Milky Way galaxy—a view
now formalized by the so-called Principle of Mediocrity, which holds that we are
not the one planet with life but one of many. Various estimates for the number
of other intelligent civilizations range from none to 10 trillion.
Page xxvii
If it is found to
be correct, however, the Rare Earth Hypothesis will reverse that decentering
trend. What if the Earth, with its cargo of advanced animals, is virtually
unique in this quadrant of the galaxy—the most diverse planet, say, in the
nearest 10,000 light-years? What if it is utterly unique: the only planet with
animals in this galaxy or even in the visible Universe, a bastion of animals
amid a sea of microbe-infested worlds? If that is the case, how much greater
the loss the Universe sustains for each species of animal or plant driven to
extinction through the careless stewardship of Homo sapiens?
Welcome
aboard.
Page xxviii
DEAD
ZONES OF
THE UNIVERSE
Early Universe The most distant known galaxies are too young
to have enough metals for formation of Earth-size inner planets. Hazards
include energetic quasar-like activity and frequent supernova explosions.
Globular clusters Although they
contain up to a million stars they are too metal-poor to have inner planets as
large as Earth. Solar-mass stars have evolved to giants that are too hot for
life on inner planets. Stellar
encounters perturb outer planet orbits.
Elliptical galaxies Stars are too
metal-poor. Solar-mass stars have evolved into giants that are too hot for life
on inner planets.
Small galaxies Most stars
are too metal-poor.
Centers of galaxies Energetic
processes impede complex life.
Edges of galaxies Many stars are too metal-poor.
Page xxix
Planetary systems with Inward spiral of giant planets drives the
inner planets “hot Jupiters” into the central star.
Planetary systems with Environments too unstable for higher
life.
giant
planets eccentric
orbits Some planets lost to space.
Future stars Uranium, potassium and thorium are perhaps too rare
to provide sufficient heat to drive plate tectonics.
Page xxx
RARE EARTH FACTORS
Right distance from star
Habitat for complex
life.
Liquid water near
surface.
Far enough to avoid tidal lock.
Right mass of star
Long
enough lifetime.
Not
too much ultraviolet.
Stable planetary orbits
Giant planets do
not create
orbital chaos.
Right planetary mass
Retain atmosphere
and ocean.
Enough heat for plate
tectonics. Solid/molten core.
Jupiter-like neighbor
Clear out comets
and asteroids.
Not too close, not too far
A Mars
Small neighbor as possible life
source to seed
Earth-like planet, if
needed.
Plate tectonics
CO2 –silicate
thermostat. Build up
land mass. Enhance
biotic diversity. y. Enable
magnetic field.
Ocean
Not too much. Not
too little.
Large Moon
Right
distance. Stabilizes tilt.
Page
xxxi
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