THE BOOK Privileged Planet—— a large thick book that is technical science, in showing blue planet Earth maybe very privileged and rare in the universe - Keith Hunt
by Gonzalez and Richards
PRAISE FOR
The Privileged Planet
"In a book of magnificent sweep and daring, Guillermo Gonzalez and Jay Richards drive home the argument that the old cliche of no place like home is eerily true of Earth. Not only that, but if the scientific method were to emerge anywhere, Earth is about as suitable as you can get. Gonzalez and Richards have flung down the gauntlet. Let the debate begin; it is a question that involves us all."
— Simon Conway Morris, author of Life's Solution: Inevitable
Humans in a Lonely Universe
"This thoughtful, delightfully contrarian book will rile up those who believe the 'Copernican Principle' is an essential philosophical component of modern science. Is our universe designedly congenial to intelligent, observing life? Passionate advocates of the search for extraterrestrial intelligence (SETI) will find much to ponder in this carefully documented analysis."
— Owen Gingerich, Harvard-Smithsonian Center for Astrophysics,
author of The Book Nobody Read: Chasing the Revolution of Nicolaus Copernicus
"Not only have Guillermo Gonzalez and Jay Richards written a book with a remarkable thesis, they have constructed their argument on an abundance of evidence and with a cautiousness of statement that make their volume even more remarkable. In my opinion, The Privileged Planet deserves very careful attention."
— Michael J. Crowe, Cavanaugh Professor Emeritus at the Univer
sity of Notre Dame and author of The Extraterrestrial Life Debate,
"The Privileged Planet will surely rattle, if not finally dislodge, a pet assumption held by many interpreters of modern science: the so-called Copernican Principle. Gonzalez's and Richards's argument is so carefully and moderately presented that any reasonable critique of it must itself address the astonishing evidence. I expect this book to renew the whole scientific and philosophic debate about Earth's cosmic significance. It is a high-class piece of work that deserves the widest possible audience."
— Dennis Danielson, Professor of English at the University of British Columbia and editor of The Book of the Cosmos
The Privileged Planet
How Our Place in the Cosmos Is designed for Discovery
Guillermo Gonzalez and Jay W. Richards
Table of Contents
Foreword xi
Introduction xvii
Section 1. Our Local Environment
Chapter 1: Wonderful Eclipses 1
Chapter 2: At Home on a Data Recorder 21
Chapter 3: Peering Down 45
Chapter 4: Peering Up 65
Chapter 5: The Pale Blue Dot in Relief 81
Chapter 6: Our Helpful Neighbors 103
Section 2. The Broader Universe
Chapter 7: Star Probes 119
Chapter 8: Our Galactic Habitat 143
Chapter 9: Our Place in Cosmic Time 169
Chapter 10: A Universe Fine-Tuned for Life and Discovery 195
Section 3. Implications
Chapter 11: The Revisionist History of the Copernican Revolution 221
Chapter 12: The Copernican Principle 247
Chapter 13: The Anthropic Disclaimer 259
Chapter 14: SETI and the Unraveling of the Copernican Principle.... 275
Chapter 15: A Universe Designed for Discovery 293
Chapter 16: The Skeptical Rejoinder 313
Conclusion: Reading the Book of Nature 331
Appendix A: The Revised Drake Equation 337
Appendix B: What about Panspermia? 343
Notes- 347
Acknowledgments 413
Figure Credits 415
Index 417
Foreword
The gratuitous beauty of the starry heavens above. The chimerical appearance of rainbows. The austere splendor of solar eclipses. These inspire people in every time and place. For most of human history they were also mysteries.
In a sense, we've now removed the mysteries. Scientists routinely measure the distances to the stars. We know how sunlight passing through millions of suspended water droplets in the atmosphere produces a rainbow. We can predict the timing and location of solar eclipses years in advance anywhere on Earth to within one second.
Yet, these discoveries point to even deeper mysteries. Why is our world, from our local and galactic environment to the constants of physics, set up so we can see the stars, rainbows, and solar eclipses? After all, our ability to see these things is not logically required for our existence. Surely, the universe could have been otherwise.
In 2004, when this book was first published, we connected the dots. We argued that those rare places in the universe best suited for complex life are also the places best suited for scientific discovery. Moreover, we argued that this is evidence of a cosmic conspiracy, rather than a mere coincidence. We set out to test this hypothesis against the best evidence from natural science. We didn't try to immunize our argument against future discoveries. Instead, we put it at risk by making predictions about future discoveries.
So, how has our argument fared? It would require a new edition to take into account all the new evidence. But here's a brief survey of what nature has revealed over the last decade and a half.
We opened the book by pondering why we can observe perfect solar eclipses. We enjoy them because the Sun and Moon appear to be the same size from the Earth's surface. While this coincidence has been long known, it still befuddles scholars. Seven years after The Privileged Planet was published, John Gribbin observed in Alone in the Universe: Why Our Planet Is Unique:
Just now the Moon is about 400 times smaller than the Sun, but the Sun is 400 times farther away than the Moon, so that they look the same size on the sky. At the present moment of cosmic time, during an eclipse, the disc of the Moon almost exactly covers the disc of the Sun. In the past the Moon would have looked much bigger and would have completely obscured the Sun during eclipses; in the future, the Moon will look much smaller from Earth and a ring of sunlight will be visible even during an eclipse. Nobody has been able to think of a reason why intelligent beings capable of noticing this oddity should have evolved on Earth just at the time that the coincidence was there to be noticed. It worries me, but most people seem to accept it as just one of those things.1
Obviously, either Gribbin didn't read The Privileged Planet, or didn't want to admit that he had.
Although we explored this topic in detail, we did leave two issues unresolved. First, while we showed that solar eclipses viewed from the surface of the Earth are better than those viewed from other planets with moons, we didn't consider solar eclipses viewed from other moons. So-called mutual eclipses occur when one moon eclipses another moon in orbit around the same planet—as they do in some outer planets such as Jupiter that have many moons. One of us (Guillermo) studied mutual eclipses in the solar system and found that Earth's solar eclipses are indeed the best. The results of this research were published in 2009.2
While we explained why the apparent sizes of the Sun and Moon closely match, others have since noted other important ramifications. The same year that The Privileged Planet came out, Dave Waltham of the University of London published a paper arguing that the relatively large size of the Moon to the Earth is explained by the Moon's mass being finely calibrated to stabilize the Earth's rotational axis.3 In a later paper, he buttressed the case that the Moon's size is also perfect for stabilizing the Earth's climate.4
In this book we discuss some of the ways that Earth is a good platform for scientific discovery. For instance, we argue that Earth's accessible, abundant, and diverse minerals and fossil fuels prepared the way for technology. Robert Hazen, a geologist at George Mason University and at the Carnegie Institution's Geophysical Laboratory, has quantified just how special Earth's minerals resources are. In his 2012 book, The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet, Hazen notes that Earth has the greatest diversity of mineral species of any body in the Solar System.5 Earth has more than 4,600 mineral species. Mars has about 500 and Venus about 1,000. What's more, Hazen discovered that life processes formed about two-thirds of Earth's mineral species.
There is, of course, much more to be said on the topic. Our discussion is, at best, an appetizer. Michael Denton's recent books in his Privileged Species series—Fire-Maker: How Humans Were Designed to Harness Fire and Transform our Planet (2016),6 The Wonder of Water: Water's Profound Fitness for Life on Earth and Mankind (2017),' and Children of the Light: Astonishing Properties of Sunlight that Make Us Possible (2018),8 —provide the three-course meal. If our treatment merely whets your appetite, then we encourage you to check out Denton's trilogy.
In our own book, we move beyond the Solar System and discuss exo-planets. Our discussion is brief and tentative because, when The Privileged Planet was first published, only about 100 such planets were known.9 The number of confirmed exoplanets has risen steeply since then, doubling about every twenty-seven months. Today, we have confirmed discoveries of more than 4,000 exoplanets.
In 2004, we didn't know whether Earth was uniquely habitable. We still don't know that in 2019. For many people, the sheer number of new planet discoveries may seem to guarantee the presence of other planets like Earth in the Milky Way Galaxy. But other research in astrobiology over the last fifteen years counts against this impression, and has instead affirmed our view that Earth-like planets are very rare.10 Turning to the broader cosmos, we should note that the theory of General Relativity received new confirmations from the first direct measurement of gravity waves in 2015 and the first (synthetic) image of the region around a supermassive black hole in 2019. Why do these discoveries matter? Because General Relativity is the foundational theory for Big Bang cosmology, which points to a beginning. Our conclusion in this book is that we live, not just in what we dub the Cosmic Habitable Age —which is no surprise—but during the best time to do cosmology, because we are best placed to detect the beginning of cosmic expansion.
In 2007 we received support for this idea from an unlikely source. Atheist cosmologist Lawrence Krauss (along with Vanderbilt cosmologist Robert Scherrer) published an award-winning paper on how in the distant future important information about the cosmos will be lost. Their concluding paragraphs are worth quoting at length:
The remarkable cosmic coincidence that we happen to live at the only time in the history of the universe when the magnitude of dark energy and dark matter densities are comparable has been a source of great current speculation, leading to a resurgence of interest in possible anthropic arguments limiting the value of the vacuum energy.... But this coincidence endows our current epoch with another special feature, namely that we can actually infer both the existence of the cosmological expansion, and the existence of dark energy. Thus, we live in a very special time in the evolution of the universe: the time at which we can observa-tionally verify that we live in a very special time in the evolution of the universe!
Observers when the universe was an order of magnitude younger would not have been able to discern any effects of dark energy on the expansion, and observers when the universe is more than an order of magnitude older will be hard-pressed to know that they live in an expanding universe at all, or that the expansion is dominated by dark energy. By the time the longest lived main sequence stars are nearing the end of their lives, for all intents and purposes, the universe will appear static, and all evidence that now forms the basis of our current understanding of cosmology will have disappeared.11
While it had escaped our notice in 2004, Tony Rothman and George F. R. Ellis had thought about alternate worlds where cosmology would go wrong in 1987. "It is even possible," they wrote in the final sentence of their paper, "that universes could exist in which life arises only at times when observations lead to a deceptive understanding of cosmology."12
These all, in one way or another, support elements of our original argument. And in the last year, we've come up with another important element. In brief, the Earth (and the broader Solar System) is an excellent platform for space travel.13 Many factors have converged to permit us to put men on the Moon and send probes to all the planets in the Solar System. Earth contains one of the best chemical rocket propellants (hydrogen plus oxygen) in the form of water. In contrast, inhabitants on planets only a bit more massive than Earth would have a much harder time getting rockets with large payloads into space. People on planets within the habitable zones of less massive host stars would find it much harder to launch interstellar missions. As it happens, our Solar System is in the best place in its 225-million-year orbit around the center of the galaxy for interstellar travel to nearby stars. This is just at the moment when such travel has become conceivable.
When we were working on this book at the dawn of the new millennium, we not only wanted to propose a new hypothesis. We wanted to add to the growing case for purpose and design in the universe. Our argument hinges on the eerie overlap between the conditions for life and for scientific discovery. Those rare places where observers can exist are, as it happens, the best overall places for observing. This pattern, we argue, makes much more sense if the universe is designed for discovery than if it is not. So, far from being anti-science, our design argument implies that the world is made for scientific discovery!
We were, perhaps, a bit naive about the sheer hostility and metaphysical panic this book would provoke among self-appointed defenders of science and atheist religion professors. These hostile attacks have had serious repercussions for us over the years —especially for Guillermo, a research and teaching astronomer.
We won't recount these trials and tribulations here. But we will note that none of these bad faith attacks have touched our argument or its supporting evidence. On the contrary, evidence for our hypothesis has continued to emerge. We're happy to trust its fate to future discoveries, and are grateful that our argument, and this book, continues to be read and debated.
Introduction
The Privileged Planet
Discovery is seeing what everyone else saw and thinking what no one thought. —Albert von Szent-Gyorgyi1
On Christmas Eve, 1968, the Apollo 8 astronauts —Frank Borman, James Lovell, and William Anders —became the first human beings to see the far side of the Moon.2 The moment was as historic as it was perilous: they had been wrested from Earth's gravity and hurled into space by the massive, barely tested Saturn V rocket. Although one of their primary tasks was to take pictures of the Moon in search of future landing sites —the first lunar landing would take place just seven months later—many associate their mission with a different photograph, commonly known as Earthrise. (See Plate 1.)
Emerging from the Moon's far side during their fourth orbit, the astronauts were suddenly transfixed by their vision of Earth, a delicate, gleaming swirl of blue and white, contrasting with the monochromatic, barren lunar horizon.3 Earth had never appeared so small to human eyes, yet was never more the center of attention.
To mark the event's significance and its occurrence on Christmas Eve, the crew had decided, after much deliberation, to read the opening words of Genesis: "In the beginning, God created the heavens and the Earth.”
The reading, and the reverent silence that followed, went out over a live telecast to an estimated one billion viewers, the largest single audience in television history.
In his recent book about the Apollo 8 mission, Robert Zimmerman notes that the astronauts had not chosen the words as parochial religious expression but rather "to include the feelings and beliefs of as many people as possible."4Indeed, when the majority of Earth's citizens look out at the wonders of nature or Apollo 8's awe-inspiring Earth rise image, they see the majesty of a grand design. But a very different opinion holds that our Earthly existence is not only rather ordinary but in fact insignificant and purposeless. In his book Pale Blue Dot, the late astronomer Carl Sagan typifies this view while reflecting on another image of Earth (see Plate 2.), this one taken by Voyager 1 in 1990 from some four billion miles away:
Because of the reflection of sunlight... Earth seems to be sitting in a beam of light, as if there were some special significance to this small world. But it's just an accident of geometry and optics. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.5
But perhaps this melancholy assumption, despite its heroic pretense, is mistaken. Perhaps the unprecedented scientific knowledge acquired in the last century, enabled by equally unprecedented technological achievements, should, when properly interpreted, contribute to a deeper appreciation of our place in the cosmos. In the following pages we hope to substantiate that possibility by means of a striking feature of the natural world, one as widely grounded in the evidence of nature as it is wide-ranging in its implications. Simply stated, the conditions allowing for intelligent life on Earth also make our planet strangely well suited for viewing and analyzing the universe.
The fact that our atmosphere is clear; that our moon is just the right size and distance from Earth, and that its gravity stabilizes Earth's rotation; that our position in our galaxy is just so; that our sun is its precise mass and composition—all of these facts and many more not only are necessary for Earth's habitability but also have been surprisingly crucial to the discovery and measurement of the universe by scientists. Mankind is unusually well positioned to decipher the cosmos. Were we merely lucky in this regard?
Scrutinize the universe with the best tools of modern science and you'll find that a place with the proper conditions for intelligent life will also afford its inhabitants an exceptionally clear view of the universe. Such so-called habitable zones are rare in the universe, and even these may be devoid of life. But if there is another civilization out there, it will also enjoy a clear vantage point for searching the cosmos, and maybe even for finding us.
To put it both more technically and more generally, "measurability" seems to correlate with "habitability."6 Is this correlation simply a strange coincidence? And even if it has some explanation, is it significant? We think it is, not least because this evidence contradicts a popular idea called the Copernican Principle, or the Principle of Mediocrity. This principle is far more than the simple observation that the cosmos doesn't literally revolve around Earth. For many, it is a metaphysical extension of that claim. According to this principle, modern science since Copernicus has persistently displaced human beings from the "center" of the cosmos, and demonstrated that life and the conditions required for it are unremarkable and certainly unintended. In short, it requires scientists to assume that our location, both physical and metaphysical, is unexceptional. And it usually expresses what philosophers call naturalism or materialism —the view that the material world is "all that is, or ever was, or ever will be," as Carl Sagan famously put it.7
Following the Copernican Principle, most scientists have supposed that our Solar System is ordinary and that the emergence of life in some form somewhere other than Earth must be quite likely, given the vast size and great age of the universe. Accordingly, most have assumed that the universe is probably teeming with life. For example, in the early 1960s, astronomer Frank Drake proposed what later became known as the Drake Equation, in which he attempted to list the factors necessary for the existence of extraterrestrial civilizations that could use radio signals to communicate. Three of those factors were astronomical, two were biological, and two were social. They ranged from the rate of star formation to the likely age of civilizations prone to communicating with civilizations on other planets.8 Though highly speculative, the Drake Equation has helped focus the debate, and has become a part of every learned discussion about the possibility of extraterrestrial life. Ten years later, using the Drake Equation, Drake's colleague Carl Sagan optimistically conjectured that our Milky Way galaxy alone might contain as many as one million advanced civilizations.
This optimism found its practical expression in the Search for Extraterrestrial Intelligence, or SETI, a project that scans the skies for radio transmissions containing the "signatures" of extraterrestrial intelligence. SETI seeks real evidence, which, if detected, would persuade most open-minded people of the existence of extraterrestrial intelligence. In contrast, some advocates (and critics) of extraterrestrial intelligence rely primarily on speculative calculations. For instance, probability theorist Amir Aczel recently-argued that intelligent life elsewhere in the universe is a virtual certainty. He is so sure, in fact, that he titled his book Probability One: Why There Must Be Intelligent Life in the Universe.9
Although attractive to those of us nurtured on Star Trefa'nd other fascinating interstellar science fiction, such certainty is misplaced. Recent discoveries from a variety of fields and from the new discipline of astrobiology have undermined this sanguine enthusiasm for extraterrestrials. Mounting evidence suggests that the conditions necessary for complex life are exceedingly rare, and that the probability of them all converging at the same place and time is minute. A few scientists have begun to take these facts seriously. For instance, in 1998 Australian planetary scientist Stuart Ross Taylor challenged the popular view that complex life was common in the universe. He emphasized the importance of the rare, chance events that formed our Solar System, with Earth nestled fortuitously in its narrow habitable zone.10 Contrary to the expectations of most astronomers, he argued that we should not assume that other planetary systems are basically like ours.
Similarly, in their important book Rare Earth: Why Complex Life Is Uncommon in the Universe,"paleontologist Peter Ward and astronomer Donald Brownlee, both of the University of Washington, have moved the discussion of these facts from the narrow confines of astrobiology to the wider educated public. 12Ward and Brownlee focus on the many improbable astronomical and geological factors that united to give complex life a chance on Earth.
These views clearly challenge the Copernican Principle. But while challenging the letter of the principle, Taylor, Ward, and Brownlee have followed its spirit. They still assume, for instance, that the origin of life is basically a matter of getting liquid water in one place for a few million years. As a consequence, they continue to expect "simple" microbial life to be common in the universe. More significant, they all keep faith with the broader perspective that undergirds the Copernican Principle in its most expansive form. They argue that although Earth's complex life and the rare conditions that allow for it are highly improbable, perhaps even unique, these conditions are still nothing more than an unintended fluke.13 In a lecture after the publication of Rare Earth, Peter Ward remarked, "We are just incredibly lucky. Somebody had to win the big lottery, and we were it."
But we believe there is a better explanation. To see this, we have to consider these recent insights about habitability—the conditions necessary for complex life — in tandem with those concerning measurability. Measurability refers to those features of the universe as a whole, and especially to our particular location in it—in both space and time — that allow us to detect, observe, discover, and determine the size, age, history, laws, and other properties of the physical universe. It's what makes scientific discovery possible. Although scientists don't often discuss it, the degree to which we can "measure" the wider universe — not just our immediate surroundings — is surprising. Most scientists presuppose the measurability of the physical realm: it's measurable because scientists have found ways to measure it. Read any book on the history of scientific discovery and you'll find magnificent tales of human ingenuity, persistence, and dumb luck. What you probably won't see is any discussion of the conditions necessary for such feats, conditions so improbably fine-tuned to allow scientific discoveries that they beg for a better explanation than mere chance.
Our argument is subtle, however, and requires a bit of explanation. First, we aren't arguing that every condition for measurability is uniquely and individually optimized on Earth's surface. Nor are we saying that it's always easy to measure and make scientific discoveries. Our claim is that Earth's conditions allow for a stunning diversity of measurements, from cosmology and galactic astronomy to stellar astrophysics and geophysics; they allow for this rich diversity of measurement much more so than if Earth were ideally suited for, say, just one of these sorts of measurement.
For instance, intergalactic space, far removed from any star, might be a better spot for measuring certain distant astronomical phenomena than the surface of any planet with an atmosphere, since it would contain less light and atmosphere pollution. But its value for learning about the details of star formation and stellar structure, or for discovering the laws of celestial mechanics, would be virtually worthless. Likewise, a planet in a giant molecular cloud in a spiral arm might be a great place to learn about star formation and interstellar chemistry, but observers there would find the distant universe to be hidden from view. In contrast, Earth offers surprisingly good views of the distant and nearby universe while providing an effective platform for discovering the laws of physics.
When we say that habitable locations are "optimal" for making scientific discoveries, we have in mind an optimal balance of competing conditions. Engineer and historian Henry Petroski calls this constrained optimization in his illuminating book Invention by Design: "All design involves conflicting objectives and hence compromise, and the best designs will always be those that come up with the best compromise."14 To take a familiar example, think of the laptop computer. Computer engineers seek to design laptops that have the best overall compromise among various conflicting factors. Large screens and keyboards, all things being equal, are preferable to small ones. But in a laptop, all things aren't equal. The engineer has to compromise between such matters as CPU speed, hard drive capacity, peripherals, size, weight, screen resolution, cost, aesthetics, durability, ease of production, and the like. The best design will be the best compromise. (See Figure 0.1) Similarly, if we are to make discoveries in a variety of fields from geology to cosmology, our physical environment must be a good compromise of competing factors, an environment where a whole host of "thresholds" for discovery are met or exceeded.
For instance, a threshold must be met for detecting the cosmic background radiation that permeates the universe as a result of the Big Bang. (Detecting something is, of course, a necessary condition for measuring it.) If our atmosphere or Solar System blocked this radiation, or if we lived at a future time when the background radiation had completely disappeared, our environment would not reach the threshold needed to discover and measure it. As it is, however, our planetary environment meets this requirement. At the same time, intergalactic space might give us a slightly better "view" of the cosmic background radiation, but the improvement would be drastically offset by the loss of other phenomena that can't be measured from deep space, such as the information-rich layering processes on the surface of a terrestrial planet. An optimal location for measurability, then, will be one that meets a large and diverse number of such thresholds for measurability, and which combines a large and diverse number of items that need measuring. This is the sense in which we think our local environment is optimal for making scientific discoveries.b In a very real sense the cosmos, our Solar System, and our exceptional planet are themselves a laboratory, and Earth is the best bench in the lab.
Even more mysterious than the fact that our location is so congenial to diverse measurement and discovery is that these same conditions appear to correlate with habitability. This is strange, because there's no obvious reason to assume that the very same rare properties that allow for our existence would also provide the best overall setting to make discoveries about the world around us. We don't think this is merely coincidental. It cries out for another explanation, an explanation that suggests there's more to the cosmos than we have been willing to entertain or even imagine.
Section 1
Our Local
Environment
…..THE PHYSICS OF THE MOON
First, consider a little-known fact: A large moon stabilizes the rotation axis of its host planet, yielding a more stable, life-friendly climate. Our Moon keeps Earth's axial tilt, or obliquity—the angle between its rotation axis and an imaginary axis perpendicular to the plane in which it orbits the Sun — from varying over a large range.6 A larger tilt would cause larger climate fluctuations.' At present, Earth tilts 23.5 degrees, and it varies from 22.1 to 24.5 degrees over several thousand years. To stabilize effectively, the Moon's mass must be a substantial fraction of Earth's mass. Small bodies like the two potato-shaped moons of Mars, Phobos and Deimos, won't suffice. If our Moon were as small as these Martian moons, Earth's tilt would vary not 3
……
Figure 1.2: Earth's axis currently tilts 23.5 degrees from a line perpendicular to the plane formed by the Earth's orbit around the Sun, and varies a modest 2.5 degrees over thousands of years. Such stability is due to the action of the Moon's gravity on Earth. Without a large Moon, Earth's tilt could vary by 30 degrees or more, even 60 degrees, which would make Earth less habitable.
……
degrees but more than 30 degrees. That might not sound like anything to fuss over, but tell that to someone trying to survive on an Earth with a 60-degree tilt. When the North Pole was leaning sunward through the middle of the summer half of the year, most of the Northern Hemisphere would experience months of perpetually scorching daylight. High northern latitudes would be subjected to searing heat, hot enough to make Death Valley in July feel like a shady spring picnic. Any survivors would suffer viciously cold months of perpetual night during the other half of the year. But it's not just a large axial tilt that causes problems for life. On Earth, a small tilt might lead to very mild seasons, but it would also prevent the wide distribution of rain so hospitable to surface life. With a 23.5-degree axial tilt, Earth's wind patterns change throughout the year, bringing seasonal monsoons to areas that would otherwise remain parched. Because of this, most regions receive at least some rain. A planet with little or no tilt would probably have large swaths of arid land.
The Moon also assists life by raising Earth's ocean tides. The tides mix nutrients from the land with the oceans, creating the fecund intertidal zone, where the land is periodically immersed in seawater. (Without the Moon, Earth's tides would be only about one-third as strong; we would experience only the regular solar tides.) Until very recently, oceanogra-phers thought that all the lunar tidal energy was dissipated in the shallow areas of the oceans. It turns out that about one-third of the tidal energy is spent along rugged areas of the deep ocean floor, and this may be a main driver of ocean currents.8 These strong ocean currents regulate the climate by circulating enormous amounts of heat.9 If Earth lacked such lunar tides, Seattle would look more like northern Siberia than the lush, temperate "Emerald City.”………
Habitability varies dramatically, depending on the sizes of a planet and its host star and their separation. There are good reasons to believe that a star similar to the Sun is necessary for complex life.15 A more massive star has a shorter lifetime and brightens more rapidly. A less massive star radiates less energy, so a planet must orbit closer in to keep liquid water on its surface. (The band around a star wherein a terrestrial planet must orbit to maintain liquid water on its surface is called the Circumstellar Habitable Zone.) Orbiting too close to the host star, however, leads to rapid tidal locking, or "rotational synchronization," in which one side of the planet perpetually faces its host star. (The Moon, incidentally, is so synchronized in its orbit around Earth.) This leads to brutal temperature differences between the day and night sides of a planet. Even if the thin boundary between day and night, called the terminator, were habitable, a host of other problems attend life around a less massive star (more on this in Chapter Seven).
If a planet's moon were farther away, it would need to be bigger than our Moon to generate similar tidal energy and properly stabilize the planet.14 Since the Moon is already anomalously large compared with Earth, a bigger moon is even less likely. A smaller moon would have to be closer, but then it would probably be less round, creating other problems.
As for the host planet, it needs to be about Earth's size to maintain plate tectonics, to keep some land above the oceans, and to retain an atmosphere (more on these requirements in Chapter Three). To maintain a stable planetary tilt, a planet needs a minimum tidal force from a moon. A larger planet would require a larger moon. So indirectly, even the size of Earth itself is relevant to the geometry of the Earth-Sun-Moon system and its contribution to Earth's habitability. In short, the requirements for complex life on a terrestrial planet strongly overlap the requirements for observing total solar eclipses……..
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THIS IS JUST A LITTLE ABOUT THE UNIQUENESS OF OUR MOON—— A VERY IMPORTANT PART OF EVERYTHING THAT MAKES OUR PLANET PRIVILEGED FOR ANIMAL AND HUMAN LIFE. THE AUTHORS SHOW FROM SCIENCE AND REASONABILITY, OUR EARTH COULD VERY WELL BE UNIQUE IN THE UNIVERSE - Keith Hunt
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