Introduction to The Privileged Planet

Discovery is seeing what everyone else saw and thinking what no one thought.

Albert von Szent-Györgyi

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. 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.

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. 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.” Indeed, when the majority of Earth’s citizens look out at the wonders of nature or Apollo 8’s awe-inspiring Earthrise 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, 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.

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. Indeed, 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.” 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.

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. 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.

Although attractive to those of us nurtured on Star Trek and 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. 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. Ward 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. 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.” 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. 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. 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.

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