But unbridled enthusiasm for ETs has obscured the obvious. Liquid water is a necessary condition for life, but not nearly a sufficient one. Life doesn’t just spring up spontaneously out of water. It’s not as if life has only one instruction in its recipe: “Just add water.” In fact, what is striking is not that Mars once had lots of liquid water on its surface, but that, although it was probably bathed by Earthly microbes during the same time, there’s no evidence that life prospered on the Red Planet. That’s the most important but almost overlooked lesson of our study of Mars.
Consider how our expectations for Mars have diminished in the last century. In 1908, H.G. Wells published a non-fiction article in Cosmopolitan magazine about the civilization that he thought inhabited the planet. Around the same time, Percival Lowell built an observatory to gather evidence of that civilization--canals built by Martian engineers. (The “canals” turned out to be the result of optical illusions and an active imagination.) Even as late as the 1950’s, some scientists thought Mars was home to intelligent life.
In the decades that followed, the Mariner, Viking, and Sojourner missions to Mars revealed a barren and hostile environment. This dampened enthusiasm for Martian civilizations. We are now reduced to looking, not even for microbial Martians, but for the existence of one of life’s necessary conditions sometime in the distant past. Mars has more in common with Earth than does any other known body in the universe. Yet, so far as we know, it still doesn’t harbor life. And the life Earth sent there clearly didn’t “terraform” the planet. Any remaining life would be the microscopic vestiges of a dying planet.
Yet the expectation that life is everywhere lives on. Why this pervasive opinion among scientists, the media, and the public at large? Among scientists, at least, it comes not so much from scientific discovery as from an assumption called the Copernican Principle or Principle of Mediocrity. Martian life enthusiast Percival Lowell summed up the basic idea in 1895: “That we are the sum and substance of the capabilities of the cosmos is something so preposterous as to be exquisitely comic. . . . [Man] merely typifies in an imperfect way what is going on elsewhere, and what, to a mathematical certainty, is in some corners of the cosmos indefinitely excelled.” According to Carl Sagan, Lowell’s enthusiasm “turned on all the eight-year olds who came after him, and who eventually turned into the present generation of astronomers.”
In his book Pale Blue Dot, Sagan reflected on a famous image of Earth taken by a Voyager satellite from some four billion miles away. He made clear that the Copernican Principle is no mere scientific hypothesis, but an offshoot of the materialistic worldview:
Following the Copernican Principle, most scientists have supposed that our Solar System is typical and that the origin and evolution of life must be quite likely, given the vast size and great age of the universe. Accordingly, most have assumed that the universe is probably teeming not just with life, but complex, intelligent life.
But the scientific evidence has stubbornly pointed in the opposite direction. We’re now learning how much must go right to a get a habitable planet. The list gets longer all the time. Complex life in particular probably needs many of the things that we Earthlings enjoy: a rocky terrestrial planet similar in size and composition to the Earth, with plate tectonics to recycle nutrients, and the right kind of atmosphere; a large, well placed moon to contribute to tides and stabilize the tilt of the planet’s axis. That planet needs to be just the right distance from the right kind of single star, in a nearly circular orbit--to maintain liquid water on its surface.
It also needs a home within a stable planetary system that includes some outlying giant planets to protect the inner system from too many deadly comet impacts. That planetary system must be nestled in a safe neighborhood in the right kind of galaxy, with enough heavy elements to build terrestrial planets. And that planet will need to form during the narrow habitable window of cosmic history. (This is to say nothing of having a universe with a fine-tuned set of physical laws to make stars, planets, and people possible in the first place. But that’s another long and complicated story.)
Since the mid-1990s, astronomers have been able to detect planets around other Sun-like stars. And they have taught us an important, if unadvertised lesson. Planetary systems are not all alike. In fact, mounting evidence suggests that the conditions needed for complex life are exceedingly rare, the probability of them all occurring at the same place and time, minuscule.
So argued Peter Ward and Donald Brownlee in their best-selling book Rare Earth: Why Complex Life is Uncommon in the Universe. Ward and Brownlee obviously challenge the letter of the Copernican Principle. But they don’t challenge its spirit. Intuitively, you might think that such a precise configuration of life-friendly factors suggests that Earth is part of some cosmic design. Ward and Brownlee, however, argue that although the conditions that allow for complex life are highly improbable, perhaps even unique, these conditions are still nothing more than an unintended fluke. The universe, after all, is a big place, with some 1022 stars in the part we can see. With so many opportunities, maybe at least one habitable planet will turn up just by chance.
But what if we’re not merely the winners of a blind cosmic lottery? What if our existence is the result of a conspiracy rather than a coincidence? Is there any way we could tell? We argue that there is. It turns out that the same rare, finely tuned conditions that allow for intelligent life on Earth also make it strangely well suited for viewing, analyzing and discovering the universe around us.
The fact that we inhabit a terrestrial planet with a clear atmosphere and water on its surface; that our moon is just the right size and distance from Earth to stabilize the tilt of Earth’s rotation axis; that our position in our large spiral galaxy is just so; that our sun is its precise mass and composition: all of these and many more are not only necessary for Earth’s habitability; they also have been surprisingly crucial for scientists to discover the universe.
To put it more technically and more generally, “measurability” seems to correlate with habitability. Measurability refers to those features of the universe as a whole, and especially to our particular location in it--both in 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.
Those rare pockets of habitability in our universe, as it happens, also allow for the most measurement. They’re the best overall places for scientific discovery. This is strange because there’s no obvious reason to assume that the very same rare properties that allow for observers would also provide the best overall setting for observing the world around them.
Of course, justifying such a claim requires a lot of evidence. But a couple of examples should be enough to illustrate what we mean by a “correlation between habitability and measurability.”
A rare convergence of events allows Earthlings to witness not just solar eclipses, but perfect solar eclipses, where the Moon just barely covers the Sun’s bright photosphere. Such eclipses depend on the precise sizes, shapes, and relative distances of the Sun, Moon, and Earth. There’s no law of physics or celestial mechanics that requires the right configuration. In fact, of the more than 65 major moons in our Solar System, ours best matches the Sun as viewed from its planet’s surface, and this is only possible during a fairly narrow window of Earth’s history encompassing the present. The Moon is about 400 times smaller than the Sun. But right now, the Moon is about 400 times closer to the Earth than is the Sun. So, the Moon’s apparent size on the sky matches the Sun’s. Astronomers have noted this odd coincidence for centuries. And, since the Sun appears larger from the Earth than from any other planet with a moon, an Earth-bound observer can discern finer details in the Sun’s chromosphere and corona than from any other planet. This makes our solar eclipses more valuable scientifically.
The recent pictures of solar eclipses sent back from the Opportunity rover on Mars nicely illustrate how much better our solar eclipses are. The two small potato-shaped Martian moons, Deimos and Phobos, appear much too small to cover the Sun’s disk, and they zip across it in less than a minute.
It’s intriguing that the best place to view total solar eclipses in our Solar System is the one time and place where there are observers to see them. It turns out that the precise configuration of Earth, Moon and Sun are also vital to sustaining life on Earth. A moon large enough to cover the Sun stabilizes the tilt of the rotation axis of its host planet, yielding a more stable climate, which is necessary for complex life. The Moon also contributes to Earth’s ocean tides, which increase the vital mixing of nutrients from the land to the oceans. The two moons around Mars are much too small to stabilize its rotation axis.
In addition, it’s only in the so-called Circumstellar Habitable Zone of our Sun--that cozy life friendly ring where water can stay liquid on a planet’s surface--that the Sun appears to be about the same size as the Moon from Earth’s surface. As a result, we enjoy perfect solar eclipses.
That alone seems fishy. But here’s the part that suggests conspiracy rather than quirky coincidence. Our ability to observe perfect solar eclipses has figured prominently in several important scientific discoveries, discoveries that would have been difficult if not impossible on the much more common planets that don’t enjoy such eclipses.
First, these observations helped disclose the nature of stars. Scientists since Isaac Newton (1666) had known that sunlight splits into all the colors of the rainbow when passed through a prism. But only in the 19th century did astronomers observe solar eclipses with spectroscopes, which use prisms. The combination of the man-made spectroscope with the natural experiment provided by eclipses gave astronomers the tools they needed not only to discover how the Sun’s spectrum is produced, but the nature of the Sun itself. This knowledge enabled astronomers to interpret the spectra of the distant stars. So, in a sense, perfect eclipses were a key that unlocked the field of astrophysics.
Second, in 1919, perfect solar eclipses allowed two teams of astronomers, one led by Sir Arthur Eddington, to confirm a prediction of Einstein’s General Theory of Relativity--that gravity bends light. They succeeded in measuring the changes in the positions of starlight passing near the Sun’s edge compared to their positions months later. Such a test was most feasible during a perfect solar eclipse. The tests led to the general acceptance of Einstein’s theory, which is the foundation of modern cosmology.
And finally, perfect eclipses give us unique access to ancient history. By consulting historical records of past solar eclipses, astronomers can calculate the change in Earth’s rotation over the past several thousand years. This, in turn, allows us to put ancient calendars precisely on our modern calendar system.
These are just three ways in which perfect solar eclipses, produced by conditions that help create a habitable planet, have fostered scientific discovery. But this is only one example of the correlation between habitability and measurability.
At the much larger, galactic, scale, we again find that the most habitable place is also the best overall location for making a diverse range of scientific discoveries.
Though the visible universe contains perhaps a hundred billion galaxies, astronomers group them into just three basic types: ellipticals, irregulars, and spirals. Our Milky Way is a spiral galaxy. Most of its stars are located in its flattened disk, its thickness is only about one percent its diameter. We live in the disk, very close to its midplane, about half way between the dangerous Galactic nucleus and its visible edge. Spiral galaxies like the Milky Way derive their popular name from the beautiful spiral pattern formed by their young stars and bright nebulae. We reside between the Sagittarius and Perseus spiral arms.
Contrary to popular impression, not all galaxies are equally habitable, since habitability depends on a galaxy’s mass, type, age, and allotment of heavy elements. Moreover, even the relatively rare, large spiral galaxies like the Milky Way, which are likely optimal for life, probably contain only a few locations within a “Galactic Habitable Zone” compatible with complex life. Galaxies are filled with dangerous radiation hazards, and many regions are either so low in heavy elements as to prohibit terrestrial planets from forming, or so high that planetary systems will be hostile to life.
This Zone is an exclusive piece of real estate. In contrast, the inner ghetto of the Milky Way suffers from greater radiation threats and comet collisions, and an Earth-size planet is less apt to form there in a stable circular orbit. The outer regions are safer, but stars there will be accompanied by only fairly small terrestrial planets, planets too small to retain an atmosphere or sustain plate tectonics.
And the spiral arms are much more hostile to planetary systems aspiring to habitability than is our location between spiral arms. While we can’t yet say how wide it is, the Galactic Habitable Zone seems to be a fuzzy ring in the thin disk at roughly the Sun’s location, a ring whose habitability is itself compromised at several points where it intersects the spiral arms. If habitability depends on proximity to the so-called corotation circle--that region in which stars orbit at about the same speed as the spiral arm--then this thin and often broken ring could be narrower still.
At the same time, our location within the Galactic Habitable Zone offers the best overall location to be a successful astronomer and cosmologist. Even though we’re near the mid-plane, there’s very little in the way of dust in our neighborhood to absorb light from nearby stars and distant galaxies. We’re far enough from the Galactic center and the disk is flat enough that it doesn’t excessively obscure our view of the distant universe. We have access to a striking diversity of nearby stars and other Galactic structures, as well as a clear view of distant galaxies and the unique cosmic microwave background radiation, both essential for discovering the astonishing facts that the universe is expanding and finite in age.
These examples are merely illustrative. To be persuasive, the argument needs more detail, more evidence, and more rigor. Properly framed and developed, however, we think the evidence for the correlation between life and discovery forms a pervasive and telling pattern, a pattern that not only contradicts the Copernican Principle, but also suggests that the universe, whatever else it is, is designed for discovery.
Design? Surely no question in science is more interesting and more controversial. But our argument has more mundane implications. If we’re right, research dollars would be better spent exploring what other factors, still undiscovered, also contribute to a planet’s habitability (and capacity for discovery). At the moment, we’re learning about habitability mostly as a spin-off of the increasingly quixotic search for extraterrestrial life, because many astronomers are still in the grip of the Copernican Principle. Another unfortunate result of that Principle is that few are inclined to ask if the universe could be designed for a purpose, let alone to seek evidence for such a possibility. But in science, as in life, things can change. Perennial questions, even when officially ignored, have a way of bubbling up.
Jay W. Richards is Vice President and Senior Fellow of the Discovery Institute in Seattle. Guillermo Gonzalez is Assistant Professor of Astronomy and Physics at Iowa State University. They are co-authors of the recently-released book The Privileged Planet: How Our Place in the Cosmos is Designed for Discovery (Regnery, 2004).