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See Inside Scientific American Volume 311, Issue 2

Is Earth’s Life Unique in the Universe?

To know whether life exists beyond Earth, we must come to terms with our own significance in the universe. Are we uniquely special or merely mediocre?



Tim Bower

In Brief

  • Earth orbits one of the hundreds of billions of stars in our galaxy, which in turn is one of hundreds of billions in the observable universe. This apparent insignificance fits with the Copernican principle that our planet is not the center of the cosmos but simply a mediocre member of a mediocre solar system.
  • Meanwhile there are reasons to think Earth and its life are special, perhaps even singular. Some evidence comes from the details of our planetary circumstances, as well as from the observation that certain fundamental constants of nature appear to be fine-tuned for life's existence.
  • Scientists must reconcile these conflicting ideas to understand where we fit—and whether we are alone—in the cosmos.

More In This Article

Adapted from The Copernicus Complex: Our Cosmic Significance in a Universe of Planets and Probabilities, by Caleb Scharf, by arrangement with Scientific American/Farrar, Straus and Giroux, LLC (US) and Penguin Press (UK). Copyright © 2014 by Caleb Scharf.

We all reside on a small planet orbiting a single, middle-aged star that is one of some 200 billion stars in the great swirl of matter that makes up the Milky Way galaxy. Our galaxy is but one of an estimated several hundred billion such structures in the observable universe—a volume that now stretches in all directions from us for more than 270,000,000,000,000,000,000,000 (2.7 × 1023) miles.

By any paltry human standard, this is an awful lot of stuff and an awfully large amount of room. Our species has sprung into existence within the barest instant of this universe's enormously long span of history, and it looks like there will be an even longer future that may or may not contain us. The quest to try to find our place, to discover our relevance, can seem like a monumental joke. We must be appallingly silly to imagine we can find any importance for ourselves at all.

Yet we are trying to do just that, despite our apparent mediocrity, which became evident when Renaissance scholar Nicolaus Copernicus decentralized Earth from the solar system around 500 years ago. His idea has been one of the greatest scientific guides for the past few hundred years and a critical signpost on our journey to discern the underlying structure of the cosmos and the nature of reality.

In our efforts to assess our significance, we face a conundrum: Some discoveries and theories suggest life could easily be ordinary and common, and others suggest the opposite. How do we begin to pull together our knowledge of the cosmos—from bacteria to the big bang—to explain whether or not we are special? And as we learn more about our place in the universe, what does it all imply for our efforts to find out if there are other living things out there? How do we take the next steps?

What We Know
In the 1600s tradesman and scientist Antony van Leeuwenhoek used his hand-built microscopes to become the first human to see bacteria, a journey that took him into the alien world of the microcosm. In that remarkable descent, sliding down the ladder of physical dimensions, into the thriving universe within us, was one of the first clues that the components of our bodies, our arrays of molecular structures, exist at one extreme end of a spectrum of biological scales. Until van Leeuwenhoek's moment of surprise, I doubt that humans had the opportunity to think about this fact in anything more than a superficial way.

There are organisms on Earth that are physically larger and more massive than we are—just look at whales and trees. Yet we are much closer to the upper limit of scale than we are to the microscopic end of life's spectrum. The smallest reproducing bacteria are a couple of hundred-billionths of a meter across; the smallest viruses are 10 times as small as that. The human body is roughly 10 million to 100 million times larger than the simplest life we know of.

Among warm-blooded terrestrial mammals we are also on the large side but not quite at the extreme top. At the opposite end of scale, the smallest of our kin are the pygmy shrews, diminutive scraps of fur and flesh barely two grams in weight. They exist at the edge of feasibility, their bodies endlessly leaking heat that they can barely compensate for by voracious eating. But most mammals are closer to this size than to our size: so much so that the global average body weight of the mammalian population is 40 grams, or less than 1.5 ounces. Our complex-celled, intelligent bodies are at the boundary of the upper extremes, with comparatively few mammalian types bigger than us.

It is an undeniable observation that we exist at this border, this interface between the complex diversity of the biologically small and the limited options of the biologically large. Consider, too, our planetary system. It is unusual in certain respects. Our sun is not one of the most numerous types of star (most of which are less massive), our orbits are at present more circular and rather more widely spaced apart than most exoplanetary systems, and we do not count a super-Earth among our planetary neighbors. Such a world, a few times more massive than Earth, is represented in at least 60 percent of all systems but not our solar system. If you were an architect of planetary systems, you would consider ours to be an outlier, a little bit off from the norm.

Some of these characteristics stem from the fact that our solar system has escaped wholesale dynamical rearrangement, compared with the majority of planetary systems. This does not mean that we are assured a quiet and peaceful future—state-of-the-art gravitational simulations indicate that a few hundred million years along, a more chaotic period could overtake our system. And another five billion years into the future the sun will inflate with the onset of a spasmodic old age and quite drastically revise the properties of its array of planets. All indications are that today we also live at an interface or border in time, a transition between a period of stellar and planetary youth and one of encroaching decrepitude. Our existence in this period of relative calm is, in retrospect, not so surprising. As with so many other aspects of our circumstances, we live in a temperate place, not too hot or cold, not too chemically caustic or chemically inert, neither too unsettled nor too unchanging.

It is also now apparent that this astrophysically calm neighborhood extends well beyond our local galaxy. In terms of the universe as a whole, we exist in a period that is many times more ancient than the fast tumult of the young, hot cosmos. Everywhere the production of stars is slowing down. Other suns, and their planets, are forming at an average rate that is barely 3 percent of that 11 billion to eight billion years ago. The stars are slowly beginning to go out across the universe. And in grand cosmological terms, only six billion or five billion years ago the universe was decelerating from the big bang. Now we are again in a period of gentle transition. Dark energy, stemming from the vacuum itself, is accelerating the growth of space, helping to quash the development of larger cosmic structures. But this means that life is ultimately condemned to a distant future of bleak isolation within an increasingly indecipherable universe.

Put all these factors together, and it is clear that our view of our inner and outer cosmos is highly constrained. It is a view from a narrow perch. Indeed, our basic intuition for random events and our scientific development of statistical inference might have been different under other circumstances of order or disorder, space and time. And the very fact that we are far isolated from any other life in the cosmos—to the extent that we have not spotted or stumbled across it yet—profoundly impacts the conclusions we can draw.

Deductions
Much of the evidence we have supports the basic Copernican view that we are mediocre. Yet at the same time, there are specifics about our environment that say otherwise. Some of these qualities have led to the so-called anthropic principle, the observation that certain fundamental constants of nature appear “fine-tuned” in a way that causes the underlying properties of the universe to be balanced near a boundary that enables Earth and its life to exist. A little too far to either side, and the nature of the cosmos would be radically different. Tweak the relative strength of gravity, and either no stars form, no heavy elements are forged—or huge stars form and are quickly gone, leaving nothing of any import in their wake, no descendants, no pathway to life. Similarly, alter the electromagnetic force, and the chemical bonds between atoms would be too weak or too strong to build the diversity of molecular structures that allows such incredible complexity in the cosmos.

What do we make of all the contradictions? I would argue that the facts are pushing us toward a new scientific idea about our place in the cosmos, a departure from both the Copernican and anthropic principles, and I think it is well along the road to becoming a principle in its own right. Perhaps we could call it a cosmo-chaotic principle, the place between order (from the original Greek kosmos) and chaos. Its essence is that life, and specifically life like that on Earth, will always inhabit the border or interface between zones defined by such characteristics as energy, location, scale, time, order and disorder. Factors such as the stability or chaos of planetary orbits, or the variations of climate and geophysics on a planet, are direct manifestations of these characteristics. Too far away from such borders, in either direction, and the balance for life tips toward a hostile state. Life like us requires the right mix of ingredients, of calm and chaos—the right yin and yang.

Proximity to these edges keeps change and variation within reach but not so close that they overwhelm a system constantly. There are obvious parallels to the concept of a Goldilocks zone, which proposes that a temperate cosmic environment for a planet around a star exists within a narrow range of parameters. But for the existence of life, the hospitable zone may be much more dynamic—it need not be fixed in space or time. Rather it is a constantly drifting, twisting, flexing, multiparameter quantity, like the paths traced by a dancer's limbs.

If it is a universal rule that life exists only under these circumstances, it raises some intriguing possibilities about our cosmic significance. Unlike strict Copernican ideas, which stress our mediocrity and therefore suggest an abundance of similar circumstances across the cosmos, the notion that life requires a varying and dynamic alignment of parameters narrows the options. The opportunities for life implied by this new view also differ from anthropic ideas, which at their most extreme predict as little as one sole occurrence of life across all space and time. Instead this new rule actually identifies the places where life should occur and the potential frequency with which it does. It specifies the fundamental characteristics necessary for life within a virtual space of many waltzing parameters—it maps out the fertile zones.

Such a rule about life does not necessarily make living things some special part of reality. Biology may be the most complicated physical phenomenon in this universe—or in any amenable universe. But that is possibly as special as it gets: a particularly intricate natural structure that arises under the right circumstances, between order and chaos. And this conceptualization of where life fits into the grand scheme of nature leads directly to a way to resolve the conundrum between the persuasive, but unresolved, arguments that life must be abundant and that it is exquisitely rare.

This article was originally published with the title "Cosmic (In)Significance."

MORE TO EXPLORE

Just Six Numbers: The Deep Forces That Shape the Universe. Martin Rees. Basic Books, 2001.

Caleb Scharf's Life, Unbounded blog: http://blogs.ScientificAmerican.com/life-unbounded

FROM OUR ARCHIVES

The Fate of Life in the Universe. Lawrence M. Krauss and Glenn D. Starkman; November 1999.

The Benevolence of Black Holes. Caleb Scharf; August 2012.

SCIENTIFIC AMERICAN ONLINE See a video on the potential for extraterrestrial life at ScientificAmerican.com/aug2014/scharf

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