We live in an era when astronomers are making amazing discoveries about planets around other stars than our sun. The prospects are good that the science of comparative planetary systems is going to keep growing. In this heady atmosphere, the question of whether there is life in these other systems begins to assume even more importance. As we find life in more and more extreme environments on Earth, we realize that other planets might be more hospitable to simple forms of life than we had realized. And yet, there are those who believe that while life may be widespread in the universe, complex organisms like ourselves may be very rare. In 2000, Peter D. Ward and Donald Brownlee published Rare Earth: Why Complex Life Is Uncommon in the Universe, which lays out the case for this viewpoint.
Habitable zones of all kinds
When I started reading this book, I knew some of the constraints on planets that can harbor life. They must be not too hot and not too cold, but rather just right, like the baby bear’s porridge; as far as we can tell, liquid water is crucial and so the temperature somewhere on the planet must support liquid water. The “habitable zone” around a star depends on what type of star it is; a hotter star will have the right temperatures further out from the star, and a cooler star will have them closer in. A hotter star will not live long enough to provide the time that it took on Earth for complex organisms like you and me to arise. (Hotter stars are generally more massive, and the most massive stars live for only hundreds of thousands of years, far less time than the billions of years of evolutionary history on this planet.)
However, the concept of habitable zones is a lot more complicated than that. It turns out that the smaller stars might be too cool, but not in quite the way you’d think. Planets could maintain the right temperature around these stars, but only if their orbit is so close to the star that they become tidally locked, like the moon is with the Earth. If that happens, the planet turns only one face to its sun all the time, leaving the other side too cold and playing havoc with the distribution of thermal energy over the surface.
Habitable zones also change with time as the energy output of a star changes (generally a slow increase), so a planet must occupy not just a habitable zone but a continuously habitable zone. As the star brightens, the habitable zone moves outward, and so a planet that is too near the inner edge will become too hot over time. (Also the ability of a planet to cope with this kind of change depends on chemistry, as described below.)
Habitable zones vary depending on life form. Many of the extremophiles that thrive in harsh environments on Earth are microbes, and they survive in places that would kill us hominids. (At a talk here at Indiana University last year, astronomer and extrasolar planet specialist Geoffrey Marcy described the robustness of bacteria by saying, “When hell freezes over, the bacteria will party.”) So the habitable zone for microbes is much larger than that for animals.
There are habitable zones on larger scales too. For example, there are better and worse places in the galaxy for the evolution of complex life. In star clusters and near the galactic center, there is enough electromagnetic radiation (not just light, but more energetic forms like UV and gamma rays) to disrupt emerging life processes. And the more stars are nearby, the greater the chance that a planet will be pasteurized by the radiation from a nearby supernova.
On the other hand, if you’re too far out from the galactic center, there might not be enough heavy elements (heavy meaning in this case heavier than hydrogen and helium). You need heavy elements to form a planet in the first place, not to mention the life forms to inhabit it. So you do need supernovae explosions to scatter the elements synthesized inside a star; you just don’t want them too close or too frequent. In short, being on the galactic fringes might provide too sparse an environment for the evolution of complex life.
And as if all that were not enough, not all galaxies are equally likely to be hospitable to life. Spiral galaxies like our own have multiple waves of star formation, so that later stars incorporate some of the material that was made in earlier generations but was missing from the galaxy until those earlier stars made it. This material is Carl Sagan’s famed “star stuff” that makes up you and me and the ground beneath our feet. Elliptical galaxies, on the other hand, are believed to have much more static stellar populations, and so perhaps lack this factor.
But wait, there’s more
Even if a planet is appropriately placed, there are a host of other factors to consider. The size of the planet is important: too small, and it lacks sufficient gravity to hang onto an atmosphere; too large, and it becomes a gravitational magnet for asteroid strikes. The composition of the planet has to be just right, not too much water (you need both land and sea) and not too much CO2 (so that there is not a runaway greenhouse effect). A planet closer to its sun might not have enough of the elements that go into making up life, because these easily evaporated elements (called volatiles) can be blown away by the solar wind.
Large outer planets are something of a mixed blessing, though.
Jupiter and Saturn might have played a vital role in scattering volatile-rich materials into the inner solar system. It’s possible that some precursors of life came in from the outer solar system as well. Large outer planets are something of a mixed blessing, though. For Earth, they provide a shield of sorts by gravitationally attracting some objects that might otherwise make their way into the inner solar system and hit Earth. On the other hand, they can disturb the orbits of comets and send them into the inner solar system. In a system that had more giant planets, or giants that are closer to the star, gravitational forces could knock hapless smaller planets out of a habitable zone and possibly even eject them from the system altogether.
The chemical composition of Earth is crucial in a variety of ways. To take one example, the minerals in the rocks can affect the content of oxygen in the air (more iron, for example, would react with oxygen and remove it from the atmosphere), and the appearance of large quantities of oxygen in the atmosphere gave complex life on Earth a major boost. There’s also something called the CO2-rock cycle, which helps stabilize planetary temperatures by shuttling carbon dioxide between surface rocks and the atmosphere depending on the temperature. (I think this process is too slow to cope with the huge amounts of CO2 that have been going into the atmosphere in industrial times, but it could help regulate temperatures over a longer time scale of slow change, like the brightening of a star described above.)
Earth’s magnetic field and its plate tectonics rely in complicated ways on its chemical composition, and both are crucial for the existence of complex life as we know it. The magnetic field arises because Earth has a partially liquid iron/nickel core (not only does it possess sufficient nickel and iron; it is big enough for them to gravitate toward the center of the planet). Having a magnetic field provides a buffer that helps protect the Earth’s inhabitants from charged particles coming in from the sun and from outer space.
The working of plate tectonics relies on radioactive elements deep in the Earth’s innards that keep the temperatures higher than they would be otherwise, and on the existence of a thin crust that is light enough to float on the material below. The manifestations of plate tectonics with which we’re probably most familiar are volcanoes and earthquakes. As destructive as these two events so often are, they recycle the elements in the Earth’s atmosphere in ways that are essential for life. Furthermore, volcanoes played an important role early in the Earth’s history. The early Earth was stripped of its first atmosphere and gained another through outgassing from volcanoes. Volcanoes also released the greenhouse gases that kept the “snowball Earth” phase from being permanent. Also, the dust that went up into the atmosphere from these volcanoes settled on the ice and then fertilized the oceans when the ice finally melted, resulting in an oxygen bloom that fostered the spread of life.
Some factors in the timing and nature of the evolution of life on Earth may also be crucial on other planets. Impacts from large objects, for example, are important, although their role is a bit ambiguous. On the one hand, a planet-wide catastrophe can wipe out any complex life that has managed to evolve. On the other hand, the terrestrial catastrophe that did in the dinosaurs also gave mammals their chance to evolve into something like us. In general, on Earth mass extinctions have been followed by an eventual increase in the diversity of life forms.
And now for the meaning…
The ideas presented by Ward and Brownlee are of course open to debate, and there has been quite a bit of discussion (see, for example, the debate on space.com from July 2002). While I feel that Ward and Brownlee make a good case, I realize that we’re generalizing from a sample of one life-bearing planet. We don’t have a good way of being sure yet what is essential and what is arbitrary in the way life arose and evolved on Earth, so we cannot be sure yet what the answer is.
In the absence of firm answers, it might make sense to withhold judgement on how the Rare Earth hypothesis or any other hypotheses about life on other worlds affects our beliefs or values. But since this subject touches on perennially vital questions of who we are and our cosmic context, we already have quite a set of beliefs and values built up, and naturally we try to figure out how (and if) the new information fits into what we already think. This is a very delicate area, to my mind, where science and religion must somehow co-exist. (I use “religion” to refer not only to organized institutions, but also to any system of beliefs involving values, meaning, and ethics.)
The methods by which we seek to answer our questions about the frequency of complex life in the universe are scientific, and there are observations and discoveries that could falsify the rare earth hypothesis. (Falsifiability is often considered to be a hallmark of science.) But it interest us in part because of its connection to existential questions of meaning and purpose. The facts that come from the scientific method can and sometimes do influence our religious beliefs. The Rare Earth hypothesis is an interesting example of this, because in some ways it is like a Rorschach test. Based partly on the facts but partly on personality, personal history, cultural environment, and other factors, people take very different messages from the science to date.
For those who believe that humans will one day roam the galaxy, and that life will somehow always find a way, the Rare Earth hypothesis is either a dousing with cold water or a call to arms. Myself, I’m highly skeptical of the optimism that assumes that the universe overall is friendly to life; some days it doesn’t even look like this planet is unambiguously friendly to complex life.
There are probably others who take it as evidence for the existence of God, because so many things have to happen just so for human life to arise. However, it did happen here, and science is by and large able to explain how it happened without invoking a deity. So this doesn’t hold water for me. On the other end of the spectrum, many of the scientific truths discovered in the twentieth century have been interpreted (and distorted, in my view, but that’s another story) to support a view of life as essentially meaningless and random.
My own take on it is very different. First, even if it turns out that some of the factors that figured in the rise of intelligent life on Earth turn out not to be absolute requirements, they were the path that led to us. We would not be here if any of a multitude of things, small and large, had gone differently. I tend to agree with Ward and Brownlee that there are not likely to be many planets with complex life. But even if there are, we are still the result of a complex and contingent history. Not only that, but we’re able to figure out significant portions of this history, despite the fact that collectively as a species we inhabit such a tiny fraction of that history.
These facts lead me to feel a deep appreciation and gratitude for the fact that I am here now. They inform my belief that to be a conscious being on this planet at this time is a blessing, in the sense that it is something wonderful and quite possibly unlikely that should not be taken for granted. I don’t see it as necessarily a god-given blessing, though; science can explain in outline and in a great many of the details how this all came to be, and we have no evidence of (or need for) a divine hand. It would be less incredible to me if I believed that a god put it all together, with man at the apex or the center. What awes me is the thought of the billions of years and countless tiny changes that have had to take place for us to be here. Think of the things that have to be happening in your brain right now for you to read and understand this; maybe I never met you and you know nothing of me but the words on the screen. And yet you can to some degree understand my thoughts, bridge time and space by knowing what is going on in my mind. How tremendous a story it is that results in our ability to do this.
Some other beliefs that matter to me do not rely on the Rare Earth hypothesis being true, but they do fit well within its framework: the belief that the living systems of this planet deserve our respect and protection; the belief that each person, each consciousness, is also worthy of respect; the belief that we should try to appreciate and experience thoroughly all that we can while we’re here, because life is brief and the forces of chaos that act against it can be strong. Even if we find that complex life is as common in the universe as chiggers in July, I would still hold the first two beliefs, but I would need to rethink part of the framework in which they rest. The belief in the fragility and briefness of life might need some more serious consideration, if we discover that we live in a cosmos that is abundant with life.
This kind of rethinking would be tougher, of course, in cases where a person’s beliefs about the existence of an immortal soul or the origins of humankind conflict with what science tells us, and this illuminates an area of tension between religion and science. With any scientific knowledge, the conclusions we draw reflect much more than just what the facts tell us. Our urge for discovering meaning leads us to begin to add layers of values and beliefs onto any new scientific truth. If possible, we’ll re-work our existing beliefs so that they fit with the new truth, but sometimes we can’t reconcile the two, we have to choose one or the other. This is one source for conflict between science and religion. The two are intertwined in that way.
But the layers of meaning that different people build on top of science are often astonishingly contradictory. The science and the beliefs are separate: Very different structures can be built on the same foundation of facts, whereas the scientific results themselves are our best attempt at an objective understanding of the physical world. Thus I agree with the Dalai Lama that if a scientific truth disproves one of our beliefs, we should choose the scientific truth and discard the belief.
The variety of ways we can draw meaning from our scientific knowledge also demonstrates that the stories we build on a scientific foundation can have at least as much meaning, inspiration, and beauty as any based on religious doctrine. Charles Darwin spoke of the grandeur in an evolutionary view of life that saw “that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
