Preface

Space, as they say, is big. After staring at a single patch of sky for 23 days scattered over the course of a decade, for example, the Hubble Space Telescope spied more than 5,500 galaxies in an area equivalent to a fraction of a square millimeter held at arm’s length (fig. P.1). Extrapolated across the entire sky, this corresponds to some 100 billion galaxies in the observable Universe. Each of these, in turn, contains an average of 400 billion stars. Space is really, really big.

If the Universe is so big, with its 50 sextillion stars, does it naturally follow that there must be life out there beyond the Earth? Or are we alone in the cosmos? The answer in popular culture, replete with its UFO sightings and benevolent—or hostile—space aliens, has always been a resounding “yes!” Even scientists are susceptible to the romance: during the heady days of the late 1960s and early 1970s, there was, briefly, a burst of enthusiasm for the scientific discipline called exobiology, aimed exclusively at the study of extraterrestrial life. But because there is no known life beyond the Earth, cynics quickly tagged it as a subject with no subject matter, and exobiology fell out of fashion.

Still, the question remains: are we alone? A new perspective on this question has arrived with astrobiology, a field that, in contrast to exobiology, studies life on our planet in the context of the possibility of life elsewhere. Astrobiology focuses on broader, perhaps more fundamental, and certainly more tractable questions about the relationship between life and the physics and chemistry of our Universe. Not surprisingly, astrobiology tends to focus on life on Earth—which is, after all, the only example we have on hand—but it attempts to understand this single example in the broadest contexts of the Universe. Using our extensive (if incomplete) knowledge of Terrestrial biology, astrobiology addresses three broad questions about life in the Universe:

Figure P.1 Galaxies like grains of sand. Every one of the 5,500 spots and smudges in this image is an entire galaxy in its own right. This image covers approximately 1/15,000,000 of the entire sky (roughly equivalent to a 1 × 0.5 mm rectangle held at arm’s length) and thus represents but a tiny fraction of the more than 100 billion galaxies in this amazing and expanding Universe. (Courtesy of NASA/STScI/ESA)

1. 1.What are the physical properties that allow our Universe and our planet to support life?
2. 2.How did the origins and evolution of life transpire on Earth, and how might they be transpiring differently elsewhere?
3. 3.Where else in our Universe might life have arisen, what might it be like, and how can we find it?*

Much of the worth of astrobiology lies in the fact that these are perhaps the most fundamental questions addressed by science today; they address the most profound issues regarding who we are, where we came from, and whether we are alone in this vast cosmos. Additional value arises from the exceptionally interdisciplinary approach that these questions demand; astrobiology encompasses a variety of scientific disciplines, ranging from cosmology, astrophysics, astronomy, geology, and chemistry to, of course, biology itself. In deference to the complexity that this lends, we endeavor in this book to outline the current status of astrobiology with only the absolutely necessary amount of scientific detail.

We begin, in chapter 1, by defining the object of our study: life. Although we can easily distinguish living from nonliving systems here on Earth, setting up a definition that strives to include all living systems imaginable in the Universe and to exclude all nonliving systems requires careful consideration. Next, in chapter 2, we need to investigate how the stage was set for life to arise in our Universe. How did the Universe come into being, and which of the crucial factors in its origins and early history distinguish it from other possible universes that may be unable to harbor any life at all? From the vastness of the Universe we zoom inward, in chapter 3, to explore the tiny blue dot that is our home planet and ask similar questions about it. Why did the third rock from the Sun become positively infested with life, while its neighbors did not? Which conditions are necessary for a planet—any planet, anywhere in the Universe—not only to become and remain habitable but to give rise to life in the first place? Zooming in closer still, chapter 4 looks at the molecular world present at the surface of the young Earth and investigates the chemical conditions and potential chemical pathways that set the stage for life to originate here.

Once the Universe, the Earth, and the molecules are all set and ready to go, the question then becomes: how did it actually happen? How did the inanimate become the animate, changing a habitable planet into an inhabited planet? The short answer is that we still don’t know. However, there are partial answers to some of our questions and constraints regarding the possible answers to others, giving us a chance to spin some, necessarily rather speculative, scenarios in chapter 5. Following the chronological history of life on our planet, in chapter 6 we move on from the first spark of self-replicating, evolving life to the first cells and organisms. Again, we know very little about what really happened, but our current knowledge allows us to put constraints on what could have happened on Earth and risk an educated guess or two about what conditions might be necessary for it to happen (or have happened) anywhere else.

The veil begins to lift somewhat when we come to the last common ancestor of today’s organisms: a single-celled organism named LUCA that was already quite evolved and had DNA, RNA, and hundreds of different proteins. From that point onward, molecular phylogeny (a word you’ll grow to respect, if not love) and, increasingly, paleontology can help us decipher the history of life on Earth as a proxy for life in our Universe, as outlined in chapter 7.

Is there life elsewhere in the Universe? Today, researchers are pondering this question with a much more concrete and, in some ways, more optimistic outlook than they did 45 years ago when the Viking missions, with the first landers to operate long term on the surface of another planet, searched for life on Mars. This newfound optimism is based on the accumulating examples of organisms thriving in what we humans would consider extremely hostile conditions, such as high pressures, water well above its nominal boiling point, and extremely salty, acidic, or alkaline brines. The discovery of these Terrestrial extremophiles, which we detail in chapter 8, has had ripple effects within the field of astrobiology; the discovery of life in many seemingly uninhabitable environments on Earth has radically expanded our perceptions of what might constitute a possible habitat on other planets. The search for life elsewhere in the Universe has thus become intimately connected to the study of diverse habitats on Earth—an important element of the definition of astrobiology that sets it apart from the earlier, “extraterrestrials only” focus of exobiology.

Having explored the origins and the limits of life on Earth, we then expand our considerations beyond the home planet. In chapter 9, we ask: if life can thrive around hydrothermal vents in the deepest depths of our oceans, in the driest, coldest valleys of the Antarctic, and even deep within the Earth’s crust, then which other places in the Solar System previously deemed inhospitable might actually be inhabited? Likewise, what does this tell us about potential habitats outside our Solar System? Ultimately, speculation on where and even whether extraterrestrial life exists frustrates the scientific mind unless it can be followed up with actual investigation of the potential habitats concerned. Thus, in chapter 10, we review the history of space exploration and conclude our brief overview of astrobiology with a survey of past, present, and future searches for extraterrestrial life.

Since the second edition of this book appeared in 2011, exploration of the Solar System and discoveries in the field of extrasolar planet exploration have advanced significantly. Earthly extremophiles have turned up in yet more remote and hostile environments, and even the eternally elusive quest for the origins of life has seen some small progress. We have revised the entire text both to reflect scientific advancements and to eliminate weaknesses that attentive readers of the earlier editions have kindly brought to our attention. It is our hope that by the end of this refreshed and improved edition, both cynics and enthusiasts alike will be convinced that, unlike exobiology, astrobiology has an identifiable subject matter accessible to direct study and furthers our collective understanding of our place in the Universe.

But, before you go, a note on nomenclature: throughout this book we capitalize the words Universe, Solar System, Sun, and Moon to denote when we’re speaking about our particular universe, solar system, sun, and moon. Similarly, we use Terrestrial to refer to something of or related to our planet, and terrestrial to refer to rocky, Earth-like planets in general. By extraterrestrial, though, we mean specifically not of our planet. All clear?

1. * And, in homage to Douglas Adams (1952–2001), author of The Hitchhiker’s Guide to the Galaxy, “Can I get it to buy me a drink?”