Earth is a proper noun, a definite and individual place. Living close to the planet, we view it as something more generic: the ground beneath us, the soil, the background. Science has begun to paint a picture, however, of a more particular place. We inhabit one planet among many, and yet a tremendously interesting planet. Life happens here, and we as living things participate in it. Earth is a living planet.

This book deals with the growing field of astrobiology, the study of life in space. As we begin to explore the Solar System and the wider universe, we must...

Scientists and nonscientists often fall into the trap of believing that all science proceeds in the same way. Yet a number of very clever people conduct extraordinary research in a variety of ways. One of the biggest challenges in developing astrobiology as a field has been to get all the clever people to speak the same language long enough to make prog ress.¹ Nevertheless, most scientific in quiry proceeds from a certain set of basic assumptions.

What image comes to mind when you think about science? If it is a serious man in a white coat and glasses writing equations...

What makes a thing alive? When you encounter something new, how do you tell? The question is surprisingly difficult. Start with an example so old and so common it might not have occurred to you to think of it. Are eggs alive? The straightforward answer is yes . . . and no. Let us stick with chicken eggs for the time being. I would be willing to bet that almost all of my readers have encountered chicken eggs and the majority of you have eaten them. Are chicken eggs alive?

One immediate answer is yes. Of course eggs are alive....

Looking at life in space, we see that our terms can have a major impact on how we ask and answer questions. Before we look for life out there we should ask what we mean by “out there.” So let us review what can be known about the universe at the most fundamental level. These ideas form the basis for science in general and astrobiology in particular.

The question on the table is this: Does the universe behave itself? Does it follow its own rules? Are there consistent laws that allow us to make predictions about distant times and places?...

Chapter 4 dealt with the idea that the universe we observe behaves itself. A comprehensible cosmos turns out not to be unreasonable. This chapter deals with a parallel question: “How well behaved are we when we observe the universe?” When humans construct models about how the universe operates, we come with certain assumptions and biases. They may be built in and they may simply reflect the position we hold in the universe, but it has become clear that humans cannot be entirely impartial.

I intend to discuss two ways in which we must be partial: the anthropic principle and the...

Life exists in an ordered cosmos. No matter which way we look at the universe and no matter where we go in it, the same fundamental rules apply. Life as we know it also follows certain rules that allow us to see it as part of the ordered whole. In this chapter I leave behind ruminations about possible life and discuss the fundamental rules of terrean life.

I’ll divide this chapter into the medium, matter, and method of life as we know it. The medium with which we are familiar always involves water, so I will address the special characteristics...

We now turn to the question of where life in space may be found. It requires a fairly precise balance of energy, so we would expect to find life orbiting stars. Too little energy, and an organism cannot maintain itself or reproduce. There is not enough energy to do work. Too much, and an organism cannot hold itself together. The molecules bounce around too quickly to prevent undesirable reactions. As temperature increases, organisms would have to devote more and more of their resources to keeping ev ery thing in order and preventing unwanted reactions. In or near a star, energy...

Life requires a very special place in the universe. Water, carbon, and energy constitute the basics and they must be abundant. In this chapter, I turn to the spe cific locations where we can find these commodities. The vast majority of the cosmos consists of a cold void with a few atoms of hydrogen and helium wandering around; we can say with some certainty that life (as we know it) would not be found there. No, life needs a warmer place with a little more furniture. Stars provide a universal heating system. They not only produce energy to heat up...

We now turn our attention to our own Solar System, the set of objects orbiting the local star sometimes called “Sol,” but usually referred to as the Sun. It can be divided into five regions. The first three should be well known to most readers while the last two may be somewhat unfamiliar. The first and inmost region includes four rocky planets—Mercury, Venus, Earth, and Mars. Just outside of this, a giant ring of rocks circles the Sun, the asteroid belt. Beyond the asteroid belt lie four giant planets—Jupiter, Saturn, Uranus, and Neptune—with their moons and rings....

At first glance, the outer Solar System might seem a poor place to look for life. The energy budget must be much smaller, given the additional distance from the Sun. The carbon composition of planets and moons must be low. Liquid water gets harder and harder to find. The giant planets themselves cannot support life as we know it. The incredible density felt at depth would be too high, and it can be difficult to imagine organisms living in environments of gaseous and liquid hydrogen. Minerals and heat necessary for life would be deep within the planet, but sunlight and...

Earth is not as unusual as we once thought. Within the past three de cades, astronomers have discovered more than 300 planets beyond our Solar System. The search has only just begun, but every newfound planet tells us something interesting about the way planets work. Every new planet shows us one more example of the basic principles of life. Above all, every new planet presents a new opportunity for life.

The majority of these planets will probably turn out to be giant balls of ice, dust, and gas—much like the objects in our own outer Solar System. Right now...

We turn now from generic to spe cific, from universal principles to local application. What has been a galactic survey be comes a planetary study. Up until now, I have covered issues having to do with life and the possibility of life elsewhere. Even when we looked at our own Solar System, it merely set the stage. Now we turn to life here on Earth and start to meet the players. The next seven chapters look at issues specific to terrean life and how it works.

Astrobiology encompasses a variety of disciplines. We have seen that already in the interactions...

Life as we know it resembles a skyscraper made out of building blocks. Smaller components come together to form more complex entities. One of the most fascinating things about life is that this appears to be true at every level. Like a fractal pattern, the same pattern appears at different scales: A small number of interchangeable parts can be assembled in a nearly infinite number of ways to make something larger. Part of defining life will be figuring out how life arises in scale as well as how life arises in history.

Where do we begin, and what are the...

What is an organism?The Oxford English Dictionaryde fines organism as “an individual animal, plant, or single-celled life form.” Even if we disregard the matter of fungi and other nonplant, nonanimal, multicellular life, we must admit that the dictionary has thrown us back on two other definitions. We established earlier that life can be a bit tricky. Cells form discrete packets of space and information, but we know that we want something larger than a cell—an individual. We want a category that includes humans as individuals, not to mention dogs, cats, rutabagas, and any number of other entities...

How do elements come together to form the structure of life as we know it? The first step is the construction of molecules. Almost every structure in your body was built out of one of only four types of biological molecules—carbohydrates, proteins, nucleic acids, and lipids. Each kind of “biomolecule” ranges in size from a few atoms joined together to huge strings that can involve thousands of atoms. In this chapter, I introduce some of the immense variety of biological molecules found in terrean life.

Each section begins with a discussion of how to get from atoms to simple...

We may not yet have a direct answer for how life on Earth began, but we have a good understanding of what keeps it going. Metabolism refers to the set of chemical pro cesses occurring within living organisms, particularly those that string carbons together and break them apart. These processes, divided here into carbon chemistry and electron chemistry, either store or release energy, fueling life.¹ Carbon chemistry also can be divided neatly into two parts, anabolism and catabolism, put ting life together and taking it apart. Anabolism involves stringing carbon together with other atoms to form complex organic molecules. It...

No discussion of life on Earth would be complete without some attempt to look at the diversity and interrelation of organisms. Chapter 15 introduced taxonomy, the classification and naming of organisms. In this chapter, I will go into much greater detail, looking at the history of the field as well as the state of the art. We will look at several historical systems before diving into the most popular system today: molecular phylogenetics. We will explore the three domains—Archaea, Bacteria, and Eukarya—that constitute the terrean biosphere. The next chapter addresses a few exceptional entities that blur the lines...

We have now come full circle. In Chapter 4 I asked how we de fine life. Although many interesting possibilities have been proposed, we saw that the basic answer is this—life is something like what we see on Earth. The most common definition, which I have called the pornography definition, said: “I can’t define it, but I know it when I see it.” This chapter explores whether or not that really is the case. Here we turn to specific examples from biology: viruses, prions, plasmids, and selfish genetic elements. Each entity has some but not all of the features...

What book on life in space would be complete without at least some discussion of intelligence? It may be the most important property of life on Earth. I doubt that humans have altered the planet’s chemistry as dramatically as the cyanobacteria, but we have accomplished two things that make us stand out. First, we have flooded space with information. Humans generate and broadcast tremendous quantities of information into space by way of radio and television and other signals. Second, humans have sent robots around the solar system and into deep space. We have landed instruments on Venus and Mars. In...

The story of life in space can be told as a single narrative—at least the story of life as we know it. It all started with a bang and the rapid expansion of space. What had been a solitary singularity rapidly expanded into the universe. This expansion started 13.5 billion years ago, give or take a billion years, and continues still. Within 500 million years, the dense energy began to cool down enough to form matter. Particles were born. Particles joined together to form atoms. Matter and space became different.

As matter spread out across space, it settled into...