everywhere, on average. The same laws of physics apply throughout space, holding sway over the interactions of matter and energy. For these reasons the processes that ultimately led to life on Earth could reasonably lead to life anywhere else in the universe, provided that the conditions are suitable to support life as we know it.

So what makes life precious and rare, limited only to Earth at least as far as we can tell? One answer simply may be that we have not yet looked long enough for life elsewhere or in the right places. If that's the case, the question may seem as quaint a few decades from now as the old belief that all heavenly bodies revolved around Earth. However, the answer may also lie in just how improbable it is for life to have arisen. Even the simplest organisms are incredibly advanced machines. To function and grow, an organism must maintain orderly processes within and around itself. Then, it reproduces and transfers that ability to its offspring. Such a chain of events runs strongly against a powerful tendency for systems to become more disorderly with time.

We know that tendency as one manifestation of a powerful principle called the second law of thermodynamics. According to this law, the disorder of a system--a quantity known as "entropy"--must increase with time if no energy enters or leaves the system. When entropy increases, things fall apart and lose their structure. For instance, a sugar cube dropped into a glass of water slowly dissolves. Eventually all the sugar molecules drift evenly throughout the water. We never see the reverse: a sugar cube assembling from floating molecules to appear miraculously at the bottom of the glass. At first glance the genesis of a living cell from a stew of organic molecules seems an equally miraculous violation of the second law of thermodynamics. However, the system is not isolated; it absorbs energy from the Sun and its environment. Many millions of years of mixing Earth's primitive organic ingredients with such inputs of energy led to the first cells. Billions of years of further interactions created the diverse life we see around us today.

Still, when we consider life in terms of entropy, it seems that the chances of creating a living biological machine just by following the laws of physics are ridiculously small. Yet life exists, so the probability is not zero. Does this mean life is plentiful? Will we someday make contact with other beings like ourselves? These questions often prompt spiritual responses, and understandably so. They cut to the core of our wonder about the universe and whether we are part of a grand cosmic design. Debating that metaphysical issue is easy; resolving it scientifically is impossible. However, we can and should use science in another way: to examine the sequence that matter and energy must follow to create a technological civilization out of formless interstellar gas. How plausible is each step along that path? The American astronomer Frank Drake posed that question mathematically in what is now known as the Drake equation. His formula offers a way to estimate the number of civilizations in our Milky Way galaxy that have the technology needed to talk to one another. The equation is a string of numbers and fractions. Put into words, it reads something like this:

Start with the number of stars that form in the galaxy each year. Multiply that rate by the fraction of stars with planets. Multiply by the number of planets or moons in each planetary system with conditions suitable for life. Multiply by the fraction of such planets upon which life has evolved. Multiply by the fraction of life-bearing planets with intelligent societies. Multiply by the fraction of those societies that have developed the technology to communicate across space. Finally, multiply by the average number of years that a technological civilization survives.

This long string of multiplications yields not a solid number but a range of numbers that depend on the assumptions you make at each step. Astronomers know some of the quantities fairly well, but they can only guess at others. For example, the galaxy gives birth to about 10 new stars each year. The growing number of planet discoveries seems to show that many stars, if not most, have planets. How frequently life arises on these planets is much less clear. Biologists believe that life took hold on Earth within a few hundred million years of the planet's birth, soon after a steady bombardment by large comets and meteorites died down. We don't yet know which chemical reactions spawned the first living cells. But chemistry is, at its roots, a special kind of physics that applies to atoms and molecules interacting with their environment in large groups. If physics is uniform throughout the universe--and if the Copernican principle is indeed a fundamental tenet of the cosmos--the chemistry of life may bubble forth in more places than many of us suspect.

Even so, planets experience a vast range of physical conditions. Which conditions are just right for life to arise? Here, we can draw lessons from the tale of Goldilocks. Her mischievous exploration of the bears' cabin led her to taste three bowls of porridge.