Adam Frank for McGraw-Hill

In Hollywood's vision of the Universe, you can't sneeze without hitting an alien life form. There are humanoids, intelligent reptilians, animated heads of broccoli and plain, old amorphous (but usually sinister) bags of protoplasm inhabiting every possible corner of every possible world. Life, in the popular culture's version of the cosmos, is anything but rare. Ask an average Joe or Jane on the street and it's likely you would find a similar opinion. Even most scientists, at least those that aren't directly involved in studying the problem, would likely agree. There are just so many stars out there, (one hundred billion of them in our galaxy alone), how could our Star be the only one to have nurtured Life. Surely it must be abundant. That intuitive sense of life's proliferation is exactly what two University of Washington scientists challenge in a remarkable new book called Rare Earth.

Donald Brownlee and Peter Ward make a powerful team. Brownlee is a professor of astronomy at the UW and one of the worlds leading experts on interplanetary matter. Ward is a geology professor who has spent his life studying the planetary biosphere interactions. They are no lightweights when it comes to asking about astrophysics, geophysics and life. That is what makes their conclusions all the more startling. Brownlee and Ward are not simple naysayers in the alien life debate. In fact, the most remarkable aspect of their argument is they believe life IS abundant in the Universe. It is not, however, the kind of life that makes for good science fiction scripts.

Microbes, Brownlee and Ward argue, are likely to present everywhere. Scientists have recently discovered entire communities of life around deep-sea hydrothermal vents. These are places where no light can penetrate and the temperatures are close to boiling. The presence of these communities based on super-hardy microbes has made scientists realize that an entirely new class of bacteria, or bacteria-like, organisms can survive in the most extreme environments. Conditions which used to be thought too hot, too cold, too over pressured or too poisonous for life have all given way as microbes are found thriving in new and unexpected environments. One of the most startling discoveries came when bacteria were found living in rocks pulled up from four-mile deep wells. The presence of life forms in such hostile conditions has led many scientists to conclude that microbial life is hardy enough to exist on a wide range planetary environments. The fact that some form of microbial life appeared on Earth relatively quickly after its formation (within 500 million to 1 billion years of its 5 billion year age) also leads many scientists to conclude that creating life many not be so difficult a task. If it were difficult, then wouldn't life have taken longer to form? Thus it seems that life may be neither delicate nor rare. So what is the problem?

While microbial life may be abundant, everything more complicate (i.e. plants and animals) may be exceedingly rare. Animal life (including us) may be so difficult to create, in fact, that Earth may be the only planet in the galaxy to harbor it. This is the essence of Browlee and Ward's new take on the extra-terrestrial life debate. Bacteria are hardy and can survive the most extreme changes in environmental conditions. Plants and animals are, on the other hand, hard to create and easily threatened. It doesn't take much to throw them into a survival crises.

Remember that about 65 million years ago the planet was covered in dinosaurs. They were so abundant and so successful that they managed to dominate the planet for hundreds of millions of years. Then, all it took was one good collision with an asteroid and the dinosaurs became a planetary memory. Comet and asteroid collisions can be expected to happen periodically on any planet harboring life. Each collision can have a devastating impact on the higher forms of life, which require full ecosystems to support them. These impacts can be powerful enough to literally sterilize the surface of the planet. Based on the new discoveries, bacterial life is likely to survive these impacts.

Earth has, essentially, been lucky in many different ways, which has allowed animal life to develop and thrive. One example of our luck is that we live in a solar system with giant planets like Jupiter in the right place to gravitationally vacuum up much of the comet and asteroid debris, which might otherwise cause an apocalypse. Move Jupiter to another place in the solar system or take it away all together and it is highly unlikely that Earth would be covered with anything more than microbial slime.

Asteroid and comet impacts are just form of danger posed to animal life. A long term stable climate, a location in the galaxy that is not to close to any exploding stars, a good size moon to keep the rotation axis from flopping around, these and a laundry list of other factors are needed to make a planet suitable for the development of higher forms of life. Brownlee and Ward argue that a sober assessment of facts can lead to only one conclusion. There will be very few planets on which everything went right and stayed right. Animal life, and hence intelligent life, is not likely to appear easily or very often. Unless you include the blue green algae, we may be alone after all.