It's rare because to create it, a planet needs to meet a long list of exacting requirements. It needs to be in the habitable zone, e.g. not so close to its sun that the water boils off nor so far away that it freezes. It needs to have a large moon to stabilize its axial tilt, keeping its climate from varying catastrophically. It needs to be in a solar system with a large gas giant that hurls asteroids into the outer regions, keeping it safe from repeated collisions. It needs to have plate tectonics to build up landmasses. It needs to have enough of the heavy metals that are essential for animal metabolism, such as copper and iron.

After that it's a numbers game. They argue that very, very few planets will meet all of those requirements - and therefore that we are very unlikely to find anyone else out there.

I heard a variation of this argument recently at the 2011 AAAS meeting here in Washington D.C. Howard Smith of the Harvard Smithsonian Center for Astrophysics argued that even if plenty of planets generate complex life, the odds are low that any two civilizations will be close enough in time and space to discover each other, let alone communicate. Therefore, we are alone in the universe "for all practical purposes." In a cute turn of phrase, he dubbed this the "Misanthropic Principle."

At the other end of this spectrum - way at the other end - are writers who think that not only is there complex life in the universe, it's already here and visiting us. Leslie Kean's book UFOs: Generals, Pilots, and Government Officials Go On The Record presents numerous accounts of sightings, including one written by Fife Symington III, the governor of Arizona from 1990 to 1997. Symington writes of seeing a "massive, delta-shaped craft silently navigating over the Squaw Peak in the Phoenix Mountain preserve...as a pilot and former Air Force officer, I can say with certainty that this craft did not resemble any man-made object I had ever seen" (p. 262.)

Whether you believe this pretty much comes down to whether you believe Fife Symington III. As for me, I'd have to see one with my own eyes. Until then, it's all just strange stories accompanied by blurry photos. 

Lacking such a happy event, I can only go by arguments based on chemistry and biology, which unfortunately are also fiercely disputed. In First Contact, Marc Kaufman writes of Gil Levin, the scientist who designed one of the Viking lander's experiments in 1976 to detect the presence of bacterial life on Mars. It scooped up some soil, squirted nutrients into it that had been labeled with radioactive carbon-14, and waited to see if gases were emitted that carried that radioactive label.

If labeled gases were emitted, it would suggest that some bug had eaten the nutrient and emitted a waste product. And in fact, that was exactly what was detected: a surge of radioactive carbon dioxide. As a control, the soil was then baked to high temperatures in an effort to kill off anything that might be living, and then the nutrients were added again. This time, no carbon dioxide appeared. It looked very much like a confirmation. Something in the soil had been alive, now it wasn't.

However, other tests carried on Viking, such as one for organic molecules, were negative. Faced with these conflicting results, a consensus formed that Levin's results had to be from some chemical rather than biological process. But ever since, Levin has been arguing that these other tests were flawed; that, for instance, the organic-molecules instrument couldn't detect low concentrations that are now known to be capable of supporting life.

Kaufman also writes about apparent fossils of bacteria on the Murchison and ALH84001 meterorites, which originated in outer space and Mars, respectively. Here, the debate is about whether the organic material is of extraterrestrial or earthly origin; contamination is always a possibility. The key is to find evidence proving that the bacteria cannot be of earthly origin, such as unusual ratios of isotopes or organic molecules of the "wrong" handedness.

So far, there's no consensus on the Viking results or the claims of asteroidal microbes. It seems as if we will have to get uncontaminated rocks direct from Mars or outer space in order to settle the issue.

These debates would instantly be rendered moot, though, if we found intelligent aliens. This possibility is at the heart of Paul Davies' book The Eerie Silence. He asks, once bacterial life has emerged, how likely is it that will evolve into intelligent life? And after, that, into technological life that can manage communication between stars?

Here, the challenge is to avoid naively assuming that evolution is teleological - that is, that it inevitably progresses toward intelligence. Davies points out the possibly discouraging fact that tool-using intelligence has emerged only once on this planet in 4.5 billion years, out of the many millions of species that have come and gone. Dinosaurs were around far longer than the human species, but never evolved language and tool use. Nor has any other species living today, despite the fact that its history is exactly as long as ours.

And, Davies, writes, it is not a foregone conclusion that an intelligent species will develop the kind of science and technology it needs to communicate across stellar distances. Despite having been civilized for far longer than Europe, the Chinese didn't develop the habits of inquiry and thinking that led to the discovery of Maxwell's equations or radio astronomy. Had the West never risen, would the Chinese eventually have discovered them?

Davies says that historians of science are fond of pointing out that science is intimately dependent on culture; that, for example, science is rooted in the belief that the universe is ruled by transcendent laws rather than by animate spirits. "Unless you expect there to be an intelligible order hidden in the processes of nature...there would be no motivation to embark on the scientific enterprise in the first place," he writes (p. 74).

So that's where studying the august authorities leaves you: still completely without an answer, but at least better informed on what's known so far and what the fundamental issues are.

As for me, I'm hopeful. Microbial life appears to be incredibly tenacious and opportunistic. It appeared not long after the Earth's crust cooled and thereafter spread to occupy literally every square millimeter of it. Thousands of tons of rock were transported by meteorites between the planets every year for billions of years, giving those microbes every chance to spread.