If there is one scientific topic that provokes strong opinions in the absence of any direct knowledge whatsoever, it is life on other planets. Some scientists feel certain Earth is the only place in the universe where life resides; others insist the question is not whether life exists in other places, but where. You will find sane, rational people with full command of the facts coming out on either side of this debate.
I lean toward the second possibility. The notion that in the entire universe, the only planet with living things just happens to be the only one we've ever seen up close, seems a stretch to me. But I admit I have no proof. It's just a hunch.
"Just a hunch" is the best we can do when it comes to these arguments, which never stand up to logical scrutiny. Some scientists point to evidence that life arose on Earth quite soon after the planet's initial formation (say, only several hundred million years), and if it can happen that quickly, it must have happened in other parts of the universe. The problem with this argument (aside from a rather fluid definition of "quickly") is that we still don't know how life arose here, or even whether it did--one theory holds that it came from Mars via meteorite. If we don't know what happened to cause life to exist, then we have no way of knowing whether it could have happened elsewhere.
Other scientists believe the origin of life on Earth was a rare, freakish occurrence in the universe's history, because it depended on a huge number of factors in combination, including Earth's exact composition, temperature, rotation, orbit, and so on. If any one of these factors had been slightly different, so the argument goes, life on Earth either would not have started or would have perished long before it had a chance to evolve into more complex creatures like ourselves.
The answer to this objection is twofold. First, extraterrestrial life could differ from Earth life in ways we have trouble imagining. Second, the universe is just really big. What scientists call the observable universe, or the portion we have the capacity to detect because the light has had enough time to reach us since the beginning, is estimated to have over one hundred billion galaxies. Our own galaxy alone, which remains virtually unexplored, contains hundreds of billions of stars. However unlikely the conditions necessary for life may seem, there is a great deal of space out there for those conditions to be repeated.
In the movie Contact, a scientist played by Jodie Foster argues that there must be life beyond Earth, for otherwise there would be "an awful waste of space." You could interpret this statement different ways. For religious believers, the question is why God would have created all those countless galaxies when only one planet of one star would be inhabited. Perhaps it is so that people in the future will have places to go after Earth is used up or on the brink of destruction. Still, that's an awful lot of space....
A more secular version of the same argument might go something like this: In the vastness of outer space, our solar system is a mere speck in a sea of galaxies that all look more or less like ours. Given the relative uniformity of everything from afar, our default assumption should be that the basic conditions of our vicinity are not unique. This reasoning might not convince those of us who suspect the origin of life was miraculous, but it ought to be good enough for those of a more secular mindset.
One comparison is instructive. Most people have long assumed that there are planets beyond our solar system. Yet until the 1990s, with the discovery of a star that wobbled (suggesting it was being orbited by a large object), there was no concrete evidence that any of these so-called extrasolar planets existed. For all scientists knew, our sun could have been the only star in the entire universe that harbored planets. That possibility runs contrary to intuition for much the same reason the idea that we are alone does. It proved to be incorrect, and I suspect the second idea will also.
Some people will say, "Okay, maybe there's life in other places. Perhaps things on the level of bacteria or even plants have sprouted elsewhere in the universe. But intelligent life? Advanced civilizations? How likely is that?" Skeptical scientists point to some discouraging facts. Although bacteria appear fairly early in the Earth's fossil record, there was nothing but bacteria for the next three billion years--complex, multicellular life doesn't appear until the last billion. Furthermore, current scientific consensus holds that an asteroid caused the extinction of the dinosaurs, and if not for that fortuitous event, brainy mammals would never have come to dominate the planet.
Science fiction movies always assume that alien life must look more or less similar to us. In the real world there are creatures as weird as sea anemones living right here on Earth, but movie aliens invariably have a more standard appearance. Take the recent Avatar. In that film, a crew of Earthlings is investigating the exotic life forms that we're supposed to believe evolved independently in an entirely different solar system. And just how exotic are these creatures? Other than their possessing tails, blue skin, a ten-foot stature, and a few other odd features, they basically look human, sound human, and act human, right down to the mating rituals. The movie's plot is sort of an amped-up Dances with Wolves, only instead of a white man becoming part of a tribe of Lakota Sioux Indians, it has a white man becoming part of a tribe of giant alien smurfs.
I shouldn't make fun. The director, James Cameron, has said he intended the film as "science fantasy" rather than science fiction, and that they'd first designed the aliens as less human-looking before worrying it would turn off audiences. At least the film is a touch more plausible than a lot of older sci-fi like Star Trek, where aliens are shown as humans with pointy ears or ridged skin. But it's striking that more recent flicks like Independence Day and Signs continue to fall back on the most worn cliches about extraterrestrials, depicting them all as green and warty and malevolent.
The most versatile aliens depicted on screen are still probably those in Star Wars. I never fully appreciated that fact until I read Jeanne Cavelos's The Science of Star Wars (1999). Herself an astrophysicist (who also wrote The Science of The X-Files), Cavelos makes a noble effort to find scientific basis for all the major components of the series, including the ships, the weapons, the planets, the aliens, the droids, and even the Force. She doesn't always succeed--but she does explore much of the current research on subjects ranging from A.I. to quantum mechanics. (For a broader discussion about how far along we are toward inventing the technologies of science fiction, I highly recommend the 2008 book Physics of the Impossible by Michio Kaku, one of my favorite science writers, and one of the scientists Cavelos interviews.) Her chapter on aliens begins with the following observation about the films: "Almost anywhere you go in 'a galaxy far, far away,' alien life is there. You'll either land in it, step on it, or get eaten by it.... Even in environments as inhospitable as Tatooine, Hoth, or an asteroid, life finds a way to survive."
Of course Star Wars has always been one of the more unabashedly unscientific sci-fi franchises. There are serious difficulties in explaining certain things such as how all the creatures breathe comfortably no matter what planet they're on, which seems unlikely unless they're using some sort of technological aid we never see. Cavelos points out that planetary atmospheres may be unique like fingerprints, which would mean that no life form from one planet can breathe naturally on another. Avatar gets this detail right, having the human explorers only able to breathe with gas masks or in air-tight buildings or inside their Avatar bodies.
Still, the sheer diversity of the aliens in Star Wars is impressive, especially compared to earlier films. As Dr. Kaku explains, "the aliens don't look like us anymore. They tried to have aliens with different architectures. In that sense, Star Wars is more realistic than some of the stuff I've seen." And yet many of these aliens are simply analogues of Earth animals: dogs (Wookiees), slugs (Hutts), pigs (Gamorreans), bats (mynocks), and buffalo (banthas). Some of the organisms even look identical to what's on Earth, as in Endor's redwoods or Dagobah's snakes. Most of the invented creatures are humanoid, and those that aren't, like Jabba, are still anthropomorphic. How likely is it that aliens would look that similar to what we see around us?
The book quotes one scientist who argues that most of the visible features that humans possess, such as five-digit hands, are intrinsically efficient characteristics that evolution would tend to produce even on other planets, similar to the way aquatic mammals like dolphins acquired a fish form. Most scientists disagree. As Dr. Jack Cohen puts it, "Finding another planet with our kind of dinosaurs or people is more unlikely than finding a remote Pacific island on which the natives speak perfect German."
Science fiction literature has had the opportunity to envision alien life more broadly than the movies do. Strange life forms appear in hard sci-fi novels such as Robert Forward's Dragon's Egg, which speculates about what the intelligent inhabitants of a neutron star would look like. Part of the reason why movies aren't as daring is budgetary constraints. Commercial films have the most resources for showing aliens. The problem, as James Cameron tells us, is that mass audiences aren't interested in seeing anything truly weird.
Life as we understand it can mean different things, from simple bacteria to advanced civilizations. But since our only frame of reference is what we see around us, we have trouble visualizing aliens without comparing them to familiar creatures. If aliens exist, they might be more bizarre than anything we've ever thought of, much less seen. We can't possibly be certain what lies beyond our immediate knowledge.