Is Anybody (like Us) out There?
Tyson, Neil de Grasse, Natural History
The recent discovery of about half a dozen planets around stars other than the Sun has triggered tremendous public interest. Attention was generated not so much by the discovery of extrasolar planets but by the prospect of their hosting intelligent life. In any case, the media frenzy that followed was somewhat out of proportion to the events.
Why? Because planets cannot be all that rare in the universe if the Sun happens to have nine of them. Also, the newly discovered planets are all oversized, gaseous giants that resemble Jupiter, which means they have no convenient surface upon which life as we know it could exist. And even if the planets were teeming with buoyant aliens, the odds against these life forms being intelligent are astronomical.
Ordinarily, there is no riskier step that a scientist (or anyone) can take than to make a sweeping generalization from just one example. At the moment, life on Earth is the only known life in the universe, but compelling arguments suggest that we are not alone. Indeed, most astrophysicists accept the probability of life elsewhere. The reasoning is easy: if our solar system is not unusual, then the number of planets in the universe would, for example, outnumber the sum of all sounds and words ever uttered by every human who has ever lived. To declare that Earth must be the only planet in the universe with life would be inexcusably bigheaded of us.
Many generations of thinkers, both religious and scientific, have been led astray by anthropocentric assumptions and simple ignorance. In the absence of dogma and data, it is safer to be guided by the notion that we are not special, which is generally known as the Copernican principle. It was the Polish astronomer Nicholas Copernicus, who, in the mid-1500s, put the Sun back in the middle of our solar system where it belongs. In spite of a third century B.C. account of a Sun-centered universe (proposed by the Greek philosopher Aristarchus), the Earth-centered universe has been by far the most popular view for most of the past 2,000 years. In the West, it was codified by the teachings of Aristotle and Ptolemy and the preachings of the Roman Catholic Church, and the geocentric theory was generally accepted. That Earth was the center of all motion was self-evident: it not only looked that way, but God surely made it so.
The Copernican principle comes with no guarantees that it will guide us correctly for all scientific discoveries to come. But it has revealed itself in our humble realizations that Earth is not in the center of the solar system, the solar system is not in the center of the Milky Way galaxy, and the Milky Way galaxy is not in the center of the universe. And in case you are one of those people who think that the edge may be a special place, we are not at the edge of anything either.
A wise contemporary posture would be to assume that life on Earth is not immune to the Copernican principle. How then can the appearance or the chemistry of life on Earth provide clues to what life might be like elsewhere in the universe?
I do not know whether biologists walk around every day awestruck by the diversity of life. I certainly do. On our planet, there coexist (among countless other life forms) algae, beetles, sponges, jellyfish, snakes, condors, and giant sequoias. Imagine these seven living organisms lined up next to one another in size-place. If you didn't know better, you would be hard pressed to believe that they all came from the same universe, much less the same planet.
Given the diversity of life on Earth, one might expect diversity among Hollywood aliens. But I am consistently amazed by the film industry's lack of creativity. With a few notable exceptions-such as life forms in The Blob (1958) and in 2001: A Space Odyssey (1968)-Hollywood's aliens look remarkably humanoid. No matter how ugly (or cute) they are, nearly all of them have two eyes, a nose, a mouth, two ears, a neck, shoulders, arms, hands, fingers, a torso, two legs, two feet-and they can walk. …