ARE WE ALONE?
by Art Hobson
Are we alone? Humankind has speculated on this question for at least 2000 years. In the first century B.C., Roman poet Lucretius suggested that, just as life originated by spontaneous chemical interactions on Earth, "we must acknowledge that such combinations of other atoms happen elsewhere in the universe to make worlds such as this one. …So we must realize that there are other worlds in other parts of the universe, with races of different men and different animals."
Nineteenth-century essayist Thomas Carlyle has a darkly humorous take on the issue: "A sad spectable. If the stars be inhabited, what a scope for misery and folly. If they be not inhabited, what a waste of space." And for contemporary architect Buckminster Fuller, "Sometimes I think we're alone. Sometimes I think we're not. In either case, the thought is staggering."
Today we can do much more than speculate on extraterrestrial life. We've discovered planets orbiting 108 (and counting) other stars. These "extra-solar planets" are nearly impossible to spot visually amidst the glare from their "parent" star, so we've detected them by observing the small back-and-forth motions of their parent stars caused by the motion of the planet around the star. To date we've managed to detect only large Jupiter-sized planets, but there's every reason to expect that these other solar systems also harbor smaller planets. There are theoretical reasons to believe that about 10 percent of the stars similar to our sun have Earth-like planets in orbit around them.
Three upcoming space missions will bear on the existence of Earth-like planets and life on them. If an extra-solar planet's orbit happens to be oriented exactly edge-on as seen from Earth, then each orbit of the planet will cross directly in front of the star. This causes a slight dip in the star's light as seen from Earth. NASA will launch a satellite in 2007 that will search for such dips in the light from 100,000 stars in a small patch of sky. This method will tell us what fraction of the 100,000 stars have Earth-sized planets.
If that fraction is as high as 10 percent, it will bode well for further missions. In 2010, another satellite will monitor 2000 "nearby" stars for the kinds of subtle back-and-forth motions that Earth-sized planets would cause. This survey might reveal 200 or more nearby planets for the third mission to zero in on.
That multibillion-dollar mission might be launched jointly by NASA and the European Space Agency around 2015. In one design, four free-flying but carefully synchronized mirrors in space will combine slightly different views of each "parent" star so as to just cancel the star's light and reveal the much dimmer reflected light from planets orbiting the star. This method promises to detect not only Earth-like planets, but also the basic ingredients of their atmospheres. If there is another pale blue dot out there harboring Earth-like life, such telescopes could detect its chemical imprint.
There is plenty of reason to think there's life out there. The carbon-based chemicals that build life on Earth are widespread throughout our galaxy. Liquid water appears abundant in our solar system and probably in others. There is evidence for watery oceans during the history of Mars and today on three of Jupiter's moons, and for icy oceans on the planet Pluto, on Neptune's moon Triton, and on Saturn's moon Titan (now being studied by NASA's Cassini spacecraft). Earthly life in extreme environments indicates that life could exist at the bottom of ice sheets where the ice is heated from below by underground radioactivity.
Much is known about the formation of our solar system and Earth 4.56 billion years ago. Scientists agree that present life started surprisingly early, 3.8 billion years ago. Many experiments indicate that life's chemical building blocks --nucleic acids and amino acids--form surprisingly easily from gases similar to the early Earth's atmosphere. One plausible route for the "chemical evolution" from non-living to living forms involves ribonucleic acid, which combines features of both DNA and proteins and could have performed the functions of both in the early biological world. Similar processes could easily operate on other watery planets. Thus, nearly all scientists who have studied the question believe that life is abundant in our galaxy and throughout the universe.
But is there other intelligent life out there? That is a far harder question. Some 40 percent of our galaxy has now been scanned for intelligent radio signals from other planets, searching in places and in radio frequencies that seem most plausible for communication, with negative results. It is beginning to look as though we are alone, or nearly alone, in our galaxy. Thus, we might be the only form of matter in our galaxy capable of understanding such things as galaxies--the only form through which the universe can begin to understand itself.
But it's hard to believe that intelligent life arose only here, when our solar system appears so non-unique. What, then, explains this absence of extraterrestrials? One explanation is especially compelling: Perhaps intelligent life, once it develops technology, is unable to overcome its own biological instinct to procreate and to amass power and thus "grows" itself to extinction.
This "short lifetime hypothesis," first suggested 50 years ago by the great physicist Enrico Fermi, is a kind of lesson from the stars, urging us not to misuse technology. The short lifetime hypothesis suggests that, in our present technological period, we had better use our brains, rather than following our instincts.