“To consider the Earth the only populated world in infinite space,” the Greek philosopher Metrodoros the Epicurean wrote about 300 BC, “is as absurd as to assert that in an entire field sown with millet only one grain will grow.”

I believe that too, although the only evidence I have for its truth is the same as Metrodoros’: simple common sense. He knew of five planets aside from our own—Mercury, Venus, Mars, Saturn, and Jupiter —and perhaps he thought they were inhabited, but also he could see that the sky was full of stars. He didn’t have any telescopes to show him the existence of worlds of other stars, and neither do I. But the universe is infinitely large, as Metrodoros understood two and a half millennia ago, and that which is infinite contains an infinite amount of everything. It is a place in which not merely uncountable numbers of suns exist, but uncountable numbers of galaxies. To me it seems unlikely, to say the least, that in all that literally unthinkable multitude of galaxies there is only one planet, one little dinky world, on which living beings can be found.

For many centuries the concept of a multiplicity of worlds was dangerous heresy in Christian Europe. A literal interpretation of the Bible had produced the belief that God had created the world—flat, floating on water—in six days, and had placed the sun in the sky to provide light, and in that same week had created man and woman and various subordinate creatures to populate it. Earth was unique and at the center of the universe. There was nothing in Scripture about other worlds or other forms of life; therefore, such things did not exist.

The work of the sixteenth-century Polish mathematician Nicolaus Copernicus began the process of undermining the concept of a geocentric cosmos. Copernicus showed that the Earth and its sister planets must move in orbit around the sun, rather than the sun going around us, as it appeared to do; but he mistakenly thought the orbits were circular, and it remained for the German astronomer Johannes Kepler, building on the work of a Dane, Tycho Brahe, to show that the planetary orbits were in fact elliptical. With the mathematical foundations now in place, the modern view of the universe began quickly to emerge. I suppose there are still some believers in the pre-Copernican theory of the universe, but very few, I suspect, are readers of this magazine; the rest of us have no difficulty with the notion that Earth travels around the sun and is just a speck in a vast universe full of stars and—very likely—a host of other planets more or less like our own.

That extrasolar planets exist is no longer theoretical. Our telescopes aren’t powerful enough to show them to us, but in one indirect way or another we have demonstrated the existence of some 330 such worlds. (For lovely and plausible paintings by Lynette Cook of what these already discovered extrasolar worlds might look like, check out extrasolar. spaceart.org/extrasol.hmtl. The ones located so far, though, have had to be big enough—Jupiter-sized, at least—so that their gravitational field perturbs the motions of the sun about which they move in a detectable way, and that means that they are too big to support the kind of life that thrives on Earth. That is not to say that no Earth-sized worlds exist out there, only that we are currently unable to detect their presence. But in an infinite cosmos there surely must be an infinite number of worlds, including some very much like Earth, and at this moment we have a space-going telescope up above us, searching for those other Earths.

It’s called, appropriately enough, Kepler. NASA launched it in March 2009, putting it into solar orbit at such an angle that Earth itself would not block its view of the galaxy. Because it is located in space, the images it collects are not subject to atmospheric blurring (the effect that makes the stars seem to twinkle, down here on Earth). A couple of months of tweaking were necessary before Kepler became fully functional, but it has been sending back data since July 3, 2009, and very likely some interesting revelations will have come forth by the time you are reading this piece, nine months or so after I’ve written it.

Kepler is not the only space-based telescope capable of spotting extrasolar planets. The French Space Agency and various European partners launched one in December 2006 called, in a nice Gallic cultural touch, COROT—COnvection ROtation and planetary Transits. Within five months it had sent back data on its first discovery. But COROT is best suited for detecting planets greater in diameter than Earth. As for our Hubble telescope, it was designed for other uses than the quest for extrasolar planets, and does not remain focused on any one star group long enough to gather the kind of information needed.

Kepler, though, has a fixed field of view that continuously observes more than 100,000 stars in the constellations of Cygnus, Draco, and Lyra. These constellations were chosen because they lie outside the plane of the ecliptic and thus will not be hidden from Kepler’s eye by sunlight. During its lifespan of some 3.5 years, Kepler will be looking for planetary transits, the passing of a planet in front of its star. Such a transit would cause a temporary reduction in the star’s apparent brightness; the transit of an Earth-sized planet, for example, would briefly reduce the observed magnitude of its star by 0.01 percent. We can’t detect such a minute fluctuation with Earth-based telescopes. But Kepler can, and because its gaze is constantly fixed on the same stars it not only can take note of the movements of such relatively small worlds but keep track of the interval between transits, from which the size of the planet’s orbit can be calculated and even some conclusions thus drawn about its climate.

Kepler will need to record at least three planetary transits to make certain that it is a planet that is causing the dimming, and not some random fluctuation of the star. Thus the first reports from Kepler probably will tell us about planets hundreds of times as big as ours, moving in orbits relatively close to their stars, since those are easiest to detect. Anything living on such a world would have to put up with immense gravitational forces and searing solar radiation, and, therefore, whatever kinds of life-forms such planets might have—animated balls of plasma, drifting networks of pure energy, whatever—will not be anything like those of Earth. For the time being, thinking about such beings must remain in the realm of pure speculative fantasy. Even when Kepler begins locating smaller worlds, the ones that have the sort of gravitational pull and geological structure that would make them habitable by Earth-type life, it will be necessary to consider that those that move in orbits extremely distant from their suns are likely to be chilly places, unsuitable also for Earth-type life, and those that are very close will be too hot. We know enough about our own solar system to understand that only one of its nine planets (or eight, if you are a Pluto-denier) can support our kind of life. But there’s no doubt that the universe is full of planets, and that one chance out of nine, if it carries through everywhere, yields the realization that the cosmos is teeming with habitable worlds.

What conditions are needed, after all?
There are three basic requirements for the development of life on any planet: a building-block substance chemically able to join with other elements to form complex compounds, a solvent in which atoms and molecules can move about freely to take part in chemical reactions, and the chemical reactions themselves, resulting in the production of energy, so that the vital processes of life can occur. On Earth the building-block substance is carbon, the solvent is water, and the energy-producing chemical reactions are brought about by oxygen. Other combinations are possible, though: carbon and ammonia and nitrogen, for example.

Temperature is also a factor: if a planet is too cold its water turns to ice, too hot and it turns to a gas. Therefore the planet must be at the proper distance from its sun, and the sun itself be neither too bright or too dim. If we discard from our reckoning the stars that are too small and the stars that are too big, we find that we have rejected seven stars out of eight as possible sites in our galaxy for life-bearing planets. But that still leaves a nifty twelve billion stars, just in this one galaxy. If half of them have solar systems, and half of those have planets that lie at the right distance to maintain water in its liquid state, and half of these are the right size to retain an atmosphere, a billion and a half planets remain. Rejecting half a billion of those because they are too big, because they don’t rotate on their axes, because they have no water, or because they are otherwise unsuitable, we still have five hundred million Earth-type planets in our galaxy alone! And there are millions of galaxies.

Leaving out of account the chances of the existence of really alien life-forms—based on silicon instead of carbon, or using ammonia rather than water as a solvent, say—the probability still seems overwhelming that the universe is teeming with life. Some worlds may be populated only by simple one-celled creatures, others may be swarming with fish and crustaceans, others may be worlds of insects, of amphibians, of reptiles, and some may hold intelligent civilized beings, possibly far beyond us in their achievements. All of that is probable. What isn’t probable is that there’s nobody else around at all.

You may ask—especially if you are the sort of person who keeps the makers of flying-saucer movies busy—why they haven’t been in touch with us, then. The simplest answer is that the universe is very big, traveling between stars would take so much time that it seems hardly worthwhile (or even feasible), and the chances that any of those distant interstellar civilizations would notice that we are here and worth visiting are very small. Until the development of faster-than-light space travel, itself a very low-probability concept, we ought not to expect any contact with the other intelligent races of the galaxy.

But it still would be of some interest to know where they might live. The first step in finding out where our neighbors could be has now been taken. Perhaps by the time this column sees print, the Kepler telescope will have sent back the first snapshots of those other Earths far off in the heavens.