Speculation about the possibility of a "plurality of worlds" goes back at least as far as Epicurus in the fourth century BC. Admittedly, Epicurus' definition of a "world" was closer to what we would currently regard as a solar system — but he imagined innumerable such spheres, each containing a system of planets, packed together. "There are," he declared, "infinite worlds both like and unlike this world [i.e., solar system] of ours."
The same question was subsequently considered by astronomers and philosophers over the course of many centuries; within the past half century, the idea of the existence of other planets has become a science fiction staple, and people have started looking for evidence of extraterrestrial civilizations. Like Epicurus, many people have concluded that there must be other solar systems out there, consisting of planets orbiting other stars. But they didn't know for sure. Today, we do.
The first "extrasolar" planet (ie, beyond the solar system) was found in 1995 by two Swiss astronomers, and since then another 48 planets have been found orbiting dozens of nearby sun-like stars. This figure is subject to change, because planets are now being found at an average rate of more than one per month; more planets are now known to exist outside the solar system than within it. Furthermore, one star is known to have at least two planets, and another has at least three. We can, in other words, now draw maps of alien solar systems — maps that were previously restricted to the realm of science fiction.
The discovery that there are other planets out there has not, however, caused as much of a fuss as might have been expected, for two reasons. First, decades of Star Trek and its ilk meant that the existence of other worlds was assumed; the discovery has merely confirmed what has lately become a widely-held belief. And second, none of these new planets has actually been seen. Instead, their existence has been inferred through the tiny wobbles that they cause in the motion of their parent stars. The first picture of an extrasolar planet is, however, probably just a few years away. Like the first picture of Earth from space, it is likely to become an iconic image that once again redefines the way we as humans think about our place in the universe.
Incidentally, none of these new planets has a name yet, because the International Astronomical Union, the body which handles astronomical naming, has yet to rule on the matter. But the two astronomers who found the first extrasolar planet have proposed a name for it anyway, and one that seems highly appropriate: they think it should be called Epicurus.
TOM STANDAGE is technology correspondent at The Economist in London and author of the books The Victorian Internet and The Neptune File, both of which draw parallels between episodes in the history of science and modern events. He has also written for the Daily Telegraph, The Guardian, Prospect, and Wired.He is married and lives in Greenwich, England, just down the hill from the Old Royal Observatory.