The Earth is a perfectly ordinary planet in most respects, but a most extraordinary planet in one respect: it harbors life.

Its ability to do so is largely a matter of happenstance. Unlike Jupiter, Saturn, Uranus and Neptune, the "gas giant" planets of the outer solar system, Earth has a solid surface where oceans can accumulate. And unlike Mercury, Venus and Mars, its fellow rocky planets of the inner solar system, Earth is at just the right distance from the Sun to keep those oceans from freezing or boiling away. The result is a cosmic oasis: one of the very few places in our otherwise sterile solar system that has abundant liquid water, which is an essential prerequisite to life as we know it. (The one other possibility is Jupiter’s moon Europa, which is thought to have liquid oceans under a thick crust of ice.)

The obvious question, then, is whether other planets around other stars have enjoyed similar good fortune. Are there other potential homes for life in the universe?

Until recently, astronomers could only answer, "probably." Their argument was that solar systems ought to be roughly as common as stars themselves, since they presumably formed out of the same kind of interstellar gas and dust. And since there are lots of stars out there—billions of them in our own Milky Way galaxy alone—there ought to be lots of life-friendly planets. But the fact was, nobody knew for sure.

In 1991, however, Alexander Wolszczan of Pennsylvania State University announced that he had discovered the first three planets outside our solar system. Wolszczan didn’t actually see the planets directly; instead, working at the Arecibo radio telescope with support from NSF, he monitored radio signals coming from a rapidly rotating neutron star, or "pulsar," and saw small, regular variations in the signals that were caused by the planets swinging around and around in their orbits. Nor did Wolszczan’s new planets seem likely to harbor life; they were roughly the right size—two of them were similar in mass to the Earth, and the other about equal to the mass of our moon—but their tiny pulsar "sun" was constantly blasting them with deadly radiation. Nonetheless, Wolszczan’s discovery of planets around an object as exotic as a pulsar proved that other solar systems were actually out there, and might even be fairly common.

That conclusion was bolstered in 1995, when two Swiss astronomers found a fourth new planet, this time orbiting a star similar to the Sun. Their discovery was confirmed by two NSF-supported astronomers, Geoffrey Marcy and Paul Butler, who went on the following year to announce the discovery of two more planets orbiting Sun-like stars.

The pace of discovery has accelerated rapidly since then, as Marcy, Butler and many other astronomers have made use of increasingly more sophisticated techniques for observation and analysis. In 1999, for example, two independent NSF-supported teams discovered that at least three planets were circling the star Upsilon Andromedae, making it the first multi-planet solar system around a Sun-like star.

Today the count of extra-solar planets stands at more than 120. (NASA’s Jet Propulsion Laboratory maintains an up-to-date list on its PlanetQuest site.) It’s still the case that none of these planets seem particularly promising for life. Most of them are too close or too far from their star. And all of them are gas giants as big or bigger than Jupiter, which is hundreds of times more massive than the Earth. Finding planets as small as our own will require specialized space-going telescopes that are still on the drawing boards. Still, it already seems clear that our Milky Way galaxy is rife with stars supporting planetary systems—which suggests that someday, in the not-too-distant future, the question of whether or not we are alone may have an answer.

Return to Overview