ONCE upon a time, Gliese 581, a dwarf star 20 light-years from Earth as the crow flies, was an unremarkable neighbor and just another point of light in the heavens. That was before European astronomers found two planets out of four orbiting within Gliese 581's "habitable zone," the distance a planet must be from a star to support oceans and, possibly, life.
Both planets in the habitable zone are smaller than Earth. One is a bit too close to Gliese 581 for comfort. Because of its proximity to its host sun, it is closer to Venus or Mercury in terms of its surface temperature than Earth. If it ever supported oceans, they've long since boiled away.
The planetary discovery that has excited scientists is called Gliese 581d. Astronomers believe that the less-than-Earth-sized planet is covered with vast oceans and gases. If it is as much like our planet as scientists believe, it may contain the fateful chemical mix from which life can emerge, much as it did from our own primordial stew millions of years ago.
Scientists working at the European Southern Observatory's 3.6-meter telescope at La Silla, Chile, discovered Gliese 581d and its three neighbors by detecting and analyzing wobbles around the star. Finding two smaller planets in the habitable zone of a distant star is an encouraging sign. The fact that one of them is Earth-like indicates that our own planet may not be as unique as we once thought.
Statistically speaking, there could be millions of planets in the Milky Way much like our own. Maybe they're all barren and lifeless, but if we only find one other planet that supports life, it will be worth the centuries of effort that went into finding it. Finding Gliese 581d is the beginning of an incredible adventure.