Gliese 876 is a modest star, just one-third the mass of our sun and only 15 light-years away, but it has a history-making planetary system all its own. In 1998 a team led by Geoff Marcy of the University of California at Berkeley detected the first sign of something interesting there: a giant planet, twice the mass of Jupiter, circling Gliese 876 once every two months, its gravity yanking the star back and forth at the speed of a jet plane. Three years later the same group found a second planet, half the mass of Jupiter and closer in, pulling the star around at the speed of a race car. Although the planets are too faint to be seen directly, their motions cause the star’s spectrum to wobble back and forth across the digital detector of an astronomical telescope.
In the past decade, announcements of Jupiter-size planets have become commonplace; about 300 of them have been found so far. In 2005, however, with the help of improved detection software, Marcy’s team turned up something else orbiting Gliese 876—something truly new. This invisible object added one more regular component to the star’s motion, like the third note, faint and high, of a piano chord. It was another planet, orbiting in just two days and pulling on the star much more gently, not at jet plane or race car speeds but at a speed a man could run. This planet, dubbed Gliese 876 d, is clearly no Jupiter, Marcy realized. It is no more than seven or eight times as massive as our own: a “super-Earth.” Until then, all the known exoplanets (planets circling other stars) were big and gaseous, but this one is probably made of rocky materials—the first world like ours found in an alien solar system.
Gliese’s super-Earth lies so close to its star that it has just about no chance of being inhabited. If it has an atmosphere at all, it probably consists of dense steam, says Greg Laughlin of the University of California at Santa Cruz, a member of the discovery team. But if we can find one rocky, Earth-like planet right in our galactic backyard, surely there must be many more. Already, the Swiss astronomers who in 1995 discovered the first Jupiter-like exoplanet—and who are the great rivals of the California group in the exoplanet hunt—said in June that they had identified not one but three super-Earths orbiting a single star 40 light-years away. The smallest is just four times as massive as Earth. “We’ll find an Earth-mass planet by 2010,” Laughlin predicts, “and an Earth-mass planet that’s potentially habitable by 2012.”
And yet we still won’t have found a true second Earth. The hallmark of Earth, after all, is not its mass, nor its rockiness, nor the fact that it is potentially habitable. The hallmark is that it is actually inhabited. These days, nobody doubts that there are other reasonably cool, rocky planets out there among the 100 billion stars in the Milky Way. Everything astronomers have learned about how stars and planets form says there must be. But is there life on any of those rocks, and if so, can we detect it? “That’s not going to happen from Earth,” Laughlin says. “It has to happen from space.”
