Robert Kunzig, Discover: 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.