There Are (Probably) Billions of Earth-Like Planets in the Universe

"When I began searching for exoplanets," Geoff Marcy told me last year, "everybody said I was crazy." Back then—in the 1990s, when "planet hunting" was new and, as its name might suggest, somewhat exotic—the notion of finding planets that existed outside our solar system seemed to have as much to do with science fiction as with science. The Times wrote articles noting that "a few skeptics still question whether these objects, called exoplanets, qualify as true planets." 

All of which makes new research, published today in the Proceedings of the National Academy of Sciences, fairly mind-boggling. Marcy and his co-authors, Erik Petigura and Andrew Howard, surveyed 42,000 sun-like stars, searching for the tell-tale visual sign of a planet: the periodic dimming that occurs when that planet crosses in front of its host star. The trio discovered, from that analysis, 603 planets, 10 of which are Earth-sized and orbit their host stars in the habitable zone (the range of distance from a host star that allows the surface temperature of a planet to be suitable for liquid water). The nearest of those planets may be within 12 light-years—a relatively close neighbor in the cosmic scheme of things.

Here's more potential evidence against notions of Earthian exceptionalism: The team estimates that 22 percent—essentially one in five—of sun-like stars have potentially habitable Earth-size planets. (Not all of those planets, however, may be rocky or feature liquid water, a presumed prerequisite for life. And it's worth noting, as well, that the margin of error for that stat is plus or minus 8 percent.) Caveats aside, however, that is still an astoundingly large number. It means, as the Washington Post points out, that the Milky Way galaxy alone "could harbor tens of billions of rocky worlds where water might be liquid at the surface." Yep, tens of billions. Which is a lot of maybe-Earths. 

And which is a reminder, as well, of how far we've come when it comes to our knowledge of the universe. It was only two years ago that Marcy and other members of the Kepler team announced their confirmation of the first known habitable zone planet, Kepler-22b. The discovery of that single planet was big news then. Now, we've expanded our gaze to include billions of potentially Earth-like planets. 

The original mission of the Kepler telescope, which launched in 2009, was to obtain an estimate of the percentage of stars with potentially habitable planets. The rough one-in-five estimate announced this afternoon represents the fulfillment of that mission. (It's work that is based on the extrapolation of data, rather than a careful census of these Earth-size planets directly, Sara Seager, an MIT astrophysicist, points out.) Kepler, having experienced a rather heartbreaking mechanical failure earlier this year, is now unable to continue the search for new exoplanets. But scientists will keep analyzing the data the telescope provided in its three-plus years of planet-hunting. And they'll keep speculating about what those data might mean. Today's finding, Marcy puts it, "represents one great leap toward the possibility of life, including intelligent life, in the universe."