Journey that began four decades before Columbus sailed for the New World finally ended when the Kepler space telescope snared a few errant photons as they shot past Earth’s orbit en route to infinity. The light had sped through space for 560 years, traveling more than three quadrillion miles from a star much like our sun. Captured by Kepler’s digital sensors, transformed into bytes of data, and downloaded to computers at NASA’s Ames Research Center near San Francisco, the processed starlight slowly revealed a remarkable story: A planet not much bigger than Earth was whipping around its native star at a blistering pace, completing an orbit—its version of a “year”—in just over 20 hours.
Aside from its size, the planet bears little resemblance to Earth. It circles so close to its star that its surface temperature probably exceeds 2,500 degrees Fahrenheit, hot enough to melt iron. Nevertheless, the planet’s detection was a technical and intellectual coup, a rite of passage for Kepler. The planet, dubbed Kepler-10 b when NASA announced its existence this past January, was the smallest world yet found beyond our solar system. Its discovery proved that the Kepler spacecraft, which was launched in March 2009, could indeed do what its designers had boldly promised: find small, Earth-size planets around distant stars, a task that once seemed so difficult as to border on the absurd.
Kepler-10 b was merely a preview. A month after the January announcement, NASA released its first full data set from the Kepler mission, and the results left astronomers straining for superlatives. “Frankly, we’re overwhelmed,” says Geoff Marcy, an astronomer at the University of California, Berkeley, and a member of the Kepler team. “What NASA is doing is akin to the transoceanic voyages of the 15th century—the voyages that opened up the whole world. With the Kepler telescope, we’re learning about the properties of planets across the cosmic ocean. This is history. It’s Armstrong stepping off the bottom rung.”
Kepler monitors 156,000 stars, less than 0.0001 percent of the galaxy’s population. Like some cosmic wildcat drilling operation, Kepler has struck a gusher, discovering 1,235 possible new worlds in its first four months of operation. That number doubles the previous total of just over 500, painstakingly gathered over the last 16 years. Prior to 1995, keeping track of all the known planets around other stars like the sun was easy—the tally stood at an even zero.
The bulk of Kepler’s data have not yet been studied, and the mission will keep going for at least two and a half more years. But it is already shredding the textbooks, showing that our galaxy (at least the fraction of it seen by the spacecraft) contains a far more exotic assortment of planets than astronomers expected to find. “We’re learning about a diversity of worlds in our universe that we had no clue about beforehand,” Marcy says. “Rocky planets, yeah, we thought there might be some of those. By the way, we’re finding some rocky planets that are even denser than Earth. But we’re also finding these mini-Neptunes, a class of planet for which we have no examples in our solar system. They’re like small Neptunes but with huge amounts of liquid water around a rocky core.”
Also on the list are 67 planets roughly the size of Earth, give or take a thousand miles or so in radius; 288 “super-Earths” up to twice Earth’s diameter; 662 Neptune-size planets; and 184 giants rivaling or exceeding Jupiter in size. The simple statistics from Kepler say that Earth-size planets are widespread.
Just two years into its mission, Kepler is well on its way toward determining whether planets like Earth are rare or common. But that is just the first domino as scientists try to topple the much bigger questions, the kinds that make the hairs stand up on the back of your neck. Does life exist on other planets? Are planets with life common? Are any other intelligent beings out there?
For the first time, we have a handle on the odds, and the numbers beaming in from Kepler are not only encouraging but staggering. “Our galaxy contains 200 billion stars,” Marcy says. “I would guess that at least 30 percent of them have an Earth-size planet. So 30 percent of 200 billion, that’s at least 60 billion Earth-size planets just in our galaxy alone”...
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