Space / New Planets

Two newly discovered planets (foreground) orbit the double star NN Serpentis.

Courtesy Stuart Littlefair/Univ. of Sheffield

A 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 1992, 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. But as William Borucki, the head of NASA’s Kepler team, points out, Earth-size does not mean Earth-like. Many of these are inhospitable, roasting-hot worlds.

The most intriguing discovery from Kepler is that 53 of those 1,200-odd planets dwell in the life-friendly “Goldi­locks” zones of their stars, regions where temperatures would be just right—not too cold and not too hot—for liquid water. Where there’s water, scientists reason, there could be life. Four of these Goldilocks planets are super-Earth size or smaller, meaning that they could actually resemble our own home. Even the giant worlds may offer hope. “Some of the Jupiter-size planets could conceivably have Earth-size moons, and those moons would of course also be in the habitable zone,” Borucki says.

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.”

When astronomers started finding planets around other stars in the 1990s, they fully expected to see the general structure of our own solar system repeated throughout the cosmos. Small, rocky planets like Mercury, Venus, Earth, and Mars would orbit close to the star. Giant, gaseous planets would occupy more distant orbits, just as Jupiter and Saturn do in our solar system.

The reasoning seemed self-evident. The heat from a parent star would tend to blast away lightweight elements like helium and hydrogen. Only rocky, sturdy planets could form nearby; giant planets would form farther out, where ices and cool gases could gather together. Astronomers had no way to check their assumptions, though, because for most of the 20th century there were no telescopes capable of detecting planets around other stars. The few astronomers who even attempted to look for them languished in obscurity, spending years in fruitless searching. Such was the state of astronomy when Borucki began his career at NASA.

Borucki, 72, is a mild-mannered man, patient with bureaucrats and unruffled by skeptics, traits that served him well in his three-decade quest to find those alien worlds. Without him there would be no Kepler spacecraft, and this year’s bumper harvest of planets would not have happened.

Borucki joined NASA’s Ames Research Center in 1962, straight out of the University of Wisconsin with a master’s degree in physics. He couldn’t imagine a better job. As a boy growing up in rural Wisconsin, he had launched homemade rockets. His first assignment at NASA—researching heat shields for the Apollo moon missions—suited him perfectly. Like everyone else who worked at the space agency in those heady days, Borucki dreamed of exploring other worlds. NASA Ames was also the headquarters for SETI, the Search for Extraterrestrial Intelligence, and Borucki became friendly with one of SETI’s founders, the visionary astronomer Carl Sagan.

After the Apollo program ended, Borucki grew fascinated with the idea that Earth-like planets might be orbiting other stars, unseen. He was particularly intrigued by a presentation on transit photometry, a theoretical technique that would make it possible to bring those planets into view. The concept was to equip telescopes with extremely sensitive electronic light detectors that could record the slight dip in brightness that occurs when a planet passes in front of a star. It was a brilliant idea, but it was a case of theory outpacing technology; existing detectors were not precise enough to measure such small variations in starlight consistently.

Borucki began studying the problem in his spare time. In 1984 he published a paper sketching a plan to launch a telescope into space to hunt for planets outside our solar system. In the clear vacuum of space, it could avoid the distorting effects of our own planet’s atmosphere.

That same year Borucki even managed to get a bit of funding from NASA to host a conference dedicated to transit photometry. During the conference, some scientists from the National Bureau of Standards suggested that silicon diode detectors might provide the kind of precision that Borucki needed for transit photometry. When struck by a particle of light, the detectors emit a single electron. And converting starlight efficiently into a detectable electronic signal was crucial for transit photometry.

The problem was that each detector could track just one star, and Borucki knew that stringing thousands of detectors together to track enough stars to make the statistics meaningful was wildly impractical. “Every year I’d go to my branch chief for a performance evaluation. Every year he would tell me, ‘Bill, this is never going to work.’ And I never got promoted. There wasn’t much support. On the other hand, no one ever tried to stop me. NASA Ames has a director’s discretionary fund, and anyone who has a new idea can apply for it. That supported me for quite a while.”

By 1992 Borucki’s efforts had advanced to the point where he had assembled a team that began making formal proposals for a fully funded mission. He had determined that chips called charge-coupled devices, or CCDs (the kind used in many digital cameras), could provide the precision needed to accurately measure extremely small changes in starlight. Equally important, they could be packed into arrays capable of monitoring thousands of stars simultaneously. Borucki’s team submitted a plan for a mission it called FRESIP, short for Frequency of Earth-Sized Inner Planets. NASA rejected it, citing lack of proof that the CCD technology would work.