Thirty million kilometers away, trailing the pale blue dot that is Earth as it orbits the sun, is a spacecraft designed to find some of the countless other pale blue dots that may speckle the galaxy. NASA launched this spacecraft, known as Kepler, in 2009 to take a census of Earth-like planets in the hopes of figuring out how common—or how rare—are the conditions under which life has thrived here.
Just over halfway into that 3.5-year campaign, Kepler has not been on the job long enough to conclusively identify any habitable worlds, but its discoveries are growing ever more tantalizing. In a field where small is good—small meaning less like Jupiter and more like Earth—the latest batch of planets netted by the space observatory includes five of the eight smallest worlds now known outside the solar system. All five of the new extrasolar planets, or exoplanets, as well as one more world whose properties are not yet fully understood, orbit a sunlike star called Kepler 11, some 2,000 light-years away. A group of researchers announced the discovery of the six-planet Kepler 11 system in the February 3 issue of Nature. (Scientific American is part of Nature Publishing Group.)
Almost simultaneously, the Kepler team was preparing a list of more than 1,200 additional objects the spacecraft has located that may also be planets. "We have a huge number of candidates," says Kepler's principal investigator, Bill Borucki of NASA Ames Research Center, Moffett Field, Calif. "It's very exhilarating," says Borucki, who first put forth the idea for Kepler in 1984 and proposed the concept several times before getting NASA's approval and funding. The data on the numerous candidates are somewhat preliminary and require validation, but a new analysis by a pair of astrophysicists at the California Institute of Technology suggests that the percentage of false positives among Kepler's candidate planets may be less than 10 percent.
If that is the case, then Kepler is well on its way to vastly augmenting the roster of known exoplanets, of which there are now 500 or so, and may have already gotten a whiff of several potentially habitable worlds. "We have 54 planets in the habitable zone of their stars," Borucki says, referring to the temperate orbital zone around a star that would allow for the existence of liquid water on a planet. "One of them is 0.9 times the radius of the Earth, and four of them are less than two Earth radii." Any of those would be the most Earth-like world ever detected outside the solar system. What is more, some of the larger, more Jupiter-like planets Kepler is sniffing at in the habitable zone might have moons, and some of those satellites would themselves be potentially habitable. "It's sort of awesome," Borucki says of Kepler's haul. "The implications are that there are an awful lot of planets out there."
For now, those 1,200-plus objects remain mere candidates requiring follow-up and confirmation. Until then the six planets around Kepler 11, which have been validated by various means, are the "Kepler spacecraft's latest—and we think its greatest—finding to date," said mission co-investigator Jack Lissauer of Ames during a January 31 teleconference with reporters. The Kepler 11 system is unique for several reasons: For starters, it is among the largest collections of worlds known outside our own solar system, and all six of the planets Kepler has found there are aligned so that their orbits carry them across the face of their host star from Kepler's vantage point.
Kepler continuously tracks more than 150,000 stars; when a planet passes in front of one of them, in a kind of mini eclipse known as a transit, the spacecraft registers a slight dip in the star's apparent brightness. Transits, though subtle and brief, carry a great deal of information about a planet: the amount of dimming conveys the diameter of the planet, and the frequency of transits provides the planet's orbital parameters. But such fortuitously aligned planets are relatively rare; aside from Kepler 11, only one star is known to host more than one transiting planet. A planetary system has to be almost perfectly planar for multiple planets to transit—Lissauer compared the astonishing flatness of the Kepler 11 system to a scaled-up vinyl LP.
What is more, the planets of the Kepler 11 system are so densely packed that all six would fit within the orbit of Venus, the second-closest planet to our own sun. (All of the newfound worlds are too close to their star and thus too hot to be plausibly habitable.) That is by far the densest planetary configuration known, Lissauer said. In fact, the five innermost worlds around Kepler 11 are so close together that gravitational interactions among them produce measurable perturbing effects on their individual orbits, allowing the researchers to make estimates of each planet's mass. The sixth, outermost planet is too far from any neighbors to exert such effects, so the Kepler scientists can only set an extreme upper limit on its mass.
Among the five worlds whose properties are well understood, relative to Earth the planets range from 2.3 times to 13.5 times its mass, and from about two times to 4.5 times its diameter. A few other planets are known to be smaller, but the initial estimates peg two of the bodies orbiting Kepler 11 as the lowest-mass worlds for which a complete set of planetary information—mass, diameter and orbit—has been obtained.
Borucki says it will be a few years yet before Kepler is able to identify a true Earth analogue—a small planet on a one-Earth-year orbit around a sunlike star. Until then, astronomers are learning a great deal about a poorly understood midsize class of planets. "In our solar system we have no planets between the mass of Earth and the mass of Uranus, which is about 15 Earth masses," Jonathan Fortney of the University of California, Santa Cruz, a planetary scientist affiliated with the Kepler mission, said during the teleconference. "Now with the Kepler 11 system we have five more of these intermediate-mass planets to add to the three from previous exoplanet discoveries. We're still learning how nature makes planets in this intermediate mass range; however, it is clear that such planets need not resemble the Earth in any way." The low densities of the Kepler 11 planets imply a bounty of light elements and a dearth of iron and rock, making the midsize worlds more akin to mini Neptunes than to terrestrial, supersize Earths.
Having so many worlds in one system—and so many planets that spill their secrets via transits—may provide an exceptional test bed for theories of planetary formation and evolution. "Planetary science is very comparative," Fortney said. "Planets are so different from one another that you really need them to be in similar environments and then compare them to each other. And so in the Kepler 11 system we have just a fantastic laboratory, better than any other planetary system yet found, to look at the planets and compare them to one another to understand how these planets have evolved over time in kind of a contained system."