The catch is that to produce a detectable blip, an exomoon would have to vastly outweigh any satellites found in our solar system. "In the very best case, Kepler could find moons down to 20 percent the mass of the Earth," Kipping says. That means Kepler would miss moons the size of, say, Ganymede and Titan, the largest moons of Jupiter and Saturn, respectively—each are only about 2 percent the mass of Earth. An easier target, if it exists, would be a moon of roughly Earth mass orbiting a giant planet. "We're looking for moons that don't exist in the solar system," Kipping acknowledges. But the solar system hardly constitutes a comprehensive sample of what nature allows—most of the worlds turned up by planetary searches of the past two decades are oddballs that do not resemble any of the familiar eight planets. "When you look at the exoplanets that have been found, there's every reason to be optimistic," he adds.
Astronomers affiliated with the Kepler mission are cheering the HEK effort on, even though no one knows if the class of exomoons that the spacecraft could detect even exists. "Plenty of things have surprised us before," says Eric Ford, an astronomer at the University of Florida and a participating scientist on the Kepler mission. "Whether or not [Earth-size moons] actually form, we don't know. That's why we should look," he adds. "Kepler has the sensitivity that if they are there, and if they're common, then we could detect them."
There is no reason to assume that the size limit of satellites within the solar system is a universal law, notes Darin Ragozzine, a postdoctoral astrophysicist at the CfA who works with Kepler. "It's certainly not out of the question that there are moons detectable by HEK," he says. "Kepler's definitely the best shot that we have at this."
Ragozzine notes that the HEK search is systematic enough that even if Kipping and company do not find any moons, they will have learned something valuable. "They are doing the study so carefully and thoroughly that even if there are zero discoveries, we will learn something about exomoons," namely that large ones are rare, he says.
Kipping and his colleagues are now preparing their first preliminary results for publication and hope to have something to say about the frequency of large moons by the end of the year. "Whether we turn up 10 moons or zero moons, my hope is we'd at least have a flavor of how common big moons are in Kepler's field of view and throughout the galaxy," he says.