Astronomers have discovered a trove of exoplanets—more than 700 worlds in orbit around distant stars, with leads on thousands of additional suspects. So now, naturally, they're beginning to ask: What moons might be in orbit about these planets?
It is a reasonable question. Most of the planets in our solar system host sizable natural satellites. And in some planetary systems, the moons of an extrasolar planet could themselves be favorable habitats for extraterrestrial life.
To answer it, a team of astronomers is now digging through publicly available data from Kepler, NASA's prolific exoplanet-finding spacecraft, in hopes of detecting the faint signal of the first known exomoon.
"It's something that I've been very passionate about for a long time," says David Kipping, who wrote his PhD thesis at University College London last year on exomoons. Now a postdoctoral scholar at the Harvard–Smithsonian Center for Astrophysics (CfA), Kipping is leading the Hunt for Exomoons with Kepler project, or HEK. He and his colleagues described the HEK campaign in a recent study posted to the preprint Web site arXiv.org that has been submitted to The Astrophysical Journal.
"When I first started this, I was just seeing what was possible," Kipping says. "As I went on with this, I realized that it wasn't just a crazy idea." He and his colleagues calculated that if large moons are common in the galaxy, Kepler might be sensitive enough to find them.
Since 2009, the Kepler spacecraft has trailed Earth in orbit around the sun, doggedly pursuing a deceptively simple mission. With a giant digital camera, Kepler keeps watch on a field of more than 150,000 stars near the constellation Cygnus. It watches those stars for so-called transits—instances where a planet passes in front of its host star, which slightly and temporarily diminishes the star's apparent brightness. So far, the mission has been incredibly productive; Kepler scientists have discovered more than 60 new exoplanets and have identified more than 2,000 likely candidates that await confirmation.
Some 50 of those candidates fall in the so-called habitable zone, the region around a star where temperatures would allow for the presence of liquid water and perhaps the emergence of life. A gas-giant planet in the habitable zone, akin to a warmer Jupiter or Saturn, would lack a solid surface and hence would not be an ideal habitat for life—but its moons might be. "There could be a lot of habitable moons out there, and we want to know about them," Kipping says.
If any Kepler planets happened to host a very large satellite, the moon's presence would have subtle but detectable effects on the planet's transits. For instance, the moon could itself pass in front of the star, blotting out a small fraction of starlight just before or just after the planet itself transits. Alternately, a massive moon could exert a gravitational tug strong enough to perturb the planet's orbit, causing the planetary transits to diverge from a steady, clocklike recurrence.
Promising signals demanding further scrutiny for the presence of a possible exomoon are selected both by analyzing where large, plausibly detectable moons could exist in stable orbits and by old-fashioned visual inspection. The latter effort is led by Allan Schmitt, a Minnesota citizen scientist and a veteran of planethunters.org, an online project in which volunteers browse through public Kepler data to uncover newfound exoplanets. "He was e-mailing me these candidate signals" of possible moons, Kipping recalls. "I said, 'You've done so much work here, why don't you join the team?' He agreed, and since that time he's been a full-time collaborator for us. He's looked through hundreds and hundreds of light curves, looking for these blips."
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.