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."