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A real-life 'Avatar'? Search begins for moons that could support life.

With scientists already looking for Earth-size planets orbiting in distant stars' habitable zones, a new project is using similar techniques to look for moons, too. 

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Kipping's team is hunting for moons in much the same way. In principle, a large moon can betray its presence as it eclipses part of the star, or even as it transits its planet as the planet crosses in front of the star.

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By analyzing the depth and duration of dimming, researchers can gain clues about the moon's size.

A moon also tugs on its planet. This would show up as slight variations in the timing of the planet's transits, yielding information about the moon's mass.

From mass and size come density, which yields hints about the moon's bulk composition.

In addition, Kipping adds, such information about the moon can be used to make similar estimates about density and composition of the planet it orbits.

But moon-hunting is not for the impatient.

Kipping says it takes about six or seven transits of a planet across its star to detect with any confidence the presence of a candidate moon. If the planet is at an Earth-like distance from its star, that means six or seven years.

Moreover, the researchers must be able to rule out with a fairly high level of confidence the likelihood that what they see is caused by something other than a planet's moon – imposters such as star spots or even the effect of other planets in a candidate-moon's system.

If the team is successful, it will be one more indication that Kepler's usefulness in tackling questions about stars and planetary systems is exceeding initial expectations.

"The amount of secondary science that can be done" using Kepler's data "is amazing," says Allan Schmitt, a member of the HEK project. "We haven't even scratched the surface."

A moon hunt begins

Kipping, who focuses his research on transiting planets and exomoons, says the formal moon-hunting effort emerged out of email exchanges between him and Mr. Schmitt, a retired software engineer who lives in Minnesota.

Last May, Schmitt took up the hunt for extrasolar planets at PlanetHunter.org. It's a citizen-science project that enlists people to help uncover planet-candidates that the Kepler team itself might have missed. The scientific "civilians" do this by evaluating graphs of how a star's light changes with time during a planet's transit.

These so-called light curves are Kepler's bread and butter.

Mr. Schmitt explains that he had been reading Kipping's formal research papers, particularly those related to a computer model the astronomer had developed to predict what an exomoon's light curve might look like.

When Schmitt compared those modeled curves with what he was seeing on PlanetHunters.org, he found what he thought were potential candidates. He forwarded his results to Kipping, and a collaboration was born.

By last September, it was clear that a hunt for moons in Kepler's data was feasible, and the project began in earnest.

Schmitt now makes the first cut at identifying potential candidate-moons for the rest of the team to study in more detail.

The team is scheduled to introduce the project to colleagues at the winter meeting of the American Astronomical Society, which begins Jan. 8 in Austin, Texas.

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