NASA's Kepler space telescope could be used to find exomoons, which are the moons of planets orbiting stars other than the Sun. That is the claim of an international team of astronomers, which says that careful analysis of data collected by Kepler could reveal if such exoplanets are circled by moons. The results could have major implications for astronomers' understanding of how moons form. It could even provide important information about the probability of there being life elsewhere in the universe.

The Kepler telescope was launched in 2009 and keeps its gaze permanently fixed on one randomly chosen area of the Milky Way that is about 10 degrees square. Its main aim is to detect exoplanets by observing the slight drop in light received from a star as one of its planets passes in front of it. So far, hundreds of exoplanets have been found this way.

Moon, or another planet?

To ensure that it really has detected an exoplanet, rather than just a random, temporary drop in the brightness of a star, Kepler looks for periodic drops in the star's output. Now, David Kipping of the Harvard-Smithsonian Center for Astrophysics and colleagues from other US universities and the Niels Bohr Institute in Copenhagen want to look for slight variations in this periodicity. The team claims that such variations could indicate that another object besides the star is influencing the planet's motion – and that object could be a large exomoon.

To claim discovery of an exomoon, however, astronomers would have to rule out other explanations for the variations – such as the presence of other planets orbiting the same star. Kipping and colleagues argue that this can be done by looking more closely at the data from Kepler. Variation in the magnitude of a light dip would provide a further indication that the planet had a moon, because the planet and moon together would block more light when side by side than when one was in front of the other. Furthermore, the change in period would be related to the gravitational pull of the moon, and hence its mass, while the change in brightness would be related to the moon's diameter. The two measurements together could therefore allow scientists to estimate the moon's density, giving some clue about its composition.

Massive moons

Kipping and his colleagues have worked out that Kepler should be able to find moons as small as 0.1 Earth masses. While this is still four times the size of the largest moon in our solar system, it is possible that such a moon could form. A smaller planet could become a large moon when captured by a larger planet, for example, or a large moon could be created when two planets collide.

Darin Ragozzine, a scientist at the Harvard-Smithsonian who was not involved in the research, says that failure to detect such large moons will be a valuable result: "My favourite part of this paper," he explains, "is that, even if nothing is found – which is a distinct possibility – they will still have scientifically valuable results because they will be able to say specifically what can be ruled out."

Lunar requisite for life?

One of the most intriguing possibilities is that Kepler may find a potential cradle for life outside our own solar system. The search for moons is particularly important in this respect for two reasons. First, very large moons, as might be found around a gas giant, for example, could, in principle, host life. Second, many scientists believe that life could not have evolved on Earth had the moon not been around to stabilize its axial tilt, preventing extreme variations in climate. Planets with a relatively large moon would therefore be more promising as habitable planets. "Without a massive moon, it is not clear how, or if, intelligent life could develop," says Ben Moore, a computational astrophysicist at the Institute for Theoretical Physics in Zurich.

Nevertheless, Kipping stresses that the project is not a quest to find alien life. "If we found a habitable moon, that would be a dream come true, but it's not a primary science objective," he says. "Principally we're just trying to find moons, whether habitable or not."

A preprint of the paper is available at arXiv:1201.0752.