While astronomers keenly await the discovery of Earth-like planets around other stars, the possibility of habitable moons should not be ruled out either, say scientists at University College London.

NASA's Kepler spacecraft launched earlier this year with the hunt for Earth-like planets the primary goal of the mission. It will make detections using the transit method – by looking for the characteristic dips in stellar brightness as a planet passes in front of its parent star.

David Kipping of University College London and colleagues have created computer simulations based on the sensitivity of Kepler's instruments to suggest that the spacecraft may also be able to seek out exomoons. An exomoon’s gravity tugs on the planet it orbits, making the planet wobble during its orbit around its host star. Kepler should be able to record the resulting changes in the position and velocity of the planet during these transits.

Kipping's simulations considered a wide range of possible planetary systems and found that low density Saturn-like planets give the best possible chance for detecting moons since their low mass means they wobble much more than comparatively heavier planets like Jupiter. An even more exciting scenario is if the Saturn-like planet orbits within the not-too-hot, not-too-cold 'Goldilocks zone' of the star then liquid water could be stable on any sufficiently large moon, and the presence of water is crucial for life as we know it.

"It seems probable that many thousands, possibly millions, of habitable exomoons exist in the Galaxy and now we can start to look for them," says Kipping.

The new study found that habitable exomoons as small as one-fifth the mass of the Earth are readily detectable with Kepler. In theory, Kepler could look for Earth-mass habitable moons around 25,000 stars up to 500 light-years away from the Sun, and across the whole sky there should be millions of stars which could be surveyed for habitable exomoons.

"For the first time, we have demonstrated that potentially habitable moons up to hundreds of light years away may be detected with current instrumentation," says Kipping.

The study will be presented in the Monthly Notices of the Royal Astronomical Society.