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The discovery of habitable planets around other stars is one of the great goals of modern astronomy. But it’s not just planets that can host life. Astronomers have long believed that moons orbiting Jupiter-like planets in the habitable zone could have Earth-like qualities. The problem is how to detect them.

One of the best ways to spot exoplanets is to look for changes in a parent star’s brightness as the they pass in front of its disc. So why not look for exomoons in the same way? A planet-moon system orbits a common centre of mass which itself orbits the star. Consequently, the planet’s position can be shifted by a small amount as it moves in front of the disc, leading to a small change in the time that a transit begins and ends. This transit timing variation (TTV) is a signal that could be used to spot an exomoon.

In theory, at least. The trouble is that that up to 98 per cent of promising exoplanet signals turn out to be false alarms caused by other effects. It turns out there is no way of teasing the exomoon signal out of this mess.

And there the field of exomoon hunting would have remained were it no for the work of David Kipping at the Harvard-Smithsonian Center for Astrophysics in Cambridge and a few buddies. Earlier this year, these guys showed that exomoons produce another signal. They pointed out that not only does an exomoon change the start time of a transit, it should also change the duration of the transit and that together, these signals can uniquely identify an exomoon.

Today, Kippling and co say this method is possible now. Earlier this year, NASA launched the Keppler Space Telescope to stare continuously at a fixed region of the sky and measure the changes in brightness of some 100,000 stars.

After crunching a few numbers, Kippling and co say that Keppler should be able to see exomoons smaller than Earth around Saturn-like exoplanets if they are orbiting any of 25,000 stars in the Keppler’s field of view.

That’s exciting news. Keppler is already sending back data. If habitable exomoons are out there, we’ll see them soon.

Ref: arxiv.org/abs/0911.5170: Pathways Towards Habitable Moons

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