Now let’s discover those alien moons!

With well over 700 planets now discovered around other stars, astronomers are looking to a fresh challenge – finding their moons. The search continues for exoplanets, with hundreds more candidates already lined up by NASA’s Kepler space telescope and other methods.

An artist’s impression of rocky moons orbiting a gas giant (NASA/JPL-Caltech)

The ultimate quest, of course, is to find worlds that are similar to Earth and also lie in that important habitable zone around a star where water can exist in liquid form. A handful of planets have been found in this so-called Goldilocks zone where it is not too hot and not too cold, but they turn out to be gas giants, more like Jupiter or Saturn than Earth. Earth-sized rocky worlds discovered so far lie in an unfriendly hot region.

But NASA’s online journal Astrobiology Magazine raises the intriguing possibility that while those gasballs themselves would not be suitable environments for life as we know it, they might be orbited by rocky moons that are.

Finding small satellites clinging close to giant worlds sounds like a challenge harder than looking for a needle in a galaxy-sized haystack. But it wasn’t so long ago that finding exoplanets themselves seemed like science-fiction. And it turns out that astronomers have developed three computer-simulation techniques to help planet-hunters pick moons out of their data.

David Kipping, of the Harvard-Smithsonian Center for Astrophysics, tells Astrobiology Magazine that a rocky moon of reasonable size “ticks all the boxes for our wish list of habitable conditions.” He is part of a team that calls itself Hunt for Exomoons with Kepler.

Kepler, which has been in space since 2009, constantly watching stars in a small patch if the sky in the constellations of Cygnus and Lyra, detects alien planets by measuring the fade in a star’s light when one passes in front of it.

Kipping points out that if a moon is also transiting the star, that will add very slightly to the dimming in starlight. But if it then passes behind its planet, the star’s light will increase by a tiny amount which could give the moon’s presence away. The other similar methods of picking out moons have been developed by Brazilian space scientists Luis Tusnski and Adriana Valio, and András Pál of Konkoly Observatory in Hungary.

Tusniki believes that a French planet-hunting satellite, CoRoT, could spot a moon with a radius 1.3 times greater than the Earth while Kepler could detect one a third our size.

Probes sent to Jupiter and Saturn show that both are orbited by rocky satellites and that some of these seem to have underground oceans of water where life might have taken hold. Maybe other alien moons have also become fertile locations for life.

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