How to spot moons far, far away

The search for life on another planet is about to be extended – to moons.

Katharine Sanderson

Some 'exomoons' might be bigger than earth - and habitable.A. McLatchie

We might soon get our first glimpse of a moon orbiting a planet in another solar system.

Little effort has so far been made to find such an 'exomoon'. Spotting other planets is hard enough; finding their moons is even harder.

But astronomer David Kipping of University College London has devised a way to spot the effects of a moon on the path of the planet it orbits.

The effect can be seen when the planet passes between us and its star. This is called a transit, and it is a common way of spotting exoplanets. If a planet has a moon, each transit has a slightly different position and velocity, creating a wobble in the planet's orbit.

Speed wobbles

Kipping says that his method complements an existing technique for spotting extrasolar bodies called transit time variation (TTV), which measures the time between consecutive transits. Variation in this time is not a smoking gun for an exomoon, because it has a number of possible causes, including another planet in the system.

Besides happening at irregular intervals, the transits of a planet with a moon will vary in their duration, speeding up and slowing down. Kipping calls this effect the transit duration variation (TDV).

Another planet, on the other hand, will always lengthen the transit, because the second planet will always be travelling in the same direction as the transiting one.

Furthermore, because velocity is related to mass, TTV and TDV can be used together to work out the mass of the moon. "The maths just drops out," says Kipping. His work is published in the Monthly Notices of the Royal Astronomical Society¹.

The TDV method will help to distinguish between the effects of moons and planets, says astronomer Jean Schneider at the Laboratory of the Universe and Theories near Paris and a developer of the TTV model. But it is by no means perfect.

"Kipping assumes that the planets' orbits about the star and the moon's orbit about the planet are in the same plane. This is not always the case," he says. This will make the analysis of data more complicated, Schneider says.

Some 'exomoons' might be bigger than earth - and habitable.A. McLatchie

Another problem will be finding a planet that transits its star frequently enough to measure the differences, but is not so close to its star that it can't be seen.

Moon shadow

Schneider is trawling through data from the Convection, Rotation & Planetary Transits (COROT) mission, the European Space Agency's space-based telescope searching for transiting exoplanets. So far he has not seen a signal that might be a moon, but the mission will run for at least another two years. "I am sure that we will detect a moon before 2015," he says.

Kipping hopes to see something sooner. He has some time on the Liverpool telescope, in Las Palmas, Spain, over the spring. He will train the telescope's eye on Gliese 436, a planet 10 parsecs away with one known transiting planet. Kipping hopes to see about 12 transits, and then will test his theories.

Once a moon is found, the question of habitability can be looked at. Most exoplanets found so far have been gas giants, which are inhospitable to life. "One of the big problems with gas giants is they don't have rocky surfaces," says Kipping. Moons of these planets, on the other hand, could be quite different.

Schneider thinks that a distant moon could be the next big thing, literally. "I think it is time to speak about supermoons," he says, "some day we will find large moons, much larger than Titan [the largest moon of Saturn] and larger than Earth."

References : Kipping, D. M. Mon. Not. R. Astron. Soc. 392, 181-189 (2009).

http://www.nature.com/news/2009/090112/full/news.2009.16.html