Find exomoons by watching how they warp their planet's light

 作者:双训     |      日期:2019-03-06 02:16:12
NASA/JPL-Caltech By Shannon Hall Exomoons are on the horizon. A new technique may help us find Mars-sized moons circling planets beyond the solar system. Planets in our own solar system host a total of 176 moons, some of which have vast oceans – and perhaps life – buried beneath layers of ice. The possibility that moons orbiting planets in other solar systems may be similarly habitable makes them a sought-after prize in astronomy. But so far, despite the discovery of thousands of exoplanets, these “exomoons” have eluded detection. Now, Sujan Sengupta at the Indian Institute of Astrophysics and Mark Marley at NASA’s Ames Research Center in Mountain View, California, suggest that the next class of large telescopes should be able to detect an exomoon by looking at the polarised light of the planet it orbits. Most of the exoplanets discovered so far were spotted using the transit method: watching for the slight dimming of the star when a planet crosses in front of it. Sengupta and Marley suggest a similar strategy for finding moons: if we can see the light from the planet, we should be able to see transits of their moons. It’s not quite that simple, though. Instead of seeking the moons’ shadows directly, the pair suggest looking for their effect on the polarisation, or the light waves’ preferred plane of vibration. Take a pair of polarised sunglasses and look at the sky on a clear day. The polarisation of the sky will change depending on where you look. It’s so reliable than animals use it to navigate. But take a snapshot of the entire sky and those changes cancel each other out. This is what astronomers expect to see when they look at a distant exoplanet through a polarised filter: nothing (or very little). But if an exomoon passes in front of that planet, it will block some of the light and cause a spike in polarisation – the signature of an exomoon. It’s worth remembering that these planets are so far away that you’re never going to have an image of a planet with a little moon next door, so you have to find these tricky ways of figuring out that there’s a moon present,” says Marley. A competing technique, designed by David Kipping, now at Columbia University in New York, suggests that exomoons could be detectable just before or after their planet transits the star. Like their host exoplanet, they too would block the star’s light – if only by a little. Already, Kipping and others have combed through data from the Kepler space telescope to look for this signature. So far, they’ve come up empty-handed, ruling out moons more massive than Earth. The polarisation method, however, could probe exomoons that are as light as Mars. But the work is going to be tricky. Eric Agol at the University of Washington points out that it requires a small signal from a very faint source. Additionally, it requires gaining access to the world’s most sought-after telescopes. Still, it’s a testable hypothesis. And the fact that it occurs every time the moon crosses in front of the face of the planet, means that the signal will repeat once every orbit period and allow astronomers to confirm their detection. “Despite the challenges, it may eventually be realised with a gargantuan telescope, precise instrumentation, a bit of luck and a generous time allocation committee,” he says. “The more methods at our disposal the better,” says Kipping. “I still believe the transit method provides the best chances of success in the short term, but science works best when approached from many different perspectives.” Whichever method ends up working, Marley thinks that with bigger telescopes and better instruments, we’ll detect a handful of exomoons by 2030. Journal reference: The Astrophysical Journal, DOI: 10.3847/0004-637X/824/2/76 More on these topics: