Astronomers teased with first hint of finding alien ‘exomoon’

Peering into the depths of the galactic bulge at the centre of the Milky Way, a team of astronomers has scored a fleeting glance of what could be the very first exomoon they've ever seen.

The search for planets around other stars ('exoplanets') has been coming along rather nicely over the past two decades, with astronomers netting thousands of candidates and now having nearly 1,800 confirmed planets tucked away in their databases, but one thing has so far eluded them: an exomoon. This isn't some kind of rare, exotic astronomical object (at least we don't think so), though. It's just a moon — like Earth's moon, or Jupiter's Ganymede, or Saturn's Titan — but it happens to be orbiting one of these distant exoplanets. The problem with finding them, though, is that it's difficult enough to notice an Earth-sized planet in the data, let alone an object less than half the size of our planet.

However, an international team of astronomers may have actually found one now. Using telescopes in New Zealand and Tasmania, the team scanned across the core of our galaxy using a method known as 'gravitational microlensing' to look for planets. This method takes advantage of a 'quirk' of the universe where the very fabric of spacetime itself is warped into a lens shape by the gravity of a large object (like a star or planet), and this gravity lens magnifies the light from anything on the other side of the object. What's on the other side of the lens isn't so important though; that's just the light source. What they're really looking at is the exact shape of the lens formed by the object or objects in front of that light source. If there's just one object there, the lens will be shaped roughly like a sphere (or circle from our perspective), but if there's more than one (like a star with planets or a planet-moon combo), the lens will appear distorted by the gravity of the other object(s).

In this case, when the team panned past a particular background star, they saw one of these gravity lenses show up. Taking a closer look at it, they figured out that whatever is creating the lens is actually a system of two objects — a small one orbiting something 2000 times bigger than it. It's possible that what they've spotted is actually just a very dim star, about a tenth the size of our sun, with a planet just a little bigger than Neptune orbiting it. However, another possibility is that this is a rogue exoplanet (that is, an exoplanet that no longer orbits a star), roughly 4 times the size of Jupiter, with an exomoon around two-thirds the mass of Earth orbiting it.

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The key to knowing which this is — star-exoplanet or exoplanet-exomoon — lies in the distance between us and the pair. If it's closer to us, that would favour the exoplanet-exomoon scenario, while if it's further away, that would point towards the star-exoplanet solution. That's really not so difficult to figure out. In fact, astronomers using the Hubble telescope just figured out how to expand how far we can measure these distances by a factor of 10 (out to 10,000 light years). However, the problem is that these microlensing events don't last very long, so it's not always possible to get followup measurements to tell the distance. Also, with the way our galaxy is constantly in motion, it's highly unlikely that they'll see this system show up again.

The researchers are realistic about the chances on this, of course. They've done their homework on analyzing exactly how likely it is that they've actually found an exomoon, and while their model of the system definitely favours the exoplanet-exomoon idea, they're quick to point out that, based on what we've seen so far in the galaxy, Occam's razor would suggest that it is just a star-exoplanet pair. Still, this potential find gives hope that we will be able to find more of these in the future (possibly even in data already collected by the Kepler Telescope). Given how many moons are orbiting the planets in our own solar system (we know of 173, not including the ones around asteroids and dwarf planets), these other planetary systems must have them!

The research on this was published in the most recent issue of The Astrophysical Journal. However, if you've already heard some of this, it's because their paper showed up in the arXiv.org pre-print database (before it was peer-reviewed), back in December.

(Image courtesy: NASA/JPL-Caltech)

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