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 First exomoon glimpsed – 1800 light years from Earth

Pandora and Endor, eat your hearts out. The first known moon outside of our solar system may have been found, and it seems weirder than we ever could have imagined.

Exomoons have long been predicted to exist – some may even be habitable worlds – but until now, no one had detected any. "This is the first serious candidate from any survey that I am aware of," says astronomer David Kipping of Harvard University, who was not involved in the discovery.

Unlike the exomoons that feature in the films Avatar and Return of the Jedi, not to mention the moons in our solar system, the new moon and its exoplanet seem to be adrift in the cosmos, far from any star.

The two new objects were detected using an unusual method. Most of the 1000 or so exoplanets discovered to date were found by analysing changes in the light of their star, but a select few have been seen using a technique called gravitational microlensing. When an object passes in front of a distant star as seen from Earth, the object's gravity bends the light from the background star, focusing it like a lens – and making the star temporarily appear brighter if observed from a particular angle.
Massive moon

In a paper posted online earlier this week, David Bennett of the University of Notre Dame, Indiana, and colleagues report that they spotted a microlensing event in 2011, using a number of telescopes around the world. First they saw the distant star's light amplified to 70 times its normal brightness. An hour later came a second, smaller increase in brightness.

That suggests that a large object passed in front of the star, followed by a smaller one. However, it is unclear whether these two objects are a planet and its moon as the team came up with two possible scenarios that fit the microlensing data.

In one, the pair of objects is relatively near to our solar system, at a distance of about 1800 light years, and consists of a planet around four times the mass of Jupiter and a moon about half the mass of Earth – and thus many times more massive than our moon. If this is true, then the team have discovered the first exomoon.

In the other scenario, the pair of objects is much further away and consists of a very small star or a failed star known as a brown dwarf, orbited by a Neptune-mass planet.

If the planet-moon scenario is correct, it would be unlike anything predicted by theorists or sci-fi authors.
Gravitational bust-up

As well as being massive, the exomoon would be orbiting at around 20 million kilometres from its planet, according to the model – a huge distance by the standards of our solar system. Jupiter's largest moon, Ganymede, which is also the largest in the solar system, is about 1 million kilometres out – and has just 2 per cent of the mass of Earth.

Perhaps strangest of all, the moon and planet aren't near any other objects, and are definitely too far from the star that was used to detect them to orbit it. This suggests they have been ejected from their original star system. Astronomers have previously seen such rogue planets but they have never had accompanying moons.

It's possible that the planet-moon system could originally have been two planets that got ejected from a binary-star system in a gravitational bust-up. One of the planets could have got too close to the stars and been flung out, and on an unstable trajectory it might have picked up another planet. "It almost begs the question as to whether we can really call these objects 'moons' or whether some other name is more apt," says Kipping.

Will we ever know if the moon-planet scenario is correct? Probably not, because there is no way to distinguish between the two solutions. Microlensing events are orchestrated by the cosmic dance of stars and planets, so there probably won't be another chance to observe this system again. However, we should be on the lookout for other chances to spot similar systems, say the authors.

Even if astronomers never confirm this moon, it is an exciting glimpse of what is to come. Exomoons in more normal planetary systems might even support life, so the priority is to find a planet-moon system that transits, or passes in front, of its star. "Then we can follow up the objects and measure their atmospheres, size, orbital characteristics," says Kipping. "For microlensing detections, we just see a snapshot and we can't perform the kind of detailed characterisation we would like."