22nd December 2013

The first "exomoon" may have been discovered

Astronomers at the University of Notre Dame have spotted what appears to be the first known "exomoon", located 1,800 light years away.

The first known moon outside of our Solar System may have been found, based on a study by Professor David Bennett and a team of international colleagues. This satellite (designated "MOA-2011-BLG-262"), with half the mass of Earth, orbits a much larger planet with four Jupiter masses. The duo appear to be rogue, free-floating objects – lacking a parent star and drifting aimlessly through interstellar space. They were possibly ejected from a binary-star system after getting too close to the stars and being flung out with an unstable trajectory.

Detection was made by an unusual method known as gravitational microlensing. This is particularly well-suited to objects that emit little or no light, and was used earlier this year to spot the first exoplanet around a brown dwarf. It works by using gravitational distortion effects to focus a background object as it passes behind another, from which the mass and other characteristics can be determined. Only a small percentage of the 1,000 exoplanets confirmed so far have used this technique; the vast majority are found by Doppler spectroscopy or transit photometry.

However, it is unconfirmed at this stage whether MOA-2011-BLG-262 is actually a moon. Another possible scenario has emerged from the initial data, suggesting the pair may in fact consist of a brown dwarf orbited by a Neptune-mass planet lying much further away than 1,800 light years.

If the first model is correct, the exomoon would have to be around 20 million km (12.5 million mi) from its planet in order to fit the data. This is similar to many of the gas giant satellites in our Solar System. For comparison, the gap between Earth and our own Moon is 385,000 km (240,000 miles) on average, about 50 times less.

Even if unconfirmed this time, many exomoons will likely be found within the next decade, the study concludes. It is available online at Cornell University Library.

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