astroengine writes "Through the technique of microlensing, a candidate exomoon has been discovered in orbit around a free-floating planet about 1,600 light-years away toward the galactic bulge. The microlensing event, MOA-2011-BLG-262, was detected by the MOA-II telescope at Mt. John University Observatory (MJUO) in New Zealand and it appears to have a mass of approximately half that of Earth. The host planet is around 4 times the mass of Jupiter. Unfortunately there cannot be further studies of his particular exoplanet-exomoon pair (as microlensing events are transient and random), so the astronomers who made the discovery are remaining cautious and point out that although the exoplanet-exomoon model fits the data the best, there's a possibility that the lensing object may have been a more distant star with a massive exoplanet in tow. Microlensing surveys are, however, sensitive to low mass exoplanets orbiting massive free-floating planets, so this is a tantalizing first-detection. The study's pre-print publication has been uploaded to the arXiv."