"One possibility is for the lensing system to be a planet and its moon, which if true, would be a spectacular discovery of a totally new type of system," Wes Traub, chief scientist for NASA's Exoplanet Exploration Program office at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in a statement.
"The researchers' models point to the moon solution, but if you simply look at what scenario is more likely in nature, the star solution wins," added Traub, who was not involved in the study.
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The team could solve the mystery if they knew how far away from Earth the lensing system, called MOA-2011-BLG-262, lies. If it's relatively nearby, MOA-2011-BLG-262 is probably a starless "rogue planet" and moon; a distant system would have to be as massive as a star to produce the same lensing effects, researchers said.
Unfortunately, the true identity of MOA-2011-BLG-262 will probably remain a mystery forever. Microlensing events are random encounters, so there will be no follow-up observations.