The announcement of a possible first sighting of an exomoon, a body orbiting a planet outside our solar system, is at once exciting and frustrating, astronomers say, because it will be impossible to ever verify.

That's because the chance alignment of two cosmic objects that suggested the existence of the exomoon was a one-time event that will not repeat, they say.

"We won't have a chance to observe the exomoon candidate again," David Bennett of the University of Notre Dame said in a release from NASA's Jet Propulsion Laboratory in Pasadena, California.

Using telescopes and taking advantage of a phenomenon known as gravitational microlensing -- where the gravity of a star or other cosmic object can magnify the light of an object passing behind it -- astronomers say the nearer object in the recently observed alignment was in fact an orbiting pair of objects.

While it may have been a star and an orbiting planet, they say, some of the observational data also suggests it could have been a free-floating planet accompanied by possibly the first exomoon every detected.

 "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," says Wes Traub, the chief scientist for NASA's Exoplanet Exploration Program office at JPL. "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."

Nevertheless, the lensing system, dubbed MOA-2011-BLG-262, could be a "rogue planet" moving through space without a parent star and carrying a rocky exomoon along with it, astronomers say.

The scenario is not all that unlikely, they say; planet-hunting programs have found a number of large planets the size of Jupiter roaming through the stars by themselves without a home star.

Such planets were probable ejected from distant planetary systems after encountering the gravitational effects of other planets or passing stars.

The discovery shows one thing for sure, scientists admit; how little we know about our universe and its contents. While astronomers have so far detected more than 1,700 alien planets, they're still searching for their first confirmed -- or confirmable -- exomoon.

The study, published in the Astrophysical Journal, was a joint effort by the Japan-New Zealand-American Microlensing Observations in Astrophysics (MOA) program and the Probing Lensing Anomalies NETwork (PLANET) programs.

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