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Faraway Moon or Faint Star? Possible Exomoon Found

Researchers have detected the first "exomoon" candidate -- a moon orbiting a planet that lies outside our solar system. Using a technique called "microlensing," they observed what could be either a moon and a planet -- or a planet and a star. This artist's conception depicts the two possibilities. Image Credit: NASA/JPL-Caltech

Researchers have detected the first "exomoon" candidate -- a moon orbiting a planet that lies outside our solar system. Using a technique called "microlensing," they observed what could be either a moon and a planet -- or a planet and a star. This artist's conception depicts the two possibilities.

Image Credit: NASA/JPL-Caltech

Titan, Europa, Io and Phobos are just a few members of our Solar System's pantheon of moons. Are there are other moons out there, orbiting planets beyond our Sun?

NASA-funded researchers have spotted the first signs of an "exomoon," and though they say it's impossible to confirm its presence, the finding is a tantalizing first step toward locating others. The discovery was made by watching a chance encounter of objects in our galaxy, which can be witnessed only once.                  

The international study is led by the joint Japan-New Zealand-American Microlensing Observations in Astrophysics (MOA) and the Probing Lensing Anomalies NETwork (PLANET) programs, using telescopes in New Zealand and Tasmania. Their technique, called gravitational microlensing, takes advantage of chance alignments between stars. When a foreground star passes between us and a more distant star, the closer star can act like a magnifying glass to focus and brighten the light of the more distant one. These brightening events usually last about a month.

If the foreground star -- or what astronomers refer to as the lens -- has a planet circling around it, the planet will act as a second lens to brighten or dim the light even more. By carefully scrutinizing these brightening events, astronomers can figure out the mass of the foreground star relative to its planet.

In some cases, however, the foreground object could be a free-floating planet, not a star. Researchers might then be able to measure the mass of the planet relative to its orbiting companion: a moon. While astronomers are actively looking for exomoons -- for example, using data from NASA's Kepler mission – so far, they have not found any.

In the new study, the nature of the foreground, lensing object is not clear. The ratio of the larger body to its smaller companion is 2,000 to 1. That means the pair could be either a small, faint star circled by a planet about 18 times the mass of Earth -- or a planet more massive than Jupiter coupled with a moon weighing less than Earth.

The problem is that astronomers have no way of telling which of these two scenarios is correct.

"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," said Wes Traub, the chief scientist for NASA's Exoplanet Exploration Program office at NASA's Jet Propulsion Laboratory, Pasadena, California, who was not involved in the study. "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."

The answer to the mystery lies in learning the distance to the circling duo. A lower-mass pair closer to Earth will produce the same kind of brightening event as a more massive pair located farther away. But once a brightening event is over, it's very difficult to take additional measurements of the lensing system and determine the distance. The true identity of the exomoon candidate and its companion, a system dubbed MOA-2011-BLG-262, will remain unknown.

The new exomoon candidate, if real, would orbit one such free-floating planet. The planet may have been ejected from the dusty confines of a young planetary system, while keeping its companion moon in tow.

Read the full story on NASA's website. 

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