Moons beguile, bewitch, and fascinate us as they dance in circles around their planets–both within and, very likely, beyond our own Solar System. Earth’s own large Moon is a lovely world that shines in our night sky with a silvery-golden light that is reflected from our dazzling Sun. In April 2014, a team of NASA-funded astronomers announced that they may have detected the first signs of an alien moon–that is, a moon that does its hypnotic dance around a planet dwelling in the family of a distant star! Although the team of astronomers may never know for certain exactly what they have found, and say that it is currently impossible to confirm its presence, the discovery is an intriguing first baby step toward locating other alien moons. The enchanting discovery was made by observing a chance meeting of objects in our Milky Way Galaxy–which can be seen only once!
“We won’t have a chance to observe the exomoon candidate again. But we can expect more unexpected finds like this,” commented Dr. David Bennett in an April 10, 2014 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Bennett, of the University of Notre Dame in Indiana, is lead author of a new paper describing the findings appearing in The Astrophysical Journal. The JPL is in Pasadena, California.
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, that made use of telescopes located in New Zealand and Tasmania. The astronomers used the technique of gravitational microlensing which takes advantage of fortuitous alignments between stars. When a foreground star travels between Earth and a more remote star, the closer star takes on the role of a magnifying lens–and both brightens and focuses the light of the more distant star. These magnifying events normally last for only about a month.
If the foreground star (the lens) has a planet in orbit around it, the planet will become a second lens to dim or brighten the distant stellar light even more. By carefully observing these brightening events, astronomers can determine the mass of the foreground star relative to its planet.
However, in some instances, the identity of the foreground object is unclear. For example, it could be a free-floating planet with no star to call its own–and not a star at all. Astronomers might then be able to measure the mass of the planet relative to its orbiting companion–a moon! Even though astronomers are very enthusiastically on the hunt for alien exomoons–for example, by using data from NASA’s highly productive but ill-fated Kepler mission–as of this writing, they have not been able to definitely confirm any.