Astronomers have found more than a thousand extrasolar planets, but so far, no moons have been found orbiting those planets. They’ve spotted signs of exomoons, but those remain unconfirmed. Surely hordes of them must be out there, and scientists suspect they might be even more habitable than the exoplanets they orbit.
A new exomoon detection technique proposed by René Heller of McMaster University relies on the eclipsing effect of the moon when viewed against its host star. Shadows are created during mini-eclipses where exoplanets (and possibly moons) cross the face of their star, as viewed from Earth. These transit events have already been used to find hundreds of exoplanets, and various efforts to find exomoons have focused on the slight variations in the timing or duration of the transit – these blips might be caused by moons blocking a bit of the starlight.
This new method, known as orbital sampling effect, looks for tiny, additional shadows that build up during multiple eclipses -- but from a statistical perspective. In this image, each dot represents the exomoon’s position as it orbits the planet. You can see how, when viewed from the side, shadows of the exomoon transits become closer and closer as they approach the left and right of the “wings.” The shadows even overlap at the outermost edges of its orbital path.
If you look from above, the dots seem evenly spaced, but from where we stand, the dots don’t appear evenly plotted. As a result, the wings look lighter at their inner edges (when the moon is closet to us), and the “wingtips” will have darker shadows. Heller thinks this phenomenon could be used to find exomoons.
The method is especially clever because it can identify exomoons using existing, publicly available data -- rather than requiring all new observations to be collected. Fortunately, before suffering failure last summer, NASA’s Kepler spacecraft had been staring at 150,000 stars for years.
What astronomers need to look for in those images is the signature of an exoplanet sporting a moon moving across a star: a small dip in starlight, followed by a much bigger dip, and then another smaller dip. That’s pictured below. When the planet and moon emerges from the transit, there’d be a small bump in brightness, then a bigger bump, then a smaller bump.
What’s different about this technique is that it allows the detection of multiple, smaller bodies (like moons in our solar system), rather than single, monstrously large ones. "Four hundred years after Galileo Galilei discovered four moons orbiting Jupiter, the first moons we knew of besides our moon, we now have the technologies and methods available to go find 'alien' moons beyond our Solar System,” Heller tells Astrobiology Magazine.
The work was publishing in the Astrophysical Journal last month.
Images: NASA (top), René Heller (all others)