Artist's conception of an exomoon and a distant exoplanet. ( NASA/JPL-Caltech)
Artist's conception of an exomoon and a distant exoplanet. ( NASA/JPL-Caltech)

First Candidate Exomoon Is a Celestial Tease

Hiding in the fleeting brightness of a distant star’s light is the signature of what could be the first candidate exomoon. If it’s there, the small moon is orbiting a lonely planet — a rogue world, a starless planet, an unlucky wanderer adrift in perpetual galactic black.

But at least that poor planet would have a little moon-friend to keep it company.

Late last year, a team of scientists looking for faraway worlds first announced that one of their candidate planets seemed to have a small companion. The team was looking for gravitational microlensing events — or the magnifying of distant starlight caused by a less-distant planet passing between the star and Earth. When this happens, the planet’s gravity warps and focuses the distant starlight, producing a recognizable peak in brightness.

This time, though, that peak in brightness looked a little smeared — there was extra bump in the light curve, suggesting that two foreground objects were involved. Using that smear, astronomers calculated the ratio in mass between the two objects: 2,000:1.

In other words, they were either looking at a small, dim star with a Neptune-size planet, or a large, gassy Jupiter with a moon smaller than Earth. 

Unfortunately, it will be impossible to confirm which of these two systems the team saw. To do that, astronomers would need to catch a glimpse of the system again and determine how far away it is. That would help the team measure the mass of the objects involved, and determine whether they’re seeing a star-and-planet, or a planet-and-moon. But because microlensing events are more or less random, the chances of spotting this system again are pretty much zero.

Which is kind of a bummer. Though exomoons are thought to exist, no concrete evidence for them has yet been found. But scientists are certain the moons are there. In addition to doing microlensing surveys, teams are busy looking for moons in the data gathered by NASA’s Kepler telescope, which stared at a starry patch of sky and watched for planets crossing the faces of their home stars.

One thing’s for sure: If exomoons are anything like the dazzling moons in our own solar system, then these tiny worlds are rich candidates for hosting extraterrestrial life. Even if they live around homeless planets.