Search for exomoons turns up fluffy, Earth-sized planet

C. Pulliam & D. Aguilar (CfA)

In December, scientists announced the serendipitous observation of an event that may represent the first discovery of an exomoon—a moon that orbits a planet outside our solar system. Unfortunately, it's equally likely that the event was caused by a distant star orbited by a large planet. Since the event required a chance alignment of stars, we're unlikely to observe the object again, which means we're stuck in scientific limbo when it comes to exomoons.

But there's a team trying to change that. The Hunt for Exomoons with Kepler (HEK) project has been searching the observatory's data for signs of a distant moon. So far, they've come up blank, but in sifting through the Kepler data more carefully, they've provided a more detailed picture of the planets it contains. In a recently released publication, they describe one that's only a bit larger than Earth but only slightly more dense than water.

Kepler searches for exoplanets by watching for signs of them transiting between their host star and Earth, which causes a temporary dimming of the light of the star at regular intervals. From the degree of dimming, we can then infer the size of the planet. But that data normally doesn't tell us much about what the planet looks like; we need a separate measure of its weight so that we can know its density and infer its composition.