Exomoon candidates from transit timing variations: eight Kepler systems with TTVs explainable by photometrically unseen exomoons

Abstract

If a transiting exoplanet has a moon, that moon could be detected directly from the transit itproduces itself, or indirectly via the transit timing variations it produces in its parent planet. There is a range of parameter space where the Kepler Space Telescope is sensitive to the TTVs exomoons might produce, though the moons themselves would be too small to detect photometrically via their own transits. The Earth's Moon, for example, produces TTVs of 2.6 minutes amplitude by causing our planet to move around their mutual center of mass. This ismore than Kepler's short-cadence interval of 1 minute and so nominally detectable (if transit timings are of comparable accuracy), even though the Moon's transit signatureis only 7% that of Earth's, well below Kepler's nominal threshold. Here we explore eight systems from the Kepler data set to examine the exomoon hypothesisas an explanation for their transit timing variations, which we compare with the alternatehypothesis that the TTVs are caused by an non-transiting planet in the system. We find that the TTVs of six of these systems could be plausibly explained by an exomoon, the size of which would not be nominally detectable by Kepler. Though we also find that the TTVsc ould be equally well reproduced by the presence of a non-transiting planet in the system, the observations are nevertheless completely consistent with a existence of a dynamically stablemoon small enough to fall below Kepler's photometric threshold for transit detection, and these systems warrant further observation and analysis.