Contributed Talk - Splinter Exoplanets

Thursday, 14 September 2023, 15:00   (H 3007)

Large Exomoons unlikely around Kepler-1625 b and Kepler-1708 b

René Heller (1) and Michael Hippke (2,3)
(1) Max-Planck-Institut für Sonnensystemforschung, Germany, (2) Sternwarte Sonneberg, Germany, (3) Visiting Scholar, Breakthrough Listen Group, UC Berkeley, USA

There are more than 200 moons in our Solar System, all of which are much smaller than Earth. Similar moons around extrasolar planets ("exomoons") are very hard to detect with current instruments. The best exomoon candidates so far are two nearly Neptune-sized bodies orbiting the Jupiter-sized transiting exoplanets Kepler-1625 b and Kepler-1708 b, but their existence has been contested. We reanalysed the Hubble and Kepler data of the two exomoon candidates with Pandora, the first publicly available (github.com/hippke/Pandora) fully automated photodynamical software to model the combined transits of exoplanets with exomoons including mutual eclipses (Hippke & Heller 2022). We find that the statistical evidence for the Kepler-1625b exomoon candidate comes almost entirely from the shallowness of one transit observed with Hubble. We interpret this as a fitting artifact in which a moon transit is used to compensate for the unconstrained stellar limb darkening. We also find much lower statistical evidence for the exomoon candidate around Kepler-1708b than previously reported. We suggest that visual evidence of the claimed exomoon transits is corrupted by stellar activity in the Kepler light curve. We conclude that neither Kepler-1625b nor Kepler-1708b are likely to be orbited by a large exomoon.