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

Abstract

There are more than 200 moons in our Solar System, but their relatively small radii make similarly sized extrasolar moons 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. Here we reanalyse the Hubble and Kepler data used to identify the two exomoon candidates employing nested sampling and Bayesian inference techniques coupled with a fully automated photodynamical transit model. We find that the evidence for the Kepler-1625 b 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-1708 b than previously reported. We suggest that visual evidence of the claimed exomoon transits is corrupted by stellar activity in the Kepler light curve. Our injection-retrieval experiments of simulated transits in the original Kepler data reveal false positive rates of 10.9% and 1.6% for Kepler-1625 b and Kepler-1708 b, respectively. Moreover, genuine transit signals of large exomoons would tend to exhibit much higher Bayesian evidence than these two claims. We conclude that neither Kepler-1625 b nor Kepler-1708 b are likely to be orbited by a large exomoon.

published in Nature Astronomy, 46 pages, 4 Figures (in color), 10 Supplementary Figures (3 in color, 7 in black/white), 5 Supplementary Tables

References in corpus (15)

• Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots
• A statistical test for Nested Sampling algorithms
• HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I
• An Exomoon Survey of 70 Cool Giant Exoplanets and the New Candidate Kepler-1708 b-i
• No Evidence for Lunar Transit in New Analysis of Hubble Space Telescope Observations of the Kepler-1625 System
• Detecting extrasolar moons akin to solar system satellites with an orbital sampling effect
• On the detection of Exomoons: A search in Kepler data for the orbital sampling effect and the scatter peak
• An alternative interpretation of the exomoon candidate signal in the combined Kepler and Hubble data of Kepler-1625
• Planet-Planet Occultations in TRAPPIST-1 and Other Exoplanet Systems
• Analytic solutions to the maximum and average exoplanet transit depth for common stellar limb darkening laws
• Transit least-squares survey IV. Earth-like transiting planets expected from the PLATO mission
• Digital color codes of stars
• Pandora: A fast open-source exomoon transit detection algorithm
• Analytic Light Curve for Mutual Transits of Two Bodies Across a Limb-darkened Star
• Signal preservation of exomoon transits during light curve folding

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• Magnetohydrodynamic simulation assessment of a potential near-ultraviolet early ingress in WASP-189b
• The CHEOPS view of HD 95338b: refined transit parameters, and a search for exomoons