Revisiting the exomoon candidate signal around Kepler-1625 b

Abstract

Context. Transit photometry of the Jupiter-sized exoplanet candidate Kepler-1625 b has recently been interpreted to show hints of a moon. This exomoon, the first of its kind, would be as large as Neptune and unlike any moon we know from the solar system. Aims. We aim to clarify whether the exomoon-like signal is indeed caused by a large object in orbit around Kepler-1625 b, or whether it is caused by stellar or instrumental noise or by the data detrending procedure. Methods. To prepare the transit data for model fitting, we explore several detrending procedures using second-, third-, and fourthorder polynomials and an implementation of the Cosine Filtering with Autocorrelation Minimization (CoFiAM). We then supply a light curve simulator with the co-planar orbital dynamics of the system and fit the resulting planet-moon transit light curves to the Kepler data. We employ the Bayesian Information Criterion (BIC) to assess whether a single planet or a planet-moon system is a more likely interpretation of the light curve variations. We carry out a blind hare-and-hounds exercise using many noise realizations by injecting simulated transits into different out-of-transit parts of the original Kepler-1625 light curve: (1) 100 sequences with three synthetic transits of a Kepler-1625 b-like Jupiter-size planet and (2) 100 sequences with three synthetic transits of a Kepler-1625 b-like planet with a Neptune-sized moon. Results. The statistical significance and characteristics of the exomoon-like signal strongly depend on the detrending method (polynomials versus cosines), the data chosen for detrending, and on the treatment of gaps in the light curve. Our injection-retrieval experiment shows evidence of moons in about 10 % of those light curves that do not contain an injected moon. Strikingly, many of these false-positive moons resemble the exomoon candidate, i.e. a Neptune-sized moon at about 20 Jupiter radii from the planet. We recover between about a third and half of the injected moons, depending on the detrending method, with radii and orbital distances broadly corresponding to the injected values. Conclusions. A ∆BIC of −4.9 for the CoFiAM-based detrending is indicative of an exomoon in the three transits of Kepler-1625 b. This solution, however, is only one out of many and we find very different solutions depending on the details of the detrending method. We find it concerning that the detrending is so clearly key to the exomoon interpretation of the available data of Kepler-1625 b. Further high-accuracy transit observations may overcome the effects of red noise but the required amount of additional data might be large.

Evidence for planetary hypothesis for PTFO 8-8695 b with five-year optical/infrared monitoring observations

“…The light curves obtained in figures 2 and 3 show two variability components, a stellar one and a transit-like dip, as noted in van Eyken et al (2012). Although out-of-transit light curves were fitted to extract dips in almost all previous studies of this object (e.g., van Eyken et al 2012;Ciardi et al 2015), this method is sensitive to an artificially defined time window (Rodenbeck et al 2018). Moreover, it was very difficult to set a correct time window because of the strong stellar variability trend and a possible split of a fading event, which we detected for the first time (see later discussion).…”

Exomoon indicators in high-precision transit light curves

“…Our aim is to provide the exoplanet community with a tool to inspect their TTV-TDV distributions for possible exomoon candidates with-Article number, page 1 of 13 arXiv:2004.02259v1 [astro-ph.EP] 5 Apr 2020 A&A proofs: manuscript no. main out the need of developing a fully consistent, photodynamical transit modelling of a planet with a moon (Kipping 2011;Rodenbeck et al 2018). We show that sufficiently large moons can distort the previously proposed ellipse in the TTV-TDV into very complicated patterns, which are much harder to discriminate from the TTV-TDV distribution of a planet without a moon.…”

Radial velocity constraints on the long-period transiting planet Kepler-1625 b with CARMENES

“…As shown by Morton et al (2016), however, the false positive probability of Kepler-1625 b of being caused by either an unblended eclipsing binary, or a hierarchical eclipsing binary, or a background/foreground eclipsing binary is 8.5 × 10 −3 (± 5.1 × 10 −4 ). Moreover, the probability of the signal emerging from the target star is 1 (Morton et al 2016) and the transit sequence has been successfully modelled as resulting from a Jupiter-sized object around Kepler-1625, be it with a moon or without a moon Rodenbeck et al 2018;Heller et al 2019). The only remaining possibility is a Jupiter-sized transiting object around Kepler-1625, which we here constrain to have a mass in the planetary regime.…”

“…If confirmed, this moon would be the first known exomoon. For now, however, the exomoon interpretation remains subject to debate (Rodenbeck et al 2018;Heller et al 2019;Kreidberg et al 2019).…”