摘要:Context. KeplerandHubblephotometry of a total of four transits by the Jupiter-sized exoplanet Kepler-1625 b have recently been interpreted to show evidence of a Neptune-sized exomoon. The key arguments were an apparent drop in stellar brightness after the planet’s October 2017 transit seen withHubbleand its 77.8 min early arrival compared to a strictly periodic orbit.Aims.The profound implications of this first possible exomoon detection and the physical oddity of the proposed moon, i.e., its giant radius prompt us to examine the planet-only hypothesis for the data and to investigate the reliability of the Bayesian information criterion (BIC) used for detection.Methods.We combinedKepler’s Pre-search Data Conditioning Simple Aperture Photometry (PDCSAP) with the previously publishedHubblelight curve. In an alternative approach, we performed a synchronous polynomial detrending and fitting of theKeplerdata combined with our own extraction of theHubblephotometry. We generated five million parallel-tempering Markov chain Monte Carlo (PTMCMC) realizations of the data with both a planet-only model and a planet-moon model, and compute the BIC difference (ΔBIC) between the most likely models, respectively.Results.The ΔBIC values of − 44.5 (using previously publishedHubbledata) and − 31.0 (using our own detrending) yield strong statistical evidence in favor of an exomoon. Most of our orbital realizations, however, are very different from the best-fit solutions, suggesting that the likelihood function that best describes the data is non-Gaussian. We measure a 73.7 min early arrival of Kepler-1625 b for itsHubbletransit at the 3σlevel. This deviation could be caused by a 1 d data gap near the firstKeplertransit, stellar activity, or unknown systematics, all of which affect the detrending. The radial velocity amplitude of a possible unseen hot Jupiter causing the Kepler-1625 b transit timing variation could be approximately 100 m s−1.Conclusions.Although we find a similar solution to the planet-moon model to that previously proposed, careful consideration of its statistical evidence leads us to believe that this is not a secure exomoon detection. Unknown systematic errors in theKepler/Hubbledata make the ΔBIC an unreliable metric for an exomoon search around Kepler-1625 b, allowing for alternative interpretations of the signal.
关键词:Key wordsenplanets and satellites: dynamical evolution and stabilitymethods: data analysisplanets and satellites: detectioneclipsesplanets and satellites: individual: Kepler-1625 btechniques: photometric