An alternative interpretation of the exomoon candidate signal in the combined Kepler and Hubble data of Kepler-1625

¹ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: heller@mps.mpg.de; rodenbeck@mps.mpg.de
² Institute for Astrophysics, Georg August University Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ INAF, Astrophysical Observatory of Catania, Via S. Sofia 78, 95123 Catania, Italy
e-mail: giovanni.bruno@inaf.it

Received: 18 December 2018
Accepted: 25 February 2019

Abstract

Context. Kepler and Hubble photometry 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 with Hubble and 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 combined Kepler’s Pre-search Data Conditioning Simple Aperture Photometry (PDCSAP) with the previously published Hubble light curve. In an alternative approach, we performed a synchronous polynomial detrending and fitting of the Kepler data combined with our own extraction of the Hubble photometry. 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 published Hubble data) 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 its Hubble transit at the 3 σ level. This deviation could be caused by a 1 d data gap near the first Kepler transit, 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 the Kepler/Hubble data make the ΔBIC an unreliable metric for an exomoon search around Kepler-1625 b, allowing for alternative interpretations of the signal.