Revisiting the exomoon candidate signal around Kepler-1625b

Abstract

Transit photometry of the exoplanet candidate Kepler-1625b has recently been interpreted to show hints of a moon. We aim to clarify whether the exomoon-like signal is really caused by a large object in orbit around Kepler-1625b. We explore several detrending procedures, i.e. polynomials and 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 data: 100 sequences with 3 synthetic transits of a Kepler-1625b-like planet and 100 sequences with 3 synthetic transits of this planet with a Neptune-sized moon. The statistical significance and characteristics of the exomoon-like signal strongly depend on the detrending method, and the data chosen for detrending, and on the treatment of gaps in the light curve. Our injection-retrieval experiment shows evidence for 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. We recover up to about half of the injected moons, depending on the detrending method, with radii and orbital distances broadly corresponding to the injected values. A $\Delta$BIC of -4.9 for the CoFiAM-based detrending indicates an exomoon around Kepler-1625b. This solution, however, is only one out of many and we find very different solutions depending on the details of the detrending method. It is worrying that the detrending is key to the interpretation of the data.

16 pages, 12 figures. Accepted for publication by A&A

References in corpus (12)

• Transit timing effects due to an exomoon
• Formation, Habitability, and Detection of Extrasolar Moons
• Precise time-series photometry for the Kepler-2.0 mission
• HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I
• Detecting companions to extrasolar planets using mutual events
• 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
• Transit of Exomoon Plasma Tori: New Diagnosis
• How to determine an exomoon's sense of orbital motion
• The nature of the giant exomoon candidate Kepler-1625 b-i
• On The Feasibility of Exomoon Detection Via Exoplanet Phase Curve Spectral Contrast
• Next Generation of Telescopes or Dynamics Required to Determine if Exo-Moons have Prograde or Retrograde Orbits

Cited by in corpus (30)

• Transit Least Squares: Optimized transit detection algorithm to search for periodic transits of small planets
• Wotan: Comprehensive time-series de-trending in Python
• No Evidence for Lunar Transit in New Analysis of Hubble Space Telescope Observations of the Kepler-1625 System
• An alternative interpretation of the exomoon candidate signal in the combined Kepler and Hubble data of Kepler-1625
• On the Detection of Exomoons Transiting Isolated Planetary-Mass Objects
• Large Exomoons unlikely around Kepler-1625 b and Kepler-1708 b
• Revised Exoplanet Radii and Habitability Using Gaia Data Release 2
• Catching a planet: A tidal capture origin for the exomoon candidate Kepler 1625b I
• Can close-in giant exoplanets preserve detectable moons?
• Presence of water on exomoons orbiting free-floating planets: a case study
• Searching for Small Circumbinary Planets I. The STANLEY Automated Algorithm and No New Planets in Existing Systems
• On the stability of satellites at unstable libration points of sun-planet-moon systems
• Impact of Tides on the Potential for Exoplanets to Host Exomoons
• Detectability of satellites around directly imaged exoplanets and brown dwarfs
• A full transit of ${\nu }^2$ Lupi d and the search for an exomoon in its Hill sphere with CHEOPS
• Exomoon indicators in high-precision transit light curves
• Measuring the variability of directly imaged exoplanets using vector Apodizing Phase Plates combined with ground-based differential spectrophotometry
• The subsurface habitability of small, icy exomoons
• YOUNG Star detrending for Transiting Exoplanet Recovery (YOUNGSTER) -- I. A search for young exoplanets in Sectors 1-5 of the TESS Full-Frame-Images
• Exploring exomoon atmospheres with an idealized general circulation model
• Evidence for planetary hypothesis for PTFO 8-8695b with five-year optical/infrared monitoring observations
• Direct detectability of tidally heated exomoons by photometric orbital modulation
• The CARMENES search for exoplanets around M dwarfs. A sub-Neptunian mass planet in the habitable zone of HN Lib
• Radial velocity constraints on the long-period transiting planet Kepler-1625 b with CARMENES
• Detection of exomoons in simulated light curves with a regularized convolutional neural network
• Discovering planets with PLATO: Comparison of algorithms for stellar activity filtering
• The Dynamics of Co-orbital Giant Exomoons -- Applications for the Kepler-1625 b and Kepler-1708 b Satellite Systems
• The habitable zone for Earthlike exomoons orbiting Kepler-1625b
• The formation of transiting circumplanetary debris discs from the disruption of satellite systems during planet-planet scattering
• Photometric Search for Exomoons by using Convolutional Neural Networks