Exomoon Candidates from Transit Timing Variations: Eight Kepler systems with TTVs explainable by photometrically unseen exomoons

Abstract: If a transiting exoplanet has a moon, that moon could be detected directly from the transit it produces itself, or indirectly via the transit timing variations it produces in its parent planet. There is a range of parameter space where the Kepler Space Telescope is sensitive to the TTVs exomoons might produce, though the moons themselves would be too small to detect photometrically via their own transits. The Earth's Moon, for example, produces TTVs of 2.6 minutes amplitude by causing our planet to move around their mutual centre of mass. This is more than Kepler's short-cadence interval of 1 minute and so nominally detectable (if transit timings can be measured with comparable accuracy), even though the Moon's transit signature is only 7% that of Earth's, well below Kepler's nominal photometric threshold. Here we examine several Kepler systems, exploring the hypothesis that an exomoon could be detected solely from the TTVs it induces on its host planet. We compare this with the alternate hypothesis that the TTVs are caused by an non-transiting planet in the system. We examine 13 Kepler systems and find 8 where both hypotheses explain the observed TTVs equally well. Though no definitive exomoon detection can be claimed on this basis, the observations are nevertheless completely consistent with a dynamically stable moon small enough to fall below Kepler's photometric threshold for transit detection, and these systems warrant further observation and analysis.

• The paper shows that transit timing variations in eight Kepler systems may indicate the gravitational effect of photometrically unseen exomoons.
• The authors use TTVFast simulations and MultiNest Bayesian analysis to distinguish exomoon signals from those caused by additional non-transiting planets.
• The findings define a critical 'green zone' in parameter space, guiding future observations and target selection for exomoon detection missions.

Examination of Exomoon Candidates from Transit Timing Variations in Kepler Systems

The paper investigates the potential of detecting exomoons by analyzing transit timing variations (TTVs) in systems observed by the Kepler Space Telescope. The authors explore an intriguing hypothesis: that exomoons might induce detectable TTVs on their host planets even if these moons are photometrically undetectable due to their size. This study is anchored on the foundation that while direct detection of exomoon transits is challenging, their gravitational influence on planets could offer indirect evidence of their existence.

Methodology Overview

The authors focus on a subset of Kepler systems, analyzing 13 systems with notable TTVs to determine if these could be explained by the presence of exomoons. They employ a comparative approach by evaluating two competing hypotheses: whether the observed TTVs are caused by a non-transiting planet or an exomoon. Utilizing numerical simulations with tools such as TTVFast for orbital dynamics and MultiNest for Bayesian inference, they explore the parameter space for possible solutions.

Key Findings and Interpretation

1. Parameter Space and Detection Feasibility: The study identifies a 'green zone' within the parameter space where an exomoon's influence could cause detectable TTVs without a noticeable photometric signal. This zone is defined by the constraints of Kepler's sensitivity limits for both photometry and transit timing accuracy.
2. Assessment of Target Systems: The analysis results suggest that eight out of the thirteen selected Kepler systems have TTVs that are consistent with the presence of an exomoon. However, for five systems, the mass and orbital parameters required for an exomoon to explain the TTVs are implausible due to stability issues or the moons' excessive size, which should have rendered them visible in transit data.
3. Comparative Hypothesis Viability: For the systems analyzed, the authors find that the additional planet hypothesis often furnishes a comparable, if not superior, fit to the observed TTVs. This suggests that while exomoons are a plausible cause for TTVs, additional planets remain a strong alternative explanation.
4. Mass Ratios and Formation Considerations: The best-fit exomoon models propose relatively high mass ratios compared to their host planets, exceeding the Moon-Earth system ratio. This finding indicates the need for theoretical insights into the formation and stability of such massive exomoons, potentially through collisional formation or capture scenarios.

Implications and Future Work

The study conveys the challenges in unequivocally attributing TTVs to exomoons given the current observational limitations. It underscores the necessity for future missions with improved precision in timing and photometry, such as ESA's PLATO mission, to better distinguish between the effects of moons and planets. The analytical framework provided can aid in prioritizing targets for detailed observations, particularly systems in the identified 'green zone.'

Furthermore, the work encourages the development of more sophisticated models that can account for the complex dynamics of multiple-moon systems and explore the potential stabilizing role of resonant moon orbits.

Conclusion

This research provides an insightful exploration into the detectability of exomoons through TTVs, highlighting both the potential and the hurdles in the indirect detection of these celestial bodies. By refining target selection based on transit data characteristics, the study lays groundwork for future efforts to robustly identify exomoons, thereby enriching our understanding of planetary systems beyond our own.