The Hunt for Exomoons with Kepler (HEK): V. A Survey of 41 Planetary Candidates for Exomoons

Abstract: We present a survey of 41 Kepler Objects of Interest (KOIs) for exomoons using Bayesian photodynamics, more than tripling the number of KOIs surveyed with this technique. We find no compelling evidence for exomoons although thirteen KOIs yield spurious detections driven by instrumental artifacts, stellar activity and/or perturbations from unseen bodies. Regarding the latter, we find seven KOIs exhibiting >5 sigma evidence of transit timing variations, including the 'mega-Earth' Kepler-10c, likely indicating an additional planet in that system. We exploit the moderately large sample of 57 unique KOIs surveyed to date to infer several useful statistics. For example, although there is a diverse range in sensitivities, we find that we are sensitive to Pluto-Charon mass-ratio systems for ~40% of KOIs studied and Earth-Moon mass-ratios for 1 in 8 cases. In terms of absolute mass, our limits probe down to 1.7 Ganymede masses, with a sensitivity to Earth-mass moons for 1 in 3 cases studied and to the smallest moons capable of sustaining an Earth-like atmosphere (0.3 Earth masses) for 1 in 4. Despite the lack of positive detections to date, we caution against drawing conclusions yet, since our most interesting objects remain under analysis. Finally, we point out that had we searched for the photometric transit signals of exomoons alone, rather than using photodynamics, we estimate that 1 in 4 KOIs would have erroneously been concluded to harbor exomoons due to residual time correlated noise in the Kepler data, posing a serious problem for alternative methods.

• The paper applies advanced Bayesian photodynamic techniques, expanding the survey to 41 Kepler objects and tripling previous efforts.
• Key findings include significant transit timing variations in seven candidates and the identification of numerous false positives due to instrumental and stellar noise.
• An empirical sensitivity analysis reveals detection capabilities for satellite-to-planet mass ratios comparable to Pluto-Charon in 40% of cases, guiding future exomoon searches.

Insights into the Survey of Exomoons with Kepler

The paper, titled "The Hunt for Exomoons with Kepler (HEK): V. A Survey of 41 Planetary Candidates for Exomoons," authored by D.M. Kipping et al., presents a comprehensive survey conducted on 41 Kepler Objects of Interest (KOIs) in pursuit of identifying potential exomoons. Utilizing advanced Bayesian photodynamic techniques, the study marks a significant expansion in the number of surveyed KOIs, tripling previous efforts. However, no compelling evidence for exomoons emerged from this expanded survey. Instead, the research observed numerous instances of exomoon false positives, largely attributed to instrumental artifacts, stellar activity, and perturbations from unseen bodies.

Methodology Overview

The researchers employed Bayesian inference methods for their analysis, known for their robustness in handling the complex multimodal parameter spaces and inherent degeneracies in the data. Particularly, the study leveraged the high computational strength of the Nested Sampling algorithm implemented through the Multinest software, which supports efficient exploration of parameter posteriors and Bayesian evidences. This was crucial for evaluating model fits robustly, given the complexity introduced by the potential presence of exomoons.

Key Findings

Null Findings and False Positives: Despite the comprehensive survey, no exomoons were confirmed. Notable is the identification of 13 KOIs yielding false positives. These findings underline the challenges in exomoon detection, such as correlated noise in data and potential perturbations by other celestial bodies.

Transit Timing Variations (TTVs): The researchers detected significant TTVs (greater than 5σ) in seven KOIs, which suggests potential perturbations from additional planetary bodies. Kepler-10c, famously termed as a “mega-Earth,” exhibited TTVs indicative of yet another planetary body affecting its orbital dynamics.

Empirical Sensitivity Analysis: The study assessed the sensitivity of their techniques to detect exomoons, concluding that their methods enable detection of satellite-to-planet mass ratios akin to or better than Pluto-Charon for about 40% of the planets surveyed. Importantly, the sensitivity extends to Earth-moon mass ratios in 12.5% of cases, and to Earth's mass in roughly a third of the cases when considering absolute masses.

Implications and Future Directions

The absence of confirmed exomoons invites a cautious approach to concluding the broader occurrence rate of exomoons. The findings prompt further vetting and analysis of the most promising candidates in future work. While the rigorous Bayesian framework provides a high degree of fidelity in ruling out false positives, the study also highlights other techniques, such as transit-only detection methods, risk high false-positive rates due to correlated noise.

Instrumental and Computational Demands: The paper emphasizes the computational intensity of photodynamical modeling, scaling with data volume. As such, future exomoon surveys will need to balance the rigorous demands of such computational techniques with practical limits on computing resources.

Correlated Noise Concerns: The high incidence of false positives due to correlated noise suggests a pressing need for enhanced data processing methods that can better account for or mitigate these noise effects.

Conclusion

The paper by Kipping et al. advances the understanding of satellite bodies around extrasolar planets, reinforcing the potential of sophisticated modeling techniques to decipher subtle features in astronomical data. While no exomoons were confirmed, the methodology offers a template for high-fidelity analysis in future surveys. The results provoke further inquiry into the intricate dynamics of distant planetary systems, sparking continued investigation into the elusive nature of exomoons.