A Reply to: Large Exomoons unlikely around Kepler-1625 b and Kepler-1708 b

Abstract: Recently, Heller & Hippke argued that the exomoon candidates Kepler-1625 b-i and Kepler-1708 b-i were allegedly 'refuted'. In this Matters Arising, we address these claims. For Kepler-1625 b, we show that their Hubble light curve is identical to that previously published by the same lead author, in which the moon-like dip was recovered. Indeed, our fits of their data again recover the moon-like dip with improved residuals than that obtained by Heller & Hippke. Their fits therefore appear to have somehow missed this deeper likelihood maximum, as well producing apparently unconverged posteriors. Consequently, their best-fitting moon is the same radius as the planet, Kepler-1625 b; a radically different signal from that which was originally claimed. The authors then inject this solution into the Kepler data and remark, as a point of concern, how retrievals obtain much higher significances than originally reported. However, this issue stems from the injection of a fundamentally different signal. We demonstrate that their Hubble light curve exhibits ~20% higher noise and discards 11% of the useful data, which compromises its ability to recover the subtle signal of Kepler-1625 b-i. For Kepler-1708 b-i it was claimed that the exomoon model's Bayes factor is highly sensitive to detrending choices, yielding reduced evidence with a biweight filter versus the original claim. We use their own i) detrended light curve and ii) biweight filter code to investigate these claims. For both, we recover the original moon signal, to even higher confidence than before. The discrepancy is explained by comparing to their quoted fit metrics, where we again demonstrate that the Heller & Hippke regression definitively missed the deeper likelihood maximum corresponding to Kepler-1708 b-i. We conclude that both candidates remain viable but certainly demand further observations.

• The paper challenges prior claims by demonstrating that consistent moon-like dips appear in reanalyzed HST light curves for Kepler-1625b.
• It re-evaluates detrending methods for Kepler-1708b, revealing that inadequate parameter exploration can obscure subtle exomoon signals.
• The study highlights the need for refined observational strategies and advanced modeling to mitigate noise and confirm exomoon candidates.

A Critical Examination of Exomoon Viability Around Kepler-1625\,b and Kepler-1708\,b

The investigation into the existence of exomoons—natural satellites orbiting exoplanets—has been of significant interest as researchers endeavor to understand and expand our insights into planetary systems beyond our solar system. The paper at hand addresses recent claims by Heller and Hippke regarding the potential existence of large exomoons around Kepler-1625\,b and Kepler-1708\,b, examining these claims and presenting counterarguments.

Kepler-1625\,b-i Analysis

In the case of Kepler-1625\,b, Heller and Hippke's arguments center around a perceived lack of evidence for an exomoon based on their analysis of the Hubble Space Telescope (HST) light curves. The authors of the present paper contend that the Hubble light curve data utilized by Heller and Hippke are identical to those previously published and still recover the moon-like dip when fitted appropriately. Importantly, they argue that the noise characteristics and data omissions in Heller and Hippke's analysis compromise the retrieval of subtle signals indicative of an exomoon. The claimed moon-like dip is shown to have been consistently recovered in multiple analyses using differing methodologies, reinforcing its credibility as a viable exomoon candidate despite contrasting interpretations.

Kepler-1708\,b-i Evaluation

The discourse extends to Kepler-1708\,b, where detrending processes and their impact on the Bayes factor have been criticized by Heller and Hippke. The authors utilize the detrended light curve and biweight filter codes from Heller and Hippke to re-evaluate the data and find that the original exomoon signal stands robustly against these processing variations. The analysis highlights discrepancies in fit metrics, indicating that the likelihood maximization by Heller and Hippke did not explore the parameter space sufficiently, hence missing the deeper signal associated with an exomoon.

Discussion and Future Implications

The paper effectively argues that both Kepler-1625\,b-i and Kepler-1708\,b-i remain plausible candidates for hosting exomoons, despite the critiques presented by Heller and Hippke. The authors stress the importance of continued observations to conclusively verify these exomoon candidates, calling for refined observational strategies as well as the development of advanced modeling and detrending techniques to mitigate systemic biases and errors in light curve analyses.

Moreover, the nuances in observational data handling, such as noise filtering and detrending techniques, underscore a critical challenge in exomoon detection. These procedural elements are crucial for ensuring that subtle transit signals are neither artificially introduced nor negligibly dismissed. As technological capabilities advance, alongside methodological enhancements in data interpretation and modeling, the study of exomoons will likely yield more tangible detections and insights into their formation and evolution.

Additionally, this ongoing dialogue emphasizes the broader implications for our understanding of planetary systems. The presence of an exomoon could have significant ramifications for the habitability of exoplanets by affecting climatic and tidal conditions, and hence, their geological and biological evolution.

Conclusion

In conclusion, the examination presented in this paper confronts the claims against the likelihood of large exomoons existing around Kepler-1625\,b and Kepler-1708\,b with rigor and presents a comprehensive counter-narrative based on methodological critique and re-analysis of observational data. The authors advocate for cautious optimism regarding exomoon candidates, urging further empirical validation through continuing observations, which will be instrumental in definitively resolving these debates and enhancing our understanding of the dynamics within complex exoplanetary systems.