The Hunt for Exomoons with Kepler (HEK): IV. A Search for Moons around Eight M-Dwarfs

Abstract: With their smaller radii and high cosmic abundance, transiting planets around cool stars hold a unique appeal. As part of our on-going project to measure the occurrence rate of extrasolar moons, we here present results from a survey focussing on eight Kepler planetary candidates associated with M-dwarfs. Using photodynamical modeling and Bayesian multimodal nested sampling, we find no compelling evidence for an exomoon in these eight systems. Upper limits on the presence of such bodies probe down to $\sim0.4M_{\oplus}$ in the best case. For KOI-314, we are able to confirm the planetary nature of two out of the three known transiting candidates using transit timing variations. Of particular interest is KOI-314c, which is found to have a mass of $1.0_{-0.3}<sup>{+0.4}M_{\oplus}$, making it the lowest mass transiting planet discovered to date. With a radius of $1.61_{-0.15}<sup>{+0.16}R_{\oplus}$, this Earth-mass world is likely enveloped by a significant gaseous envelope comprising $\geq17_{-13}<sup>{+12}$% of the planet by radius. We find evidence to support the planetary nature of KOI-784 too via transit timing, but we advocate further observations to verify the signals. In both systems, we infer that the inner planet has a higher density than the outer world, which may be indicative of photo-evaporation. These results highlight both the ability of Kepler to search for sub-Earth mass moons and the exciting ancillary science which often results from such efforts.

• The paper applies rigorous photodynamical modeling and Bayesian nested sampling to establish upper mass limits for potential exomoons around eight M-dwarf systems.
• It confirms exoplanet candidates, notably KOI-314b and KOI-314c, and limits exomoon masses down to 0.4 Earth masses using transit timing variations.
• Though no exomoons were detected, the study’s robust methodology lays the groundwork for future research on exomoon occurrence in similar planetary systems.

Insights into the Search for Exomoons Around M-Dwarfs Using Kepler Data

The publication titled "The Hunt for Exomoons with Kepler (HEK): IV. A Search for Moons around Eight M-Dwarfs" investigates the possibility of detecting exomoons orbiting planets associated with M-dwarf stars. Utilizing Kepler archival data, the study adopts advanced photodynamical modeling and Bayesian analysis to examine eight planetary candidates. Despite comprehensive analysis, the findings indicate no substantial evidence for the existence of exomoons within the studied systems.

Key Findings

The study implements a rigorous methodology combining photodynamical modeling and Bayesian nested sampling to analyze the transit data effectively. Their results set upper boundaries for potential exomoon masses, achieving limits as low as 0.4 Earth masses in certain scenarios.

• Exoplanet Confirmations: Within the candidate pool, two exoplanets, KOI-314b and KOI-314c, were confirmed using transit timing variations (TTVs). KOI-314c is noted for its low mass of 1.0 Earth masses and a radius of approximately 1.61 Earth radii, suggesting a substantial gaseous envelope composing a considerable portion of its radius.
• Photo-Evaporation Evidence: The investigation into the densities of inner versus outer planets within systems like KOI-314 and KOI-784 suggests potential evidence of photo-evaporation, where the inner planet displays higher density characteristics.

Detailed Analysis Approach

The research employs precise light curve detrending using methods specifically developed for exomoon detection. Moreover, the researchers leverage multimodal nested sampling for fitting models to the light curves, allowing for a comprehensive exploration of the parameter space. The delineation of purely photometric versus spurious dynamical detections is crucial in segregating false positive signals potentially attributable to noise or interplanetary gravitational interactions.

Implications and Future Directions

The derivation of robust exomoon mass limits in this study corroborates the capability of the Kepler data to probe for sub-Earth mass moons, although actual detection remains elusive. This exploratory work implies that sizable exomoons are not ubiquitous among the surveyed M-dwarfs. Future research endeavors may focus on expanding this sample size for more statistically significant occurrence rate estimates.

• Atmospheric Characterization: KOI-314c presents an intriguing opportunity for atmospheric study due to its expected gaseous envelope. Given its brightness in the infrared spectrum, it is suggested that transmission spectroscopy could potentially characterize its atmospheric properties.
• TTV and TDV Analysis: The methodology extends beyond exomoon searches, illustrating the utility of TTV and TDV analysis in confirming planet candidates and understanding the structure of planetary systems.

In conclusion, while the investigation did not confirm any exomoons, it underscores the value of meticulous data analysis and sets the foundation for future discoveries. Expanding the subject pool and integrating observational strategies might unlock a deeper understanding of exomoon prevalence, which remains an intriguing aspect of extrasolar planetary science.