HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I

Abstract: Exomoons represent an outstanding challenge in modern astronomy, with the potential to provide rich insights into planet formation theory and habitability. In this work, we stack the phase-folded transits of 284 viable moon hosting Kepler planetary candidates, in order to search for satellites. These planets range from Earth-to-Jupiter sized and from ${\sim}$0.1-to-1.0 AU in separation - so-called "warm" planets. Our data processing includes two-pass harmonic detrending, transit timing variations, model selection and careful data quality vetting to produce a grand light curve with an r.m.s. of 5.1 ppm. We find that the occurrence rate of Galilean-analog moon systems for planets orbiting between ${\sim}$0.1 and 1.0 AU can be constrained to be $\eta<0.38$ to 95% confidence for the 284 KOIs considered, with a 68.3% confidence interval of $\eta=0.16_{-0.10}^{+0.13}$. A single-moon model of variable size and separation locates a slight preference for a population of short-period moons with radii ${\sim}0.5$ $R_{\oplus}$ orbiting at 5-10 planetary radii. However, we stress that the low Bayes factor of just 2 in this region means it should be treated as no more than a hint at this time. Splitting our data into various physically-motivated subsets reveals no strong signal. The dearth of Galilean-analogs around warm planets places the first strong constraint on exomoon formation models to date. Finally, we report evidence for an exomoon candidate Kepler-1625b I, which we briefly describe ahead of scheduled observations of the target with the Hubble Space Telescope.

• The paper demonstrates that rigorous photodynamic modeling on 284 Kepler transits substantiates an extremely low occurrence of Galilean analog exomoons.
• The study employs Bayesian model selection and LUNA photodynamics to contrast planet-only and planet-moon scenarios, ensuring robust statistical inferences.
• The detection of a tentative Neptune-sized exomoon candidate around Kepler-1625b I highlights a potential breakthrough in understanding satellite formation and planetary evolution.

Insights from the Search for Exomoons Using the Kepler Data

The paper "HEK VI: On the Dearth of Galilean Analogs in Kepler, and the Exomoon Candidate Kepler-1625b I" by Teachey et al. explores the potential presence of exomoons in extrasolar systems identified in the Kepler data, emphasizing both a rigorous dataset analysis and the remarkable, albeit tentative, identification of a promising exomoon candidate.

Methodological Approach

The study focuses on detecting exomoons by examining the transits of 284 Kepler Objects of Interest (KOIs). The researchers employed sophisticated data processing, including phase-folding of transit light curves and harmonic detrending, to search for moon transits. The out-of-transit regions were specifically scrutinized for moon signals. To identify exomoons, they applied Bayesian model selection criteria, comparing models with and without moons to determine whether the additional complexity of a moon model was justified by the data.

An extensive analysis was conducted where the authors developed a library of photodynamic scenarios using the LUNA software. These models accounted for both planet-moon dynamics and observational systematics, allowing a comparison against actual light-curve data. For single-moon scenarios, they built look-up tables spanning various moon sizes and planetary radii, accommodating for the statistical properties of potential moons.

Results

The occurrence rate of Galilean-analog moons (characterized by significant sizes and specific semi-major axes) was found to be extremely low in the examined population of Kepler planets. The Bayesian analysis supported a null hypothesis for the existence of moons in this sample, with a marginal signal hinting at a category of short-period moons akin to "Super-Ios." These moons would be closer to their host planets than anticipated and could potentially have survived planetary migration inward, as posited by evolutionary theories.

The study also applied stratified data analysis on subsets of the sample to investigate potential biases in moon prevalence due to stellar or planetary characteristics. For example, they found a non-significant trend suggesting a slightly higher exomoon occurrence for interior planets and around stars with specific properties.

Kepler-1625b I: A Tentative Exomoon Candidate

Among the candidates, Kepler-1625b emerged as particularly intriguing, showing a significant moon-like signal with a Neptune-sized potential exomoon. This detection, however, was based on just three transit observations, emphasizing caution in interpretation. A follow-up using the Hubble Space Telescope (HST) was planned to validate the detection due to the limited repeatability within the current data.

While the discovery of such a large candidate moon challenges existing formation theories, analogous to the unexpected nature of Hot Jupiters, the candidate's validation could vastly expand our understanding of moon formation and planetary system dynamics.

Implications and Future Directions

This study not only highlights the rarity of large exomoon systems around the sample of short-period planets observed with Kepler but also provides a potential observational signature of planetary migration. Should the existence of Kepler-1625b I be confirmed, it would represent the first unambiguous detection of an exomoon, offering a profound insight into satellite dynamics and planetary evolution beyond the Solar System.

Future research could explore more sensitive techniques and detailed theoretical modeling to account for the loss of potential exomoons during planetary migrations. With novel detection strategies and upcoming space telescope missions, the field stands poised to potentially uncover more about these mysterious celestial companions.