The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project

Abstract: Two decades ago, empirical evidence concerning the existence and frequency of planets around stars, other than our own, was absent. Since this time, the detection of extrasolar planets from Jupiter-sized to most recently Earth-sized worlds has blossomed and we are finally able to shed light on the plurality of Earth-like, habitable planets in the cosmos. Extrasolar moons may also be frequent habitable worlds but their detection or even systematic pursuit remains lacking in the current literature. Here, we present a description of the first systematic search for extrasolar moons as part of a new observational project called "The Hunt for Exomoons with Kepler" (HEK). The HEK project distills the entire list of known transiting planet candidates found by Kepler (2326 at the time of writing) down to the most promising candidates for hosting a moon. Selected targets are fitted using a multimodal nested sampling algorithm coupled with a planet-with-moon light curve modelling routine. By comparing the Bayesian evidence of a planet-only model to that of a planet-with-moon, the detection process is handled in a Bayesian framework. In the case of null detections, upper limits derived from posteriors marginalised over the entire prior volume will be provided to inform the frequency of large moons around viable planetary hosts, eta-moon. After discussing our methodologies for target selection, modelling, fitting and vetting, we provide two example analyses.

• The paper introduces the Hunt for Exomoons with Kepler (HEK) project, a systematic effort utilizing Kepler mission data to detect elusive extrasolar moons.
• HEK employs rigorous target selection and Bayesian model comparison using algorithms like MultiNest to differentiate between planet-only and planet-with-moon transit signals.
• The search for exomoons has significant implications for identifying potentially habitable environments and understanding planetary system evolution, with null results providing crucial constraints on satellite formation.

An Overview of the Hunt for Exomoons with Kepler Project

The paper "The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project" by Kipping et al. introduces an ambitious initiative to systematically search for extrasolar moons using the data gathered by the Kepler mission. The project, termed HEK, represents a pioneering effort to detect exomoons, which have been notably elusive despite the extensive discovery of exoplanets over the past decades. This endeavor is significant as exomoons could potentially host environments conducive to life and provide crucial insights into planetary system evolution.

The HEK project takes advantage of the vast catalog of Kepler's transiting exoplanet candidates, identified largely by photometric methods. At the commencement of the project, there were 2326 candidates—each one a potential host for moons. The selection process involves filtering these candidates to focus on the most promising ones using a multi-criteria approach that includes visual inspection, automatic parameter-based selection, and identification of targets of opportunity.

Methodology

The methodology of the HEK project involves sophisticated modeling and data analysis techniques to identify exomoon signatures:

1. Target Selection: The project employs a rigorous target selection strategy, balancing availability, reliability, capability, and detectability of potential hosts. This process aids in narrowing down the vast list of candidates to a manageable number that has been optimized for exomoon detection.
2. Detection Strategy: The detection of an exomoon is framed as a problem of Bayesian model selection. This means comparing the likelihood of a planet-only model against that of a planet-with-moon model. The Bayesian approach allows for incorporation of prior knowledge and inherently includes a penalty for models that are overly complex.
3. Data Fitting: The HEK project uses a combination of the analytical algorithm called ** and MultiNest, a multimodal nested sampling algorithm, to handle the complex parameter space involved in exomoon search. This combination facilitates efficient navigation of the multimodal likelihood space typical in such kinds of problems.

Practical and Theoretical Implications

The search for exomoons is not just an extension of exoplanet science, but it holds unique implications:

• Habitability: Moons in the habitable zone around stars, particularly those orbiting gas giants, could potentially harbor life. Thus, detecting such moons could widen the scope of habitable environments beyond planets.
• Planetary System Evolution: Understanding the formation and survival of moons can offer insights into the processes governing planetary system evolution, such as migration and tidal interactions.
• Challenges and Outcomes: The project faces considerable challenges due to false positives and the less understood nature of potential exomoon signals compared to planets. A negative result will assist in placing meaningful upper limits on moon size and frequency, contributing to constraints on models of satellite formation in general.

Future Directions

The HEK project represents an exploratory foray into a relatively uncharted area of exoplanetary science. As the project progresses, it is likely that methodologies will continue to be refined, particularly as more data from Kepler becomes available and as computational methods advance. Furthermore, potential discoveries from HEK may prompt additional ground-based or space-based follow-up observations to confirm moon candidates and refine our understanding of their properties.

The results from HEK, whether positive or null, will significantly impact our comprehension of the ubiquity and characteristics of exomoons, and by extension, our understanding of planetary systems as a whole. Through this project, the astronomical community takes a crucial step towards completing the exoplanetary map with moons alongside planets, inching closer to unveiling the full complexity of cosmic archipelagos.