CHEOPS performance for exomoons: The detectability of exomoons by using optimal decision algorithm

TL;DR

This paper develops an optimized decision algorithm for CHEOPS to detect exomoons, analyzing the number of transits needed and detection probabilities, with a focus on Earth-sized moons around Neptune-sized planets.

Contribution

It introduces a step-by-step optimized detection algorithm for CHEOPS, applicable to various detection methods, and estimates the observational requirements for exomoon detection.

Findings

Detection limit is around an Earth-sized moon.

80% detection chance with 5-6 transits for large moons and Neptune-sized planets.

Detection probability decreases rapidly for smaller moons.

Abstract

Many attempts have already been made for detecting exomoons around transiting exoplanets but the first confirmed discovery is still pending. The experience that have been gathered so far allow us to better optimize future space telescopes for this challenge, already during the development phase. In this paper we focus on the forthcoming CHaraterising ExOPlanet Satellite (CHEOPS),describing an optimized decision algorithm with step-by-step evaluation, and calculating the number of required transits for an exomoon detection for various planet-moon configurations that can be observable by CHEOPS. We explore the most efficient way for such an observation which minimizes the cost in observing time. Our study is based on PTV observations (photocentric transit timing variation, Szab\'o et al. 2006) in simulated CHEOPS data, but the recipe does not depend on the actual detection method, and it can be substituted with e.g. the photodynamical method for later applications. Using the current state-of-the-art level simulation of CHEOPS data we analyzed transit observation sets for different star-planet-moon configurations and performed a bootstrap analysis to determine their detection statistics. We have found that the detection limit is around an Earth-sized moon. In the case of favorable spatial configurations, systems with at least such a large moon and with at least Neptune-sized planet, 80\% detection chance requires at least 5-6 transit observations on average. There is also non-zero chance in the case of smaller moons, but the detection statistics deteriorates rapidly, while the necessary transit measurements increase fast. (abridged)