Balancing Variety and Sample Size: Optimal Parameter Sampling for Ariel Target Selection

Targeted astrophysical surveys are limited by the amount of telescope time available, which makes it difficult to observe every single object of interest. In order to maximize the scientific return, we need a well thought-out strategy for selecting the observational targets, in our case exoplanets. This study evaluates various strategies for selecting exoplanet targets within limited observation windows, focusing specifically on the selection of exoplanet targets for Tier 2 transit spectroscopy with ESA’s upcoming Ariel mission. We define three distinct selection criteria for the selected sample—sample size, variance, and leverage—and translate them into objective functions compatible with modern optimization algorithms. The performance of these methods is demonstrated through three practical exercises across one, two, and three parameters of diversity. Each of the three objective functions represents a unique trade-off between sample size, diversity, and total observation time. While a time-greedy approach maximizes the quantity of planets, it fails to capture diversity. Conversely, variance-greedy selection prioritizes diversity but introduces significant drawbacks: it oversamples rare cases and undersamples typical planets, ultimately reducing the total number of targets observed. Leverage-based selections emerge as the most effective middle ground, successfully balancing sample diversity with a robust sample size. Specifically, when maximizing sample leverage, we test five heuristics: “leverage greedy,” “simulated annealing,” “ K -means clustering,” “regular classes,” and “quantile classes.” This work supports the broader community effort to ensure that Ariel delivers the most diverse and scientifically valuable sample of exoplanet atmospheres within mission limits.