Maximizing Ariel's Survey Leverage for Population-Level Studies of Exoplanets

TL;DR

This paper introduces a quantitative measure called leverage to optimize the selection of exoplanet targets for Ariel's population studies, emphasizing the importance of diversity and target confirmation to improve trend precision.

Contribution

It defines leverage as a metric for survey effectiveness and analyzes target selection strategies to maximize this leverage for Ariel's exoplanet survey.

Findings

Leverage predicts the precision of population trend measurements.

Dividing targets into a few classes maximizes overall leverage.

Confirming candidate planets significantly enhances survey leverage.

Abstract

ESA's Ariel mission will be uniquely suited to performing population-level studies of exoplanets. Most of these studies consist of quantifying trends between an Ariel-measured quantity, y, and an a priori planetary property, x; for example, atmospheric metallicity as inferred from Ariel transit spectroscopy vs. planetary mass. The precision with which we can quantify such trends depends on the number of targets in the survey and their variance in the a priori parameter. We define the leverage of a survey with N targets as L = sqrt(N)stdev(x) and show that it quantitatively predicts the precision of population-level trends. The target selection challenge of Ariel can therefore be summarized as maximizing L along some axes of diversity for a given cumulative observing time. To this end, we consider different schemes to select the mission reference sample for a notional three year transit spectroscopy survey with Ariel. We divide the exoplanets in the mission candidate sample into logarithmic classes based on radius, equilibrium temperature and host star temperature. We then construct a target list by cyclically choosing the easiest remaining target in each class. We find that the leverage on a single axis of diversity can be increased by dividing that axis into many classes, but this sacrifices leverage along other axes of diversity. We conclude that a modest number of classes, possibly only one, should be defined when selecting Ariel targets. Lastly, we note that the statistical leverage of the Ariel transit survey would be significantly increased if current candidate planets were confirmed. This highlights the urgency of vetting and confirming the easiest transmission and emission spectroscopy targets in the Ariel mission candidate sample.