Metrics for Optimizing Searches for Tidally Decaying Exoplanets

TL;DR

This paper investigates how to optimize observational strategies for detecting orbital decay in short-period exoplanets caused by tidal interactions, emphasizing the importance of timing precision and observation frequency.

Contribution

It identifies key astrophysical and observational parameters that maximize the likelihood of detecting tidal decay and evaluates effective observational strategies for long-term surveys.

Findings

Moderately frequent observations can detect tidal decay within a few years.

Timing uncertainties and decay rates influence detection timescales.

Thresholds for confirming tidal decay are established based on observational parameters.

Abstract

Tidal interactions between short-period exoplanets and their host stars drive orbital decay and have likely led to engulfment of planets by their stars. Precise transit timing surveys, with baselines now spanning decades for some planets, are directly detecting orbital decay for a handful of planets, with corroboration for planetary engulfment coming from independent lines of evidence. More than that, recent observations have perhaps even caught the moment of engulfment for one unfortunate planet. These portentous signs bolster prospects for ongoing surveys, but optimizing such a survey requires considering the astrophysical parameters that give rise to robust timing constraints and large tidal decay rates, as well as how best to schedule observations conducted over many years. The large number of possible targets means it is not feasible to continually observe all planets that might exhibit detectable tidal decay. In this study, we explore astrophysical and observational properties for a short-period exoplanet system that can maximize the likelihood for observing tidally driven transit-timing variations. We consider several fiducial observational strategies and real exoplanet systems reported to exhibit decay. We show that moderately frequent (a few transits per year) observations may suffice to detect tidal decay within just a few years. Tidally driven timing variations take time to grow to detectable levels, and so we estimate how long that growth takes as a function of timing uncertainties and tidal decay rate and provide thresholds for deciding that tidal decay has been detected.