Metrics for Optimizing Searches for Tidally Decaying Exoplanets

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, 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.

Doomed Worlds. I. No New Evidence for Orbital Decay in a Long-term Survey of 43 Ultrahot Jupiters

“…Two other planets, TOI-564 b and HIP 65 A b, had small negative nominal ΔBIC values that switched to small positive rescaled ΔBIC values. The last three planets are all sparsely sampled with large errors on individual transits and only 4-5 yr of baseline, precisely the kinds of systems where swings in ΔBIC are common (Jackson et al 2023), and more data are needed before the true patterns of these systems will become clear. We interpret the shifts in ΔBIC to be suggestive that underestimated errors alone may explain most of the marginal detections of decreasing orbital period, both here and in the literature.…”

“…2. In the ideal case, someone would observe a transit of every planet that might be experiencing orbital decay at least once a year to avoid long gaps in the timing data, following the recommendation of Jackson et al (2023).…”

“…Detecting orbital decay rewards a patient approach. Recent theoretical work by Jackson et al (2023) has explored how repeated, regular observations of transits over a long timescale will detect planets with relatively fast, WASP-12 b-like rates of orbital decay, requiring roughly two transits per year for 10+ yr to build up clear evidence. Building on previous suggestions (Liddle 2007), that work focused on the Bayesian information criterion, or BIC (Schwarz 1978), both to judge whether a data set supports tidal decay and to design an observing program to optimally detect said decay.…”