Evidence of Long-term Period Variations in the Exoplanet Transit Database (ETD)

Abstract

We analyze a large number of citizen science data and identify eight hot Jupiter systems that show evidence for deviations from a constant orbital period: HAT-P-19 b, HAT-P-32 b, TrES-1 b, TrES-2 b, TrES-5 b, WASP-4 b, WASP-10 b, and WASP-12 b. The latter system is already well known to exhibit strong evidence for tidal orbital decay and serves as an important control for this study. Several other systems we identify have disputed period drifts in the literature, allowing the results here to serve as an independent analysis. The citizen science data are from the Exoplanet Transit Database (ETD), which is a global project established in 2008 by the Variable Star and Exoplanet Section of the Czech Astronomical Society. With over 400 planets and 12,000 contributed observations spanning 15 yr, the ETD is brimming with potential for studying the long-term orbital evolution of close-in hot Jupiters. We use our results to discuss prioritization of targets for follow-up investigations, which will be necessary to confirm the period drifts and their causes.

Metrics for Optimizing Searches for Tidally Decaying Exoplanets

“…A recent analysis of TESS data confirmed this decay rate, reducing the error bars below 1 ms yr −1 (Wong et al 2022). Possible tidal decay has also been reported for several other systems, including XO-3 b (Ivshina & Winn 2022;Yang & Wei 2022), WASP-19 b (Patra et al 2020;Ivshina & Winn 2022), TrES-1 b, TrES-2 b, HAT-P-19 b (Hagey et al 2022), Kepler-1658b (Vissapragada et al 2022, and KELT-9 b (Harre et al 2023). However, definitive confirmation will likely require additional years of observations, and knowing which planets to prioritize requires understanding how decay is detected and what parameters best suit a system to exhibit detectable decay.…”

“…Naturally, this approach comes with important caveats and limitations. For instance, we have assumed a linear regression approach, but real transit data may have complex and asymmetric uncertainties (Hagey et al 2022) for which such an approach is only an approximation. Our approach also assumes that the BIC provides an accurate means for comparing models with tidal decay and those without.…”

“…For HAT-P-19 b, Hagey et al (2022) reported dP/dt = −55.2 ± 7.2 ms yr −1 and dP/dE = 606 μs orbit −1 , with tidal decay favored at ΔBIC = 8.3. This dP/dE value is almost seven times that for WASP-12 b, and the analytic ΔBIC reflects this, with a value predicted to exceed that for WASP-12 b by 2024.…”

“…These examples all involve systems for which possible tidal decay has been reported. We take observations for TrES-1 b, TrES-2 b, and HAT-P-19 b from Hagey et al (2022) and KELT-9 b from Harre et al (2023). For the calculations in this section, we take the correct (and variable) transit timing uncertainties and the corresponding orbital periods to convert epoch E to (Earth) years.…”

“…For example, TrES-2 b has been observed every four orbits, so we assume that same observing cadence going on past 2020. Hagey et al (2022) analyzed transit times reported in the Exoplanet Transit Database, 3 and, for TrES-1 b, a tidal decay rate dP/dt = −10.9 ± 2.1 ms yr −1 was favored over a constant period by ΔBIC = 9.7. The left panel of Figure 8 shows a good match between the numerical and analytic estimates for ΔBIC, and, assuming the nominal tidal decay rate, the analytic estimate suggests that ΔBIC ought to exceed 50 within the next few years.…”