A Short History of (Orbital) Decay: Roman's Prospects for Detecting Dying Planets

TL;DR

The paper addresses the detectability of orbital decay in exoplanets with the Roman GBTDS by formalizing a transit-timing framework that distinguishes a shrinking orbital period via a quadratic ephemeris. It derives analytic uncertainties for key transit-time parameters and propagates them to a forecast of the decay signal, applying a simulated planet catalog to predict yields under a baseline stellar tidal dissipation factor $Q'_{*}=10^{6}$ and a period-dependent variant $Q'_{*}\propto P^{-3}$. The main finding is that Roman could detect ~5–10 decaying planets under the constant $Q'_{*}$ assumption, with substantially fewer detections under the period-dependent model, thereby enabling constraints on the rate of planetary engulfment and the physics of tidal dissipation. The work further analyzes survey configurations and extensions, showing that target field count, cadence, and extended baselines can meaningfully improve decay constrains, and highlights the value of extended follow-up to maintain ephemerides for long-term monitoring.

Abstract

The Roman Space Telescope Galactic Bulge Time Domain Survey (GBTDS) is expected to detect ~$10^5$ transiting planets. Many of these planets will have short orbital periods and are thus susceptible to tidal decay. We use a catalog of simulated transiting planet detections to predict the yield of orbital decay detections in the Roman GBTDS. Assuming a constant stellar tidal dissipation factor, $Q^{'}_{*}$, of $10^6$, we predict ~ 5 - 10 detections. We additionally consider an empirical period-dependent parameterization of $Q^{'}_{*} \propto P^{-3}$ and find a substantially suppressed yield. We conclude that Roman will provide constraints on the rate of planet engulfment in the Galaxy and probe the physics of tidal dissipation in stars.

Figures (4)

• Figure 1: Comparison of the analytic estimate of uncertainty with the numerical results as a function of period, showing good agreement for all three parameters in the short-period regime.
• Figure 2: The reduction in uncertainty, relative to the GBTDS uncertainty $\sigma_{dP/dt,0}$, that can be achieved with an additional season of observing carried out some number of months after the conclusion of the GBTDS.
• Figure 3: The distribution of the planets from the wilson_transiting_2023 simulated planet catalog in period and radius. The color bar indicates the apparent magnitude of each planet's host star.
• Figure 4: The distributions of $dP/dt$ and $\sigma_{dP/dt}$ for each simulated planet detection. These quantities are highly correlated due to the strong dependence of each on the planet's orbital period, as shown by the color bar. WASP-12 b has ${dP/dt}\sim30$ ms/yr.