Post-Newtonian effects on some characteristic timescales of transiting exoplanets

Abstract

Some measurable characteristic timescales $\left\{t_\mathrm{trn}\right\}$ of transiting exoplanets are investigated in order to check preliminarily if their cumulative shifts over the years induced by the post-Newtonian (pN) gravitoelectric (Schwarzschild) and gravitomagnetic (Lense-Thirring) components of the stellar gravitational field are, at least in principle, measurable. Both the primary (planet in front of the star) and the secondary (planet behind the star) transits are considered along with their associated characteristic time intervals: the total transit duration $t_D$, the ingress/egress transit duration $\tau$, the full width at half maximum primary transit duration $t_H$, and also the time of conjunction $t_\mathrm{cj}$. For each of them, the net changes per orbit $\langle\Delta t_D\rangle,\,\langle\Delta\tau\rangle,\,\langle\Delta t_H\rangle,\,\langle\Delta t_\mathrm{cj}\rangle$ induced by the aforementioned pN accelerations are analytically obtained; also the Newtonian effect of the star's quadrupole mass moment $J_2^\star$ is worked out. They are calculated for a fictitious Sun-Jupiter system in an edge-on elliptical orbit, and the results are compared with the present-day experimental accuracies for the HD 286123 b exoplanet. Its pN gravitoelectric shift $\left\langle\Delta t_\mathrm{cj}^\mathrm{1pN}\right\rangle$ may become measurable, at least in principle, at a $\simeq 8\times 10^{-5}$ level of (formal) relative accuracy after about 30 years of continuous monitoring corresponding to about 1000 transits. Systematics like, e.g., confusing time standards, neglecting star spots, neglecting clouds, would likely deteriorate the actual accuracy. The method presented is general enough to be applied also to modified models of gravity.