Stellar proper motion and the timing of planetary transits

TL;DR

Proper motion causes measurable long-term variations in transit timing and duration in exoplanet systems, potentially dominating other effects, especially in wide-separation systems observed by Kepler.

Contribution

This study quantifies how systemic proper motion influences transit timing and duration, highlighting its significance compared to other known effects.

Findings

Proper motion induces transit duration variations of 10-100 ms/yr.

Proper motion causes orbital period evolution exceeding other effects in some cases.

Timing signatures from proper motion persist even in zero-eccentricity systems.

Abstract

Duration and period of transits in extrasolar planetary systems can exhibit long-term variations for a variety of reasons. Here we investigate how systemic proper motion, which steadily re-orients planetary orbit with respect to our line of sight, affects the timing of transits. We find that in a typical system with a period of several days proper motion at the level of 100 mas/yr makes transit duration vary at a rate ~10-100 ms/yr. In some isolated systems this variation is at the measurable level (can be as high as 0.6 s/yr for GJ436) and may exceed all other transit timing contributions (due to the general relativity, stellar quadrupole, etc.). In addition, proper motion causes evolution of the observed orbital period via the Shklovskii effect at a rate $\gtrsim 10$ $\mu$s/yr for the nearby transiting systems (0.26 ms/yr in GJ436), which in some cases exceeds all other contributions to $\dot P$. Earth's motion around the Sun gives rise to additional periodic timing signal (even for systems with zero intrinsic proper motion) allowing a full determination of the spatial orientation of the planetary orbit. Unlike most other timing effects the proper motion signatures persist even in systems with zero eccentricity and get stronger as the planetary period increases. They should be the dominant cause of transit timing variations in isolated wide separation (periods of months) systems that will be sought by Kepler.