Position angles and coplanarity of multiple systems from transit timing

Abstract

Aims. We compare the apparent difference in timing of transiting planets (or eclipsing binaries) that are observed from widely separated locations (parallactic delay). Methods. A simple geometrical argument allows us to show that the apparent timing difference also depends on the sky position angle of the planetary (or secondary) orbit, relative to the ecliptic plane. Results. Our calculation of the magnitude of the effect for all currently known planets (should they exhibit transits) find that almost 200 of them-mostly radial-velocity detected planets-have predicted timing effects greater than 1 s. We also compute the theoretical timing precision for the PLATO mission, which will observe a similar stellar population and find that a 1 s effect will frequently be easily observable. We also find that the sky coplanarity of multiple objects in the same system can be probed more easily than the sky position angle of each of the objects separately. Conclusions. We show that a new observable from transit photometry becomes available when very high-precision transit timing is available. We find that there is a good match between projected capabilities of the future space missions PLATO and CHEOPS and the new observable. We specify some initial science questions that this new observable may be able to address. © 2013 ESO.