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 allow us to show that the apparent timing difference depends also on the on-sky position angle of the planetary (or secondary) orbit, relative to the ecliptic plane. Results: We calculate that on-sky position angle would be readily observable using the future PLATO and CHEOPS missions data, and possibility observable already in many known radial-velocity systems (if they exhibit transits). We also find that on-sky coplanarity of multiple objects in the same system can be probed more easily than the on-sky position angle of each of the objects separately. We calculate the magnitude of the effect for all currently known planets (should they exhibit transits), finding that almost 200 of them -- mostly radial-velocity detected planets -- have predicted timing effect larger than 1 second. We also compute the theoretical timing precision for the PLATO mission, that will observe a similar stellar population, and find that a 1 second effect would be frequently readily observable. We also find that on-sky coplanarity of multiple objects in the same system can be probed more easily than the on-sky position angle of each of the objects separately. Conclusions: We show 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 give some initial science question that such a new observable may be related to and help addressing.