Context. Hitherto, the study of exoplanetary transit timing and duration variability has supposed transit light curves (TLCs) to be symmetric, suggesting a priori a spherical shape for the transiting object, for example, an exoplanet. As a result, the independent positions of transit borders are unknown. However, the borders of TLCs are most sensitive to the presence of exo-rings and/or dust formations of great interest.
Aims: For the first time we check for a timing variability of independently treated borders of transits of different types of exoplanets.
Methods: Using quadratic approximation for the start-, end-, and minimum parts of the long-cadence TLCs from the Kepler mission archive after their whitening and phase folding, we find the corresponding transit border timings: ∆ts, ∆te, respectively, and the TLC minimum time ∆tm. These parameters were found separately for each folded TLC constructed in the consequent non-overlapping time-windows with the respective medium time tw. Temporal and cross-correlation analysis of the obtained series of ∆ts(tw), ∆te(tw), and ∆tm(tw) were applied for the detection and diagnostics of variability of transit borders and TLC asymmetry.
Results: Among the considered TLCs of 98 Kepler objects of interest (KOIs), 15 confirmed giant exoplanets and 5 objects with still debatable status (probably non-planets) show variations in their transit timing parameters at timescales from ≈400 to ≳1500 days. These variations are especially well manifested as an anti-correlation between ∆ts and ∆te, indicating variability in the dimensions of transiting shadows, especially along their trajectories. There are also objects with well pronounced oscillations of transit border timing and asymmetry.
Conclusions: The discovered variability of transit timing is important as an indicator of large-scale non-stationary processes in the atmospheres of KOIs, as well as dust and aerosol generation in their upper layers and in their close vicinity. These findings highlight the need for a dedicated and detailed study.