A close companion to a hot Jupiter?

Abstract

Context. Hot Jupiters with close-by planetary companions are rare, with only a handful of them having been discovered so far. This could be due to their suggested dynamical histories, leading to the possible ejection of other planets. We identified another candidate for this group of exoplanets with a small separation to its host star. Unexpectedly, transit timing measurements from recently obtained CHEOPS observations show low amplitude transit-timing variations (TTVs). Aims. We aim to characterise these TTVs, trying to gain information about a possible companion, and to search for signs of orbital decay. Methods. We fit the newly obtained CHEOPS light curves using TLCM and extract the resulting mid-transit timings. Successively, we use these measurements to estimate the rate of tidal orbital decay for our target, as well as characterise the TTVs using the N-body code TRADES and the photodynamical approach of PyTTV. Results. We find tentative evidence at 3σ for orbital decay in the system, when we correct the mid-transit timings using the best-fitting sinusoidal model of the TTVs. We do not detect additional transits in the available photometric data, but find evidence towards the authenticity of the apparent TTVs, indicating a close-by, outer companion. Due to the fast rotation of the star, the new planetary candidate cannot be detected in the available radial velocity (RV) measurements, and its parameters can only be loosely constrained by our joint TTV and RV modelling. Conclusions. The examined system could join a small group of rare hot Jupiter systems that host close-by planetary companions, only one of which (WASP-47 b) has an outer companion. More high-precision photometric measurements are necessary to confirm the existence of this planetary companion.