A Planet Candidate Orbiting near the Hot Jupiter TOI-2818 b Inferred through Transit Timing

TOI-2818 b is a hot Jupiter orbiting a slightly evolved G-type star on a 4.04 day orbit that shows transit timing variations (TTVs) suggestive of a decreasing orbital period. In the most recent year of TESS observations, transits were observed ∼8 minutes earlier than expected for a constant period. The implied orbital decay rate is 1.35 ± 0.25 s yr⁻¹, too fast to be explained by tidal dissipation, even considering the evolved nature of the host star. Radial velocity (RV) monitoring and astrometric data make the possibility of perturbations from a long-period companion unlikely; further Doppler spectroscopy observations can efficiently confirm or rule out such a companion. Apsidal precession due to the tidal distortion of the planet is also physically implausible. The most plausible explanation for the TTVs appears to be gravitational perturbations from a hitherto undetected planet with mass ≲10 M_⊕ that is in (or near) a mean-motion resonance with the hot Jupiter. Such a planet could be responsible for the observed TTVs while avoiding detection with the available RV and transit data.