JWST and HST Confirm Transit Timing Variations for the Ultra-Hot Jupiter TOI-2109 b

Ultra-hot Jupiters are a rare yet valuable subset of exoplanets that can help elucidate both the internal properties of their host stars as well as the orbital evolution of close-in planets. An extreme among extremes, TOI-2109 b is a super-Jupiter (1.347 plus or minus 0.047 R Jup, 5.02 plus or minus 0.75 M Jup) with the shortest orbital period (0.67247479 plus or minus 0.00000028 days) and the fastest predicted orbital decay rate of any known giant planet. Previous observations hinted at a quadratic long-term trend in the transit timing variations (TTVs) consistent with orbital decay, as well as short-term sinusoidal TTVs consistent with a near-resonant planetary companion that would indicate a dynamically quiescent migration pathway. Here, we combine archival ground-based, TESS, and CHEOPS photometry with new transit and eclipse data from HST and JWST to update the TTV analysis. We find very strong evidence for sinusoidal TTVs over a linear ephemeris (Δ BIC of 33.6), consistent with a nearby planetary companion as suggested by J.-V. Harre et al. 2024, and rule out several non-planetary scenarios that could explain the sinusoidal TTVs. We also find strong evidence of a quadratic TTV trend (Δ BIC of 6.75) consistent with an orbital decay rate of-3.46 plus or minus 1.17 ms/yr and a stellar tidal quality factor of 2.13+1.18-0.61 times10 7, though we cannot rule out line-of-sight acceleration as the cause of the quadratic trend. If our results are confirmed, TOI-2109 b would be the only known exoplanet besides WASP-12 b with a decaying orbit and the most extreme of an ultra-rare class of close-in giant planets with nearby companions.