Year: 2026
Type: preprint
Abstract: 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 ($1.347 \pm 0.047 R_{\rm Jup}$, $5.02 \pm 0.75 M_{\rm Jup}$) with the shortest orbital period (0.67247479 $\pm$ 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 $\pm$ 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.
Source: arXiv (Cornell University)
Institutions: Carnegie Institution for Science, Earth and Planets Laboratory, University of California San Diego, Carnegie Observatories, Space Telescope Science Institute, University of Oxford, University of California, Santa Cruz, University of Hawaiʻi at Mānoa, University of Hawaii System, Johns Hopkins University, Jet Propulsion Laboratory
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7214160592