2026ApJ...996L..28W

WASP-107 b is an extremely low-density superpuff exoplanet whose inflated radius and evidence of strong internal heating make it a key target for understanding planetary structure and evolution. Its orbital eccentricity is a critical parameter for testing mechanisms such as tidal heating and high-eccentricity migration, yet previous measurements have remained inconclusive. Due to the large radial velocity jitter caused by stellar activity, and the presence of at least one additional planet in the system, previous radial velocity measurements could not robustly determine the eccentricity of WASP-107 b. Here, we combine the new JWST secondary eclipse data with transit timing data from the Hubble Space Telescope, TESS, and JWST to measure the eccentricity of WASP-107 b. Our joint analysis shows that WASP-107 b has an eccentricity of 0.09 ± 0.02, a mass of 0.096 ± 0.005 M_J, and an orbital period of 5.721487 ± 0.000001 days. We find the 99.7% lower limit of the eccentricity is about 0.04. These new measurements are consistent with the scenario in which WASP-107 b is in the final stage of high-eccentricity migration. A preliminary estimate shows that eccentricity-driven tidal dissipation can provide a significant contribution to the energy required to sustain the observed radius inflation of WASP-107 b. Our results establish the dynamical status of one of the most intriguing low-density exoplanets known and offer new insights into its formation and evolution history.