An absolute mass, precise age, and hints of planetary winds for WASP-121 A and b from a JWST NIRSpec phase curve

Abstract

We have conducted a planetary radial velocity measurement of the ultra-hot Jupiter WASP-121b using JWST NIRSpec phase curve data. Our analysis reveals the Doppler shift of the planetary spectral lines across the full orbit, which shifts considerably across the detector (∼ 10 pixels). Using cross-correlation techniques, we have determined an overall planetary velocity amplitude of Kp = 215.74 ± 1.05 km/s, which is in good agreement with the expected value. We have also calculated the dynamical mass for both components of the system by treating it as an eclipsing double-line spectroscopic binary, with WASP-121A having a mass of M⋆=1.330 ± 0.019 M⊙, while WASP-121b has a mass of Mp= 1.1697 ± 0.0426 MJup. These dynamical measurements are ∼ 3× more precise than previous estimates and do not rely on any stellar modeling assumptions which have a ∼5% systematic floor mass uncertainty. Additionally, we used stellar evolution modeling constrained with a stellar density and parallax measurement to determine a precise age for the system, found to be 1.11 ± 0.14 Gyr. Finally, we observed potential velocity differences between the two NIRSpec detectors, with NRS1 lower by 5.5±2.2 km/s. We suggest that differences can arise from day/night asymmetries in the thermal emission, which can lead to a sensitivity bias favoring the illuminated side of the planet, with planetary rotation and winds both acting to lower a measured KP. The planet’s rotation can account for 1 km/s of the observed velocity difference, with 4.5±2.2 km/s potentially attributable to vertical differences in wind speeds.