In NIRSpec’s wavelength range, the observed transit asymmetry is more pronounced in the longer than in the shorter observed wavelengths. This is the opposite of how transit asymmetries caused by temperature inhomogeneities on the star’s surface driven by, e.g., star spots or faculae would change with wavelength [58, 59]. Additionally, as an F6-type star [60], WASP-121 A is expected to display less activity-driven flux variations throughout its surface that could induce asymmetric transit light curves than later stellar types [61]. Indeed, previous transit observations in the optical have not revealed any hints for photometric contamination caused by stellar activity [62], making stellar activity as the cause of the observed transit asymmetries yet more unlikely. As an early spectral type, however, WASP-121 A might be a fast rotator, leading to gravity darkening of its equator relative to the poles due to the reduction of the equator’s surface gravity caused by centrifugal forces and an oblate shape of the star [63]. Together with the misalignment of WASP-121 b’s orbital axis with the star’s rotation axis [47, 60], this system configuration can induce a transit asymmetry [64]. From the nearly pole-on inclination of WASP-121 A [47], we estimate the rotation rate of the star to be between 0.15 −0.28 as a fraction of the critical rotational velocity. As such, gravity darkening of the host star should be a significant effect, causing a difference in emission intensity between the equator and pole of 6.4 −20.8% in the optical and 2 −6% in the JWST/NIRSpec passband. To quantify the possible impact of gravity darkening on our observations, we analyzed all available observations of WASP-121 b observed with TESS due to the proportionally higher contribution of gravity darkening to the transit asymmetry in the optical. We query 5 sectors of 20 s cadence light curves and 1 sector of 120 s cadence, including only data ∼ 0.25 days around the center of each transit. To model the transit of a rapidly-rotating, gravity-darkened star, we use the method described in ref. [34], but instead rewrite the spherical-harmonic decomposition of the gravity-darkened surface using Planck’s blackbody law into JAX. This allows us to use eclipsoid [65] as the model within the JAX ecosystem. We use NumPyro [66] to construct a probabilistic model for the gravity-darkened transit, using priors from ref. [21] for the stellar and planet parameters and ref. [47] for the projected orbital obliquity. We also sample in the sine of stellar inclination as an isotropic prior on the spin axis, restricted to the range 0-0.25 to ensure the star is relatively pole-on, as suggested by ref. [47], and place a uniform prior on the dimensionless stellar rotation rate ω between 0-0.3. We fix the gravitydarkening exponent β that governs what temperature difference a difference in surface gravity between the star’s equator and pole results in to 0.23 as varying it can cause it to run off towards physically unrealistic values [67]. Finally, we use Hamiltonian Monte Carlo with the NUTS sampler [68] in order to obtain posteriors on the parameters of interest. We also performed inference using the same priors but without the gravitydarkening map on the star as a comparison to a standard transit fit to examine the amplitude of a transit asymmetry the star’s gravity darkening induces.