1：Experimental Design and Method Selection:

Used the Hubble Space Telescope (HST) with the Space Telescope Imaging Spectrograph (STIS) to observe three primary transits of WASP-121b. Data reduction involved bias, dark, and flat correction using the CALSTIS pipeline, cosmic-ray removal, and spectral extraction with IRAF procedures. Light curve analysis employed Gaussian process (GP) modeling with a squared-exponential kernel to account for systematics, and transit modeling using Mandel & Agol (2002) models with linear baseline trends.

2：1b. Data reduction involved bias, dark, and flat correction using the CALSTIS pipeline, cosmic-ray removal, and spectral extraction with IRAF procedures. Light curve analysis employed Gaussian process (GP) modeling with a squared-exponential kernel to account for systematics, and transit modeling using Mandel & Agol (2002) models with linear baseline trends. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Observed WASP-121b, an ultra-hot Jupiter with specific properties (mass ~

3：18 MJ, radius ~7 RJ, equilibrium temperature >2400 K). Data from HST/STIS (G430L and G750L gratings) and previously published HST/WFC3 (G141 grism) data were used. List of Experimental Equipment and Materials:

Hubble Space Telescope (HST), Space Telescope Imaging Spectrograph (STIS) with G430L and G750L gratings, Wide Field Camera 3 (WFC3) with G141 grism, calibration frames for data reduction, IRAF software for spectral extraction.

4：Experimental Procedures and Operational Workflow:

Conducted observations over multiple HST orbits, extracted spectra, constructed white and spectroscopic light curves, performed GP and MCMC analyses to fit transit models and systematics, and derived transmission spectra.

5：Data Analysis Methods:

Used af?ne-invariant Markov chain Monte Carlo (MCMC) via the emcee Python package for parameter estimation, compared with forward models (ATMO code) and free-chemistry retrievals to interpret atmospheric properties.