A constant period fits 103 WASP-121 b TESS transit times with a shared transit shape and independent errors

We test whether six public TESS sectors require a change in the orbital period of WASP-121 b. We measure 103 transit times from the original 120-second SPOC light curves. We use a learned common transit shape and free times, local baselines, and depth scales. Under independent transit errors, a constant period gives chi-square 91.36 for 101 degrees of freedom. A quadratic model gives a period change of -1.49 +/- 4.06 milliseconds per year. Its BIC is 4.50 higher than the constant-period model. All six light-curve controls agree with a constant period. A fixed growth rate of +15.1 milliseconds per year gives delta chi-square 16.66. This result depends on the error model. An assumed independent Gaussian offset of 10 seconds per sector gives -1.22 +/- 8.56 milliseconds per year and includes +15.1 in the 95% interval. These offsets are assumptions. We do not measure their amplitude. Free sector offsets leave 0.0568% of the curvature information. We also find that a printed quadratic coefficient in a prior preprint gives +18.32 milliseconds per year under its stated definition, rather than the printed +15.1. The data are the same six sectors used in that work. We add no observing baseline and do not test its full record of ground and space times. The results condition on the shared transit shape. The package contains original data, source hashes, code, transit times, controls, recovery tests, and figures.

WASP-121 b · TESS · transit timing · orbital period change · correlated errors · reproducible photometry · hot Jupiter