What this research found
WASP-121b is an ultra-hot Jupiter that circles its F-type host every 1.27 days, close enough to be near tidal disruption. Starting only from the star's name and using no published orbital values as priors, a single sector of two-minute TESS photometry was reduced, the transit detected at a signal-to-noise ratio of 389, and an analytic transit model fitted by Markov chain Monte Carlo. The recovered period of 1.2749549 ± 0.0000111 days lands 2.54 seconds from the discovery ephemeris, and the fitted radius ratio implies a planet 1.561 ± 0.032 times Jupiter's radius.
- From the roughly 19 transits within a single 24.46-day sector, the orbital period was pinned to 1.2749549 ± 0.0000111 days, about 0.96 seconds, and the mid-transit epoch to 0.000124 days, about 10.7 seconds.
- That period differs from the published discovery value of 1.2749255 ± 0.0000020 days by 2.54 seconds, a fractional offset of 2.3 × 10⁻⁵ and a formal 2.6 sigma deviation. The gap is attributed to the short single-sector baseline against the multi-year, multi-instrument campaigns behind the literature ephemeris rather than to any physical period change.
- The planet-to-star radius ratio came out at 0.11008, with plus 0.00034 and minus 0.00039 uncertainty — a precision of 0.35% — giving a transit depth of 12,117 ppm, or about 1.21%, and a first-to-fourth-contact duration of 2.880 ± 0.008 hours.
- Applying the literature stellar radius of 1.458 ± 0.030 solar radii yields a planet radius of 1.561 ± 0.032 Jupiter radii, or 111,632 ± 2,323 km. The 2.1% fractional error is set almost entirely by the stellar radius, with the radius ratio contributing only 0.3%.
- The depth is probably biased low: at 1.21% it is roughly 12% shallower than the discovery value of about 1.5%, with the scaled semi-major axis (3.573 versus about 3.86) and inclination (83.53 versus about 87.6 degrees) correspondingly lower. That pattern is the signature of smoothing the light curve before fitting rather than modelling baseline and transit jointly; the timing quantities are unaffected.
How it was done
The host was resolved by positional cone search to its TESS catalogue entry, matching within 0.002 arcseconds, and all six available short-cadence sectors were located; the earliest, observed in 2019, was used. Of 16,416 downloaded cadences spanning 24.46 days, 16,339 carried finite calibrated flux; these were normalized by the median and flattened with a Savitzky–Golay filter using a 0.75-day window, about six times the transit duration, followed by an asymmetric sigma clip that removed 10 positive outliers while preserving the transit dips. A Box Least Squares search over 50,000 trial periods from 0.5 to 5.0 days and 16 trial durations located the signal, which seeded a seven-parameter fit of an analytic transit model — period, epoch, radius ratio, scaled semi-major axis, inclination, and two quadratic limb-darkening coefficients — assuming a circular orbit. Posterior sampling used 32 walkers for 12,000 steps with the first 3,000 discarded, and the radius conversion propagated errors through 500,000 Monte Carlo draws.
Data sources
- TESS Sector 7 short-cadence photometry from the Mikulski Archive for Space Telescopes — 16,416 cadences over 24.46 days, retrieved for TIC 22529346
- Delrez et al., MNRAS 458:4025 (2016) — discovery ephemeris and stellar radius of 1.458 ± 0.030 solar radii
Limitations
The single-sector baseline caps the achievable period precision, and detrending the light curve before fitting rather than jointly biases the transit depth and the geometry coupled to it low, so the reported depth and planet radius are best read as lower bounds. The reduced chi-squared of about 3 indicates the pipeline flux errors understate the true scatter, making the quoted parameter uncertainties lower limits, and the physical radius depends on an externally sourced stellar radius that dominates its error budget.
How this research was produced
K-Dense Web planned and ran this astronomy investigation end to end — gathering the sources, carrying out the analysis, producing the figures, and drafting the report. The full session transcript, including every intermediate step, is available to view.
