Exoplanet Atmosphere Report · JWST NIRSpec/G395H Phase Curve

WASP-121 b

An ultra-hot Jupiter so close to its star that its dayside glows hot enough to vaporize rock and metal, with a nightside measured well over 1000 K cooler. This page extracts that day-night divide from a JWST phase-resolved spectroscopy dataset, propagating each point's own posterior uncertainty rather than reporting a bare difference of averages.

Transit-discovered, 2016 NASA Exoplanet Archive parameters JWST phase curve, 36 phase bins x 349 wavelengths
Artist's concept of the ultra-hot Jupiter WASP-121 b and its glowing dayside

AI-generated artist's concept of WASP-121 b — not a real photograph. All data and figures in this report come from actual JWST NIRSpec/G395H phase-curve observations (see below).

The planet, in numbers

Queried live from the NASA Exoplanet Archive TAP service (pscomppars).

Radius19.53 Earth radii (~1.74 Jupiter radii)
Mass371.9 Earth masses (~1.17 Jupiter masses)
Orbital period1.27 days — one of the shortest known for a giant planet
Semi-major axis0.0257 AU
Equilibrium temperature2409 K
Host starWASP-121, F-type dwarf, Teff = 6628 K, 1.46 Rsun, 1.33 Msun
Distance269.9 parsecs (~880 light-years)
Discovery2016, transit photometry (WASP survey)

Why WASP-121 b is a benchmark ultra-hot Jupiter

At 2409 K equilibrium temperature and orbiting so close its shape is measurably tidally distorted, WASP-121 b sits at the extreme end of hot-Jupiter irradiation. Earlier HST work (Evans et al. 2017, 2018) found evidence for a stratospheric temperature inversion and metal oxide absorption (vanadium oxide) on its dayside — the atmospheric equivalent of Earth's ozone layer, but from a completely different chemistry driven by extreme heat rather than UV-absorbing ozone.

More recent JWST phase-curve spectroscopy (the data behind this report) tracks how the planet's emitted spectrum changes continuously as different hemispheres rotate into view across a full orbit, directly measuring how efficiently (or poorly) the atmosphere moves heat from the permanently irradiated dayside to the nightside.

Flux-space phase curves and temperature-space contrast

The top panel keeps NRS1 and NRS2 separate and averages the released planet/star flux ratios within each detector band at every phase. A first harmonic is overlaid only as a compact descriptive summary. The lower panels compare brightness-temperature spectra in phase windows nearest eclipse and transit. This separation matters: flux ratio is the measured observable, whereas brightness temperature is a wavelength-dependent transformation and its average is not an energy-budget temperature.

WASP-121 b day vs night brightness temperature spectrum and contrast from JWST NIRSpec G395H phase curve, with propagated uncertainty bands
Top: detector-separated phase curves and descriptive first harmonics. Middle: dayside- and nightside-facing brightness-temperature spectra. Bottom: their monochromatic difference. Generated from seven Zenodo v8 products by scripts/analyze_spectrum.py.
Wavelength channels349
NRS1 peak before eclipse2.934°
NRS2 peak before eclipse2.358°
Wavelength-avg. dayside T (this page)2722.1 ± 1.7 K
Wavelength-avg. nightside T (this page)1237.5 ± 3.3 K
Wavelength-avg. day-night contrast1493 ± 4 K

The first-harmonic conjunction estimates are 3915/158 ppm (NRS1) and 4910/620 ppm (NRS2), closely tracking the published broadband values from the same observation. This is a reproducibility cross-check, not independent confirmation. Reduced χ² values of 14.36 and 13.07 show that a first harmonic and diagonal channel errors are not adequate for inference; the peak locations therefore have no new physical uncertainty or detection significance attached here. The 1493.2 ± 3.8 K wavelength-averaged monochromatic contrast is likewise not a bolometric hemisphere temperature. For energy-budget work, use an analysis that models unobserved wavelengths, such as the cited NIRISS/SOSS result (2717 ± 17 K dayside, 1562 ± 19 K nightside).

Data and method notes

System parameters come from the NASA Exoplanet Archive TAP service. Seven numerical products are pinned to Zenodo v8 (record 10.5281/zenodo.20651891): phase and wavelength grids, flux-ratio values/errors, and brightness-temperature values with asymmetric errors. See source provenance, the analysis, machine-readable summary, and derived phase curves. Source identities and matrix shapes are checked by scripts/validate_data.py. Inter-channel covariance is unavailable; diagonal formal errors are not treated as physical parameter uncertainties.

References

  1. Delrez, L. et al., 2016. WASP-121 b: a hot Jupiter close to tidal disruption transiting an active F star. Monthly Notices of the Royal Astronomical Society, 458(4), pp.4025-4043.
  2. Evans, T.M. et al., 2017. An ultrahot gas-giant exoplanet with a stratosphere. Nature, 548, pp.58-61.
  3. Evans, T.M. et al., 2018. Detection of H2O and Evidence for TiO/VO in an Ultra-Hot Exoplanet Atmosphere. The Astrophysical Journal Letters, 822, L4.
  4. Evans-Soma, T.M., Sing, D.K. et al., 2025. SiO and a super-stellar C/O ratio in the atmosphere of the giant exoplanet WASP-121b. Nature Astronomy, 9(6), pp.845-861 (arXiv:2506.01771) — source paper for the phase-curve data used here.
  5. May, E.M. et al., 2023. A JWST NIRSpec Phase Curve for WASP-121b: Dayside Emission Strongest Eastward of the Substellar Point and Nightside Conditions Conducive to Cloud Formation. The Astrophysical Journal Letters, 943(1), L17 (arXiv:2301.03209) — source of the per-detector nightside brightness temperatures quoted above.
  6. Splinter, J. et al., 2025. Precise Constraints on the Energy Budget of WASP-121b from its JWST NIRISS/SOSS Phase Curve (arXiv:2509.09760) — source of the bolometric day/night temperatures quoted above.
  7. Zenodo record 10.5281/zenodo.20651891, "WASP-121b JWST NIRSpec/G395H data products."
  8. NASA Exoplanet Archive, exoplanetarchive.ipac.caltech.edu — system parameters, queried live via TAP.