Precise Constraints on the Energy Budget of WASP-121 b from Its JWST NIRISS/SOSS Phase Curve

Journal article

Ultra-hot Jupiters exhibit day-to-night temperature contrasts upwards of 1000 K due to competing effects of strong winds, short radiative timescales, magnetic drag, and H2 dissociation/recombination. Spectroscopic phase curves provide critical insights into these processes by mapping temperature distributions and constraining the planet’s energy budget across different pressure levels. Here, we present the first NIRISS/SOSS phase curve of an ultra-hot Jupiter, WASP-121 b. The instrument’s bandpass [0.6–2.85 μm] captures an estimated 50%–83% of the planet’s bolometric flux, depending on orbital phase, allowing for unprecedented constraints on the planet’s global energy budget; previous measurements with HST/WFC3 and JWST/NIRSpec/G395H captured roughly 20% of the planetary flux. Accounting for the unobserved regions of the spectrum, we estimate effective day- and nightside temperatures of Tday = 2717 ± 17 K and Tnight=1562−19+18 K corresponding to a Bond albedo of AB = 0.277 ± 0.016 and a heat recirculation efficiency of ϵ = 0.246 ± 0.014. Matching the phase-dependent effective temperature with energy balance models yields a similar Bond albedo of 0.3 and a mixed layer pressure of 1 bar consistent with photospheric pressures, but unexpectedly slow winds of 0.2 km s−1, indicative of inefficient heat redistribution. The shorter optical wavelengths of the NIRISS/SOSS Order 2 yield a geometric albedo of Ag=0.093−0.027+0.029 (3σ upper limit of 0.175), reinforcing the unexplained trend of hot Jupiters exhibiting larger Bond than geometric albedos. We also detect near-zero phase curve offsets for wavelengths above 1.5 μm, consistent with inefficient heat transport, while shorter wavelengths potentially sensitive to reflected light show eastward offsets.

Authors: Splinter, J; Coulombe, L; Frazier, RC; Cowan, NB; Rauscher, E

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: The Astronomical Journal
• Volume: 170
• Issue: 6
• Article number: 323
• Publication date: 2025-11-13
• Acceptance date: 2025-09-29
• DOI: 10.3847/1538-3881/ae0e52
• EISSN: 1538-3881
• ISSN: 0004-6256
• Language: English
• Keywords: Exoplanet atmospheres; Exoplanets; Exoplanet atmospheric dynamics; Exoplanet systems; Planetary atmospheres; Exoplanet astronomy; Exoplanet atmospheric structure