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

Abstract 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 H 2 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 μ m] 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 T day = 2717 ± 17 K and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">night</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>156</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>18</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> K corresponding to a Bond albedo of A B = 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.09</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.027</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.029</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> (3 σ 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 μ m, consistent with inefficient heat transport, while shorter wavelengths potentially sensitive to reflected light show eastward offsets.