A JWST NIRSpec Phase Curve for WASP-121b: Dayside Emission Strongest Eastward of the Substellar Point and Nightside Conditions Conducive to Cloud Formation

We present the first exoplanet phase-curve measurement made with the JWST NIRSpec instrument, highlighting the exceptional stability of this newly commissioned observatory for exoplanet climate studies. The target, WASP-121b, is an ultrahot Jupiter with an orbital period of 30.6 hr. We analyze two broadband light curves generated for the NRS1 and NRS2 detectors, covering wavelength ranges of 2.70–3.72 μ m and 3.82–5.15 μ m, respectively. Both light curves exhibit minimal systematics, with approximately linear drifts in the baseline flux level of 30 ppm hr ^−1 (NRS1) and 10 ppm hr ^−1 (NRS2). Assuming a simple brightness map for the planet described by a low-order spherical harmonic dipole, our light-curve fits suggest that the phase curve peaks coincide with orbital phases 3.°36 ± 0.°11 (NRS1) and 2.°66 ± 0.°12 (NRS2) prior to mideclipse. This is consistent with the strongest dayside emission emanating from eastward of the substellar point. We measure planet-to-star emission ratios of 3924 ± 7 ppm (NRS1) and 4924 ± 9 ppm (NRS2) for the dayside hemisphere and 136 ± 8 ppm (NRS1) and 630 ± 10 ppm (NRS2) for the nightside hemisphere. The latter nightside emission ratios translate to planetary brightness temperatures of 926 ± 12 K (NRS1) and 1122 ± 10 K (NRS2), which are low enough for a wide range of refractory condensates to form, including enstatite and forsterite. A nightside cloud deck may be blocking emission from deeper, hotter layers of the atmosphere, potentially helping to explain why cloud-free 3D general circulation model simulations systematically overpredict the nightside emission for WASP-121b.