Imaging & Spectroscopy

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

By Keith Cowing
Status Report
January 27, 2023
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A JWST NIRSpec Phase Curve for WASP-121b: Dayside Emission Strongest Eastward of the Substellar Point and Nightside Conditions Conducive to Cloud Formation
Black diamonds show the measured dayside (top panel) and nightside (bottom panel) planet-to-star emission levels. Note that the measurement uncertainties are smaller than the diamond symbols. Predictions from the cloud-free 3D GCM simulations of Parmentier et al. (2018) are also shown for heavy element enrichments of 1× solar (blue lines) and 5× solar (orange lines). Circle and square symbols show, respectively, the 1× and 5× solar GCM predictions binned to the light curve passbands, which are shown as green lines in the top panel. — astro-ph.EP

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 micron and 3.82-5.15 micron, respectively. Both light curves exhibit minimal systematics, with approximately linear drifts in the baseline flux level of 30 ppm/hr (NRS1) and 10 ppm/hr (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 deg (NRS1) and 2.66±0.12 deg (NRS2) prior to mid-eclipse. This is consistent with the strongest dayside emission emanating from eastward of the substellar point. We measure planet-to-star emission ratios of 3,924±7 ppm (NRS1) and 4,924±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 1,122±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 over-predict the nightside emission for WASP-121b.

Thomas Mikal-Evans, David K. Sing, Jiayin Dong, Daniel Foreman-Mackey, Tiffany Kataria, Joanna K. Barstow, Jayesh M. Goyal, Nikole K. Lewis, Joshua D. Lothringer, Nathan J. Mayne, Hannah R. Wakeford, Duncan A. Christie, Zafar Rustamkulov

Comments: Accepted for publication in Astrophysical Journal Letters on December 29, 2022
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2301.03209 [astro-ph.EP] (or arXiv:2301.03209v1 [astro-ph.EP] for this version)
Submission history
From: Thomas Mikal-Evans
[v1] Mon, 9 Jan 2023 08:57:18 UTC (490 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻