J/AJ/169/341 JWST transmission spectra of WASP-121b (Gapp+, 2025)
WASP-121 b's transmission spectrum observed with JWST/NIRSpec G395H reveals
thermal dissociation and SiO in the atmosphere.
Gapp C., Evans-Soma T.M., Barstow J.K., Lothringer J.D., Sing D.K.,
Ruseva D., Ahrer E.-M., Goyal J.M., Christie D., Kreidberg L., Mayne N.J.
<Astron. J., 169, 341 (2025)>
=2025AJ....169..341G 2025AJ....169..341G
ADC_Keywords: Exoplanets; Spectra, infrared
Keywords: Exoplanet atmospheric composition ; Transmission spectroscopy ;
Infrared spectroscopy
Abstract:
WASP-121 b has been established as a benchmark ultrahot Jupiter,
serving as a laboratory for the atmospheric chemistry and dynamics of
strongly irradiated extrasolar gas giants. Here, we present and
analyze WASP-121 b's transmission spectrum observed with NIRSpec G395H
on board the James Webb Space Telescope and find evidence for the
thermal dissociation of H2O and H2 on the planet's permanent dayside.
Additionally, we detect SiO at a statistical significance of
5.2σ, which is compatible with chemical equilibrium in the
atmosphere. Constraining the abundance of SiO and abundance ratios
between silicon and volatile atoms in WASP-121 b's atmosphere could
help discriminate between possible migration histories of the planet.
The three-dimensional nature of thermal dissociation on WASP-121 b's
dayside and of recombination on its nightside, however, poses a
challenge to constraining molecular abundances and elemental abundance
ratios from the transmission spectrum. To account for this, we
implemented an atmospheric model in the NEMESIS framework that splits
the planet's atmosphere into dayside and nightside. A retrieval
applying our atmospheric model to WASP-121 b's transmission spectrum
favors a higher H2O abundance on the nightside than on the dayside,
demonstrating the impact of hemispheric heterogeneity when attempting
to constrain WASP-121 b's bulk H2O inventory.
Description:
In this work, we analyze a transit of WASP-121 b observed on 2022
October 14-15 as part of the phase-curve observations of James Webb
Space Telescope (JWST) program GO-1729 (PI: Evans-Soma; co-PI:
Kataria). The observation was carried out with the NIRSpec instrument
employing G395H to obtain spectra between 2.54 and 5.15um with a
wavelength gap ranging from 3.72 to 3.82um caused by the offset
between NIRSpec's NRS1 and NRS2 detectors. The telescope pointing and
performance throughout the 37.8hr long phase-curve observation were
very stable.
To obtain WASP-121 b's transmission spectrum, we analyzed the data
from 3.5hr before to 3.5hr after the transit midtime, leaving about
2.05hr of both pre- and post-transit baseline for calibrating stellar
fluxes and instrumental systematics in addition to the transit with a
duration of about 2.9hr itself.
To refine orbital parameters that are independent of the observed
wavelengths but important for transit observations, we
wavelength-integrated the light curves over the NRS1 and NRS2
detectors separately to create two white light curves we then analyzed
jointly. After modeling the white light curves, we divided the time
series of the one-dimensional spectra into finer wavelength bins to
generate spectrophotometric light curves. We generated these light
curves at two different wavelength binning levels: first, binning over
10 detector pixel columns per spectroscopic channel, leading to a
spectral resolution of R∼600, and second, binning over two
detector pixel columns, which results in a spectral resolution of
R∼3000, both resolutions varying with wavelength.
Objects:
-----------------------------------------------------
RA (2000) DE Designation(s)
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07 10 24.06 -39 05 50.5 WASP-121 b = TOI-495.01
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
fig6a.dat 55 349 WASP-121b transmission spectra for R∼600
fig6b.dat 65 1748 WASP-121b transmission spectra for R∼3000
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See also:
VI/119 : High accuracy computed water line list - BT2 (Barber+, 2006)
J/ApJS/95/535 : Infrared CO line list for X 1Σ+ state (Goorvitch, 1994)
J/MNRAS/434/1469 : ExoMol line lists for SiO (Barton+, 2013)
J/A+A/564/A125 : AGN Torus model comparison of AGN in the CDFS (Buchner+, 2014)
J/A+A/573/A90 : STAGGER-grid of 3D stellar models. IV. (Magic+, 2015)
J/MNRAS/474/5158 : Stars with hot Jupiter exoplanets (Goyal+, 2018)
J/MNRAS/473/5324 : ExoMol line lists. XXIV. SiH (Yurchenko+, 2018)
J/A+A/635/A205 : Ultra-hot Jupiter WASP-121b transits (Bourrier+, 2020)
J/AJ/161/174 : Transit depth biases & errors for 31 planets (Morello+, 2021)
Byte-by-byte Description of file: fig6a.dat
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Bytes Format Units Label Explanations
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1- 7 F7.5 10-6m Wave Wavelength; microns
9- 15 F7.5 % TDepth Transit-only transit depth
17- 23 F7.5 % E_TDepth Upper uncertainty in TDepth
25- 31 F7.5 % e_TDepth Lower uncertainty in TDepth
33- 39 F7.5 % PDepth Phase-curve transit depth
41- 47 F7.5 % E_PDepth Upper uncertainty in PDepth
49- 55 F7.5 % e_PDepth Lower uncertainty in PDepth
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Byte-by-byte Description of file: fig6b.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.5 10-6m Wave Wavelength; microns
9- 15 F7.5 % TDepth Transit-only transit depth
17- 28 F12.5 % E_TDepth Upper uncertainty in TDepth
30- 41 F12.5 % e_TDepth Lower uncertainty in TDepth
43- 49 F7.5 % PDepth Phase-curve transit depth
51- 57 F7.5 % E_PDepth Upper uncertainty in PDepth
59- 65 F7.5 % e_PDepth Lower uncertainty in PDepth
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History:
From electronic version of the journal
(End) Preapred by [AAS], Robin Leichtnam [CDS] 20-Mar-2026