Highlights from Exoplanet Observations by the James Webb Space Telescope

The James Webb Space Telescope (JWST) has started a revolution in exoplanetary science.

From studying in exquisite detail the chemical inventories and physical processes in gas giant exoplanets, the structure and chemical diversity of the enigmatic sub-Neptune population to even providing constraints on the atmospheric make-up of rocky exoplanets, the observatory is enabling cutting-edge science that is touching virtually every sub-area in the field.

In this review Chapter, we showcase key highlights from exoplanet science being conducted with this state-of-the-art space observatory, which we believe is representative of the transformational science it is producing.

One of the key takeaways from these pioneering JWST observations is how they are starting to reshape not only how we think, study and interpret exoplanet observations — but how they are also reshaping our intuition about our very own Solar System planets.

Exoplanets being observed by JWST as of February 2025. Orbital distance in Astronomical Units (AU) versus planetary mass (in Earth’s units) of known exoplanets being observed by JWST through February 2025 (hexagons). This includes Guaranteed Time Observations (GTO), Director’s Discretionary Time (DDT), Early Release Observations (ERO), Early Release Science (ERS) and General Observer (GO) observations from Cycles 1, 2 and 3. Yellow hexagons represent JWST direct imaging/spectroscopy exoplanet observations, while purple hexagon represent transiting exoplanet observations with JWST. Grey points represent all known confirmed exoplanets. Images of Solar System planets are embedded for reference. — astro-ph.EP

Some highlights on JWST’s multi-dimensional exploration of gas giant exoplanets. (Bottom) Anotated, raw NIRSpec/G395H NRS2 phase-curve of WASP-121 b — JWST’s first phase curve event published in the refereed literature, showcasing the exquisite precision of the observatory to study these events (adapted from Mikal-Evans et al. 2023b). To match the geometry of the transit mapping illustration (morning/evening), the illustration of phases throughout the phasecurve shown on top of it show the planet as it moved clockwise around its star (assumed to be the same direction of the rotation of the planet). (Top, left) Flux map of WASP-43 b derived from fitting eclipse maps to its MIRI/LRS secondary eclipse lightcurve, which allows to obtain both longitudinal and latitudinal information about the planet’s flux distribution (adapted from Hammond et al. 2024). (Top, middle) Morning/evening spectrum of WASP-39 b obtained by separating the terminator components using NIRSpec/PRISM wavelength-dependent transit events (adapted from Espinoza et al. 2024, under a Creative Commons Attribution (CC BY) license). (Top, right) Phase-resolved emission spectroscopy of WASP-43 b, obtained by studying its MIRI/LRS phase curve — best-fit retrievals to each phase are plotted on top of the datapoints (adapted from Bell et al. 2024, under a Creative Commons Attribution (CC BY) license) — astro-ph.EP

Highlights from JWST’s atmospheric exploration of the rocky planets TRAPPIST-1 b and TRAPPIST-1 c. Left-most illustration showcases the TRAPPIST-1 star (red fraction of a circle) and the orbits of TRAPPIST-1 b (inner one) and TRAPPIST-1 c (outer one), together with the geometries of the different measurements showcased. (Top) Emission photometry and transmission spectroscopy characterization of TRAPPIST-1 b. The eclipses, and corresponding emission measurements at 12.8 and 15 µm are adapted from Ducrot et al. (2025). Eclipse depths as a function of wavelength are showcased against blackbodies with different bond albedos AB (grey curves), an ultramafic rock surace (UM in the illustration, in black), and a hazy atmosphere that creates a thermal inversion that produces a CO₂ feature in emission (orange model); also adapted from Ducrot et al. (2025) The transmission spectrum is adapted from (Lim et al. 2023), and it has been corrected for stellar contamination. The various models shown are all consistent with the data. (Bottom) Same graphical depiction of the constraints on TRAPPIST-1 c. Eclipse and model emission spectra as a function of wavelength adapted from Zieba et al. (2023), under a Creative Commons Attribution (CC BY 4.0) license. In eclipse, once again blackbodies (grey) are shown along an ultramafic surface (UM, black). Models for a cloudy, 10-bar Venus is shown in dark orange, thin orange curve shows a 0.1 bar O₂ plus 100 ppm CO₂ atmosphere, which is consistent with the data; also adapted from Zieba et al. (2023), under a Creative Commons Attribution (CC BY 4.0) license. Similarly to the transmission spectrum of TRAPPIST-1 b, the spectrum of TRAPPIST-1 c is shown below, adapted from Radica et al. (2025). Similar to TRAPPIST-1 b, the models shown (all at 100 bar) cannot be differentiated from the data. — astro-ph.EP

Néstor Espinoza, Marshall D. Perrin

Comments: Comments from the community are very welcome. To be published in: Handbook of Exoplanets, 2nd Edition, Hans Deeg and Juan Antonio Belmonte (Eds. in Chief), Springer International Publishing AG, part of Springer Nature
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2505.20520 [astro-ph.EP] (or arXiv:2505.20520v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.20520
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Submission history
From: Néstor Espinoza
[v1] Mon, 26 May 2025 20:50:52 UTC (13,131 KB)
https://arxiv.org/abs/2505.20520

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