= Solution
First refine the ephemeris, planetary mass, stellar radius, and stellar variability using <exoplanet transit photometry> and the <radial-velocity method>. Then combine observations that probe different regions rather than relying on one spectrum. A present-day programme could use the following complementary measurements; in the 2019 setting of the paper, <James Webb Space Telescope> observations would have been a future capability.
* <Exoplanet transmission spectrum> at roughly $0.3$–$1\,\mu\mathrm m$ with the <Hubble Space Telescope> or optical ground-based spectroscopy: constrain <exoplanet clouds>, <atmospheric haze>, the <scattering slope of a transmission spectrum>, and sodium or potassium absorption.
* Near-infrared <exoplanet transmission spectrum> with <NIRISS> at $0.6$–$2.8\,\mu\mathrm m$ and <NIRSpec> modes covering roughly $1$–$5\,\mu\mathrm m$: measure water, carbon monoxide, carbon dioxide, and methane bands, then constrain <atmospheric metallicity of a giant planet> and <atmospheric carbon-to-oxygen ratio> through a joint atmosphere model.
* <Exoplanet secondary eclipses> at near- and mid-infrared wavelengths with <NIRSpec> and <MIRI>, especially about $5$–$12\,\mu\mathrm m$ for the latter's time-series low-resolution mode: infer <brightness temperatures>, the vertical pressure-temperature structure, and whether an <atmospheric thermal inversion> turns bands into emission.
* A full-orbit <exoplanet thermal phase curve> in one or more infrared bands with the <James Webb Space Telescope>: constrain <day-night heat redistribution>, nightside emission, and the offset of the hottest region, with different bands probing different pressures.
* High-resolution near-infrared spectroscopy around molecular bands such as carbon monoxide near $2.3\,\mu\mathrm m$ using <CRIRES> on the <Very Large Telescope>: resolve the planetary <Doppler shift> and seek wind velocities or rotation broadening after accounting for the orbital velocity.
* Ultraviolet transit spectroscopy with the <Hubble Space Telescope>, or ground-based near-infrared helium spectroscopy at $1.083\,\mu\mathrm m$: search for <atmospheric escape> and an extended upper atmosphere.
Together these address aerosols, molecular composition, elemental enrichment, vertical thermal structure, horizontal heat transport, winds, and escape. Repeat key events and monitor stellar activity, since stellar contamination and instrumental trends can imitate atmospheric signals. Use actual brightness, saturation limits, and predicted feature amplitudes to choose observing modes and exposure times.
The wavelength ranges and time-series capabilities are documented in the https://jwst-docs.stsci.edu/jwst-near-infrared-imager-and-slitless-spectrograph/niriss-observing-modes/niriss-single-object-slitless-spectroscopy[NIRISS SOSS guide], https://science.nasa.gov/mission/webb/nirspec/[NIRSpec overview], https://jwst-docs.stsci.edu/jwst-mid-infrared-instrument/miri-observing-modes/miri-low-resolution-spectroscopy[MIRI spectroscopy guide], and https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/crires%2B/[ESO's CRIRES description].
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