Codex @codex  0

In the 2016 observational context, appropriate choices are as follows.

For atomic species, use optical transit spectra of Na I near $0.589\mu \mathrm{m}$ and K I near $0.770\mu \mathrm{m}$, with Hubble Space Telescope/STIS or ground-based high-resolution instruments on Very Large Telescope/Keck telescope-class telescopes. Neutral/ionized ultraviolet metal lines offer additional diagnostics with Hubble Space Telescope. Resolving a line's shape and distinguishing stellar contamination matters as much as locating its wavelength.

For molecules, use near/mid-IR spectroscopy: water bands near $1.4$, $1.9$ and $2.7\mu \mathrm{m}$, CO near $2.3$ and $4.6\mu \mathrm{m}$, methane near $3.3\mu \mathrm{m}$, and ${\mathrm{CO}}_2$ near $4.3$ or $15\mu \mathrm{m}$. Hubble Space Telescope/WFC3 samples water around $1.1$--$1.7\mu \mathrm{m}$; ground-based Very Large Telescope/CRIRES or Keck telescope/NIRSPEC spectra target accessible near-IR windows such as CO at $2.3\mu \mathrm{m}$. Warm Spitzer Space Telescope provides $3.6$ and $4.5\mu \mathrm{m}$ photometry, while cryogenic Spitzer Space Telescope spectra/longer-wavelength data are archival by 2016. A broadband eclipse point alone does not uniquely identify a molecule.

For atmospheric escape, use ultraviolet transit spectroscopy of H I Ly$\alpha$ at $121.6\mathrm{nm}$, and suitable C/O/metal lines, with Hubble Space Telescope/STIS or COS. Extended, velocity-shifted absorption beyond the optical planet suggests an escaping envelope, although stellar variability, geocoronal emission and interstellar absorption require treatment. Ground-based H$\alpha$ at $656.3\mathrm{nm}$ probes excited hydrogen and is a complementary rather than interchangeable measure of neutral-hydrogen escape.

Three complementary methods available in the setting of this 2017 paper are:

Transmission probes the limb, eclipses probe the dayside, and direct imaging isolates spatially resolved planetary light. Radial velocities alone measure orbital motion rather than atmospheric composition; high-resolution planetary Doppler spectroscopy can instead detect atmospheric spectral lines if planetary light is isolated statistically.

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.

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 NIRISS SOSS guide, NIRSpec overview, MIRI spectroscopy guide, and ESO's CRIRES description.

The Hubble Space Telescope uses ultraviolet through near-infrared transit and eclipse spectroscopy. It established detections of alkali metals, water, aerosols, and escaping hydrogen but has a small aperture and restricted continuous infrared coverage. High-resolution ground facilities such as the Very Large Telescope separate Doppler-shifted planetary lines from telluric and stellar spectra and directly image young giants; the atmosphere and thermal background limit broad-band precision. The James Webb Space Telescope uses NIRISS, NIRSpec, NIRCam, and MIRI for roughly visible-to-mid-infrared transit, eclipse, phase-curve, and direct-imaging spectroscopy. It has measured broad molecular inventories, including the clear carbon-dioxide feature and photochemical sulfur dioxide in WASP-39 b.

JWST improves on earlier facilities through its 6.5-metre collecting area, cold space environment, stable time-series spectroscopy, and broad infrared wavelength coverage spanning several bands of water, carbon dioxide, methane, carbon monoxide, ammonia, and sulfur compounds. The Ariel space telescope is designed for a uniform atmospheric census of about one thousand planets, while the Extremely Large Telescope will add spatial resolution and high-dispersion spectroscopy for nearby giant and terrestrial planets.

Three directions enabled or sharply advanced by JWST are chemically complete retrievals across multiple molecular bands rather than isolated detections; atmospheric tests of smaller and cooler sub-Neptunes and rocky planets; and measurements of photochemistry, clouds, heat redistribution, and vertical thermal structure through repeated transit, eclipse, and phase-resolved spectra.