Exoplanet atmospheres at high spectral resolution
Chapter in Handbook of Exoplanets, Springer (2026) 1-38
Abstract:
The spectrum of an exoplanet reveals the physical, chemical, and biological processes that have shaped its history and govern its future. However, observations of exoplanet spectra are complicated by the overwhelming glare of their host stars. Here, we focus on high-resolution spectroscopy (HRS) (R∼5,000−140,000), which helps disentangle and isolate the exoplanet’s spectrum. HRS resolves molecular features into a dense forest of individual lines in a pattern that is unique for a given molecule. For close-in planets, the spectral lines undergo large Doppler shifts during the planet’s orbit, while the host star and Earth’s spectral features remain essentially stationary, enabling a velocity separation of the planet. For slower-moving, wide-orbit planets, HRS, aided by high contrast imaging, instead isolates their spectra using their spatial separation (high contrast spectroscopy; HCS). The planet’s spectral lines are compared with HRS model atmospheric spectra, typically using cross-correlation to sum their signals. It is essentially a form of fingerprinting for exoplanet atmospheres and works for both transiting and non-transiting planets. It measures their orbital velocity, true mass, and simultaneously characterizes their atmosphere. The unique sensitivity of HRS to the depth, shape, and position of the planet’s spectral lines allows it to measure atmospheric composition, structure, clouds, and dynamics, including day-to-night winds and equatorial jets, plus its rotation period and even its magnetic field. These are extracted using statistically robust log-likelihood frameworks and match space-based instruments in their precision. This chapter describes the HRS technique in detail and concludes with future prospects with Extremely Large Telescopes to identify biosignatures on nearby rocky worlds and map features in the atmospheres of giant exoplanets.Atmospheric asymmetries in WASP-121 b revealed by rotational transits detected with JWST
Nature Astronomy (2026)
Abstract:
Close-in exoplanets are tidally locked to their host star and thus exhibit extreme atmospheric temperature gradients. It has been theorized that the fraction of star light absorbed by such planets during transit changes as a function of orbital phase as progressively hotter or colder atmospheric gas rotates into view, but this effect has not been observed so far. Here we show that two transits of the ultrahot Jupiter WASP-121 b, acquired with JWST/NIRSpec and NIRISS, exhibit asymmetric light curves caused by the planet’s rotation during transit. We observe increasing CO absorption and slightly decreasing H
Asymmetric Aerosol Distribution on the Terminators of the Warm Saturn WASP-69 b Revealed by JWST NIRISS/SOSS
Submitted to AAS
Abstract:
How aerosols form, are transported, and cycle between condensation and evaporation across exoplanet temperature regimes remains poorly understood. Recent models and observations suggest that warm giant planets near 800--1000 K may span a transition between homogeneous and longitudinally heterogeneous aerosol distributions. We present a robust detection of aerosol asymmetry in a giant planet with Teq≲1000 K, using the 0.86--2.82 μm JWST NIRISS/SOSS transmission spectrum of WASP-69 b. The evening limb shows prominent 1.4 μm H2O absorption (ΔBICH2O=+22.7), whereas H2O is not detected on the morning limb (ΔBICH2O=−8.7). Atmospheric retrievals reveal significant aerosol opacity on both limbs, with high-altitude, optically thick clouds muting molecular features on the morning limb and lower cloud opacity allowing H2O to emerge on the evening limb. The evening terminator is hotter by 304+62−91 K, consistent with morning-limb condensates partially evaporating during transport toward the evening limb. This mechanism is independently verified with 3D general circulation models. Stellar contamination or aerosols dominated by photochemical haze do not readily explain the asymmetry. From a limb-resolved analysis, we infer a stellar-to-superstellar atmospheric metallicity, with [M/H]=0.11+0.40−0.46 from the equilibrium retrieval and [O/H]=1.38+0.44−0.79 from the free retrieval. We also detect an escaping metastable-helium tail extending to 3.08+0.50−0.45Rp. WASP-69 b anchors the cooler edge of the emerging population of planets with asymmetric aerosol distributions and suggests that substantial aerosol opacity may persist on both limbs across this transition.
Characterizing Transiting Exoplanet Atmospheres in the 2030s with the Hubble Space Telescope
Hhite papers by STScI on "Building a Roadmap for Hubble science into the 2030s."
Abstract:
The Hubble Space Telescope inaugurated the era of exoplanet atmospheric characterization. While the James Webb Space Telescope has largely taken up the mantle of infrared atmospheric characterization, Hubble's unique short-wavelength capabilities remain unmatched. Recent theoretical advances in exoplanet atmospheric science combined with new observing strategies, like those offered by WFC3-UVIS/G280, have opened science cases that only Hubble can address for the foreseeable future. In this white paper, we discuss these new windows into the atmospheres of other worlds, focusing on characterization of their hydrostatic lower atmosphere, and identify the critical capabilities necessary for future observations. We highlight three overall science cases that will depend on the continued short-wavelength capabilities of Hubble: measuring aerosol scattering slopes, characterizing metal absorption in ultra-hot Jupiters, and understanding stellar activity with Transit Light Source effect decontamination and flare monitoring. Throughout, we highlight useful synergies between HST and JWST. This article is a response to the call for white papers by the Space Telescope Science Institute on "Building a Roadmap for Hubble science into the 2030s."
Cloudy mornings and clear evenings on a gas giant exoplanet
Science, Volume 392, 858-862 (2026)
Abstract:
The spectra of exoplanet atmospheres are affected byaerosols (clouds and hazes) of uncertain origin. Proposedaerosol formation mechanisms include gas condensation orphotochemical reactions. We measured the transmissionspectrum of the tidally locked gas giant exoplanet WaSP- 94a band identified asymmetry in its atmosphere. The morning limbis cooler and cloudy, whereas the evening limb is hotter andexhibits gaseous water absorption features. We interpret thisdifference as being due to the formation of cloud droplets nearthe morning limb, which evaporate during circulation to theevening limb. The dominant aerosols are clouds cyclingbetween the day and night sides of the atmosphere, notphotochemical hazes. The resulting asymmetry can severelybias chemical abundance measurements, unless limb-resolvedspectroscopy is available.