Exoplanet atmospheres

We present a study of stellar limb-darkening using JWST transit observations of seven exoplanets orbiting FGK host stars, spanning 4200-6800 K. The wide wavelength coverage and high S/N of NIRISS/SOSS and NIRSpec/PRISM enable precise constraints on the wavelength-dependent limb-darkening. Using Bayesian model selection, we find that the quadratic limb-darkening law is statistically preferred over higher-order laws for most FGK stars, recovers consistent intensity profiles for >95% of the stellar disk area, and introduces a minimal bias in the derived transit depths of only ∼14 ppm (1σ) for 6/7 targets. This contrasts with previous studies relying on stellar models. We compare the empirically derived quadratic coefficients to predictions from the PHOENIX, MPS-ATLAS, MURaM, and Stagger stellar atmosphere grids. We introduce a quadratic limb-darkening parameterization in terms of limb intensity (ℓ) and curvature at mid-μ (δ), finding that empirical FGK limb darkening is generally more linear than models predict (δ≲0.1). We identify wavelength-independent offsets between data and model quadratic coefficients, minimized by adopting μmin=0.2 in intensity calculations; we attribute this in part to models overpredicting limb-darkening near the limb where the plane-parallel approximation breaks down. For spherical PHOENIX models, we derive a μ rescaling method using a τ=1 photospheric radius. With these corrections, residual offsets are minimized and all stellar models achieve statistically acceptable fits. We provide recommended limb-darkening offset priors for use in JWST transit analyses, enabling more accurate constraints on exoplanet transmission spectra while accounting for residual stellar model uncertainties.

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.

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."

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.

A key goal of astronomers with the next generation telescopes is to detect signs of life in exoplanet atmospheres. NASA's next flagship is the Habitable Worlds Observatory (HWO). In the context of ESA's Voyage 2050 program, the Senior Committee report prioritises detecting habitable exoplanet atmospheres in the mid-IR. The most suited mission for this is the Large Interferometer for Exoplanets (LIFE) which can detect an even wider range of biosignatures than HWO and at lower concentrations. LIFE is a global science collaboration based out of ETH Zürich. With the UK's expertise in building infrared instruments we could play a leading role in realising an ambitious European-led mission. Notably, LIFE is able to detect necessary planetary context like surface temperature and pressure, along with a key discriminator molecule for biosignature false positives, methane, which will be much harder or impossible with HWO. Also, LIFE will be able to investigate many of the nearby rocky exoplanets known from radial velocity searches that are inaccessible to HWO due to its limited spatial resolution.