Exoplanet atmospheres

Abstract:

Determining the composition of an exoplanet atmosphere relies on the presence of detectable spectral features. The strongest spectral features, including dimethyl sulphide (DMS), look approximately Gaussian. Here, I perform a suite of Gaussian feature analyses to find any statistically significant spectral features in the recently published MIRI/LRS spectrum of K2-18b. In N. Madhusudhan et al., they claim a 3.4σ detection of spectral features compared to a flat line. In 5 out of 6 tests, I find the data preferred a flat line over a Gaussian model, with a χν2 of 1.06. When centering the Gaussian where the absorptions for DMS and DMDS peak, I find ln(B) = 1.21 in favour of the Gaussian model, with a χν2 of 0.99. With only ∼2σ in favour of Gaussian features, I conclude no strong statistical evidence for spectral features.

Abstract:

Measuring the atmospheric composition of hazy sub-Neptunes like GJ 1214b through transmission spectroscopy is difficult because of the degeneracy between mean molecular weight (MMW) and haziness. It has been proposed that phase-curve observations can break this degeneracy because of the relationship between MMW and phase-curve amplitude. However, photochemical hazes can strongly affect phase-curve amplitudes as well. We present a large set of general circulation model simulations of the sub-Neptune GJ 1214b that include photochemical hazes with varying atmospheric composition, haze opacity, and haze optical properties. In our simulations, photochemical hazes cause temperature changes of up to 200 K, producing thermal inversions and cooling deeper regions. This results in increased phase-curve amplitudes and adds a considerable scatter to the phase-curve amplitude–metallicity relationship. However, we find that if the haze production rate is high enough to significantly alter the phase curve, the secondary eclipse spectrum will exhibit either emission features or strongly muted absorption features. Thus, the combination of a white-light phase curve and a secondary eclipse spectrum can successfully distinguish between a hazy, lower-MMW and a clear, high-MMW scenario.

Abstract:

Sub-Neptunes, the most common planet type, remain poorly understood. Their atmospheres are expected to be diverse, but their compositions are challenging to determine, even with JWST. Here, we present the first JWST spectroscopic study of the warm sub-Neptune GJ 3090 b (2.13 R⊕, Teq,A = 0.3 ∼ 700 K), which orbits an M2V star, making it a favorable target for atmosphere characterization. We observed four transits of GJ 3090 b: two each using JWST NIRISS/SOSS and NIRSpec/G395H, yielding wavelength coverage from 0.6 to 5.2 μm. We detect the signature of the 10833 Å metastable helium triplet at a statistical significance of 5.5σ with an amplitude of 434 ± 79 ppm, marking the first such detection in a sub-Neptune with JWST. This amplitude is significantly smaller than predicted by solar-metallicity forward models, suggesting a metal-enriched atmosphere that decreases the mass-loss rate and attenuates the helium feature amplitude. Moreover, we find that stellar contamination, in the form of the transit light source effect, dominates the NIRISS transmission spectra, with unocculted spot and faculae properties varying across the two visits separated in time by approximately 6 months. Free retrieval analyses on the NIRSpec/G395H spectrum find tentative evidence for highly muted features and a lack of CH4. These findings are best explained by a high-metallicity atmosphere (>100× solar at 3σ confidence for clouds at ∼μbar pressures) using chemically consistent retrievals and self-consistent model grids. Further observations of GJ 3090 b are needed for tighter constraints on the atmospheric abundances and to gain a deeper understanding of the processes that led to its potential metal enrichment.

Abstract:

Saturn’s moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan’s northern winter and spring were investigated in detail by the Cassini–Huygens spacecraft (2004–2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan’s late northern summer. Using JWST’s mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan’s climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.

Abstract:

Over the course of the past decade, advances in radial velocity and transit techniques have enabled the detection of rocky exoplanets in the habitable zones of nearby stars. Future observations with novel methods are required to characterize this sample of planets, especially those that are nontransiting. One proposed method is the Planetary Infrared Excess (PIE) technique, which would enable the characterization of nontransiting planets by measuring the excess IR flux from the planet relative to the star’s spectral energy distribution. In this work, we predict the efficacy of future observations using the PIE technique by potential future observatories such as the MIRECLE mission concept. To do so, we conduct a broad suite of 21 general circulation model (GCM) simulations, with ExoCAM, of seven nearby habitable zone targets for three choices of atmospheric composition with varying partial pressure of CO2. We then construct thermal phase curves and emission spectra by post-processing our ExoCAM GCM simulations with the Planetary Spectrum Generator (PSG). We find that all cases have distinguishable carbon dioxide and water features assuming a 90° orbital inclination. Notably, we predict that CO2 is potentially detectable at 15 μm with MIRECLE for at least four nearby known nontransiting rocky planet candidate targets in the habitable zone: Proxima Centauri b, GJ 1061 d, GJ 1002 b, and Teegarden’s Star c. Our ExoCAM GCMs and PSG post-processing demonstrate the potential to observationally characterize nearby nontransiting rocky planets and better constrain the potential for habitability in our solar neighborhood.