Exoplanet atmospheres

Abstract:

The formation and evolution of giant planets remain incompletely understood, with mounting evidence that many close-in giants may have migrated from their birth locations. The detection of helium escaping the atmosphere of exoplanets has provided a powerful new tracer of atmospheric escape and exoplanetary evolution. Here, using high-precision spectroscopic observations from the James Webb Space Telescope (JWST) Near Infrared Imager and Slitless Spectrograph (NIRISS) in single-object slitless spectroscopy mode (SOSS) mode, we report the detection of substantial helium absorption during the pre-transit phase of WASP-107 b (17σ), as well as in the transit and post-transit phases. This unique continuous helium absorption begins approximately 1.5 h before the planet’s ingress and reveals the presence of an extended thermosphere. The observations show a maximum transit depth of 2.395 ± 0.01% near the helium triplet (36σ; at the NIRISS-SOSS resolution of ~700). Our ellipsoidal model of the planetary thermosphere matches the measured light curve well, suggesting an outflow extending to tens of planetary radii. Furthermore, we confidently detect water absorption (log10H2O = −2.5 ± 0.6), superimposed with a short-wavelength slope that we attribute to a prominent signature from unocculted stellar spots (5.2σ), rather than a small-particle haze slope. We place an upper limit on the abundance of K (log10K < −4.86, or K/H < 75× stellar) at 2σ, which is consistent with the O/H supersolar metallicity estimate. Together with the supersolar water abundance and the evidence for vigorous atmospheric escape, these findings suggest that WASP-107 b has undergone inward migration in its recent past, probably accompanied by strong tidal heating that continues to sustain its inflated atmosphere and mass loss. This investigation underscores the transformative potential of JWST for investigating planetary evolution.

Abstract:

Context. Characterizations of giant exoplanets such as β Pictoris b (hereafter β Pic b) are now routinely performed with multiple spectrographs and imagers exploring different spectral bandwidths and resolutions, allowing for atmospheric retrieval of spectra with or without the conservation of the planet spectral continuum. The accounting of data multimodality in the analysis could provide a more comprehensive determination of the planets physical and chemical properties and inform on their formation history. Aims. We present the first VLTI observations at R λ ∼4000 of β Pic b obtained for an exoplanet with GRAVITY at such a high resolution. We upgraded the forward modelling code ForMoSA to account for the data multimodality, including low-, medium-, and high-resolution spectroscopy based on both a direct model-data comparison and an analysis of cross-correlation signals. We used the ForMoSA code to refine the constraints on the atmospheric properties of the exoplanet and evaluated the sensitivity of the retrieved values to the input dataset. Methods. We obtained four high-signal-to-noise (S/N ∼ 20) spectra of β Pic b in the K band with GRAVITY at R λ ∼4000 conserving both the pseudo-continuum and the pattern of molecular absorptions. We used ForMoSA with four grids of self-consistent forward models (Exo-REM, ATMO, BT-Settl, and Sonora) to explore different T e ff , log(g), metallicity, C/O, and 12 CO/ 13 CO ratio values. We then combined the GRAVITY spectra with published 1–5 µm photometry (NaCo, VisAO, NICI, and SPHERE), low-to-mediumresolution ( R λ ≤ 700 broadband, 0.9–7 µm) spectra, and echelle spectra covering narrower bandwidths ( R λ ∼ 100 000, 2.1–5.2 µm). Results. Sonora and Exo-REM are statistically preferred among all four models, regardless of the dataset used. Exo-REM predicts T eff  = 1607.45 −6.20 +4.85 K and log(g) = 4.46 −0.04 +0.02 dex when using only the GRAVITY epochs, whereas we have T eff  = 1502.74 −2.14 +2.32 K log(g) = 4.00 ± 0.01 dex when incorporating all available datasets. The inclusion of archival data significantly affects all retrieved posteriors. When using all datasets, C/O mostly remains solar (0.552 −0.002 +0.003 ), while [M/H] reaches super-solar values (0.50 ± 0.01). We report the first tentative constraint on the isotopic ratio log( 12 CO/ 13 CO) = 1.12 −0.08 +0.11 in β Pic b’s atmosphere; however, we note that this detection remains inconclusive due to telluric residuals affecting both the GRAVITY and SINFONI data. Additionally, we estimated the bolometric luminosity as log(L/L ⊙ ) = −4.01 −0.05 +0.04 dex. Using a system age of 23 ± 3 Myr, along with this bolometric luminosity and the constraints on the dynamical mass of β Pic b, we were able to constrain the maximum of heavy element content of the planet to be on the order of 5% (20–80 M Earth ). Conclusions. The joint access to the pseudo-continuum and molecular lines in the K band provided by GRAVITY have a significant impact on the retrieved metallicity, possibly owing to the collision-induced absorption driving the continuum shape of the K band. The echelle spectra do not dominate the final fit with respect to lower resolution data covering a broader portion of the spectral energy distribution and the latter keeps encapsulating more robust information on T eff . Future multimodal frameworks should include a weighting scheme to account for the bandwidth and central wavelength of the observations.

Abstract:

Although TRAPPIST-1’s temperate planets have the highest transmission signals of any known system, flares contaminate 50%–70% of transits at the 1000 ppm level, far above 100 ppm secondary atmospheric signals. Efforts to mitigate flare contamination and assess impacts on radiation environments are each hampered by a lack of empirical spectral analysis and physics-based modeling. We present spectrotemporal analysis and radiative-hydrodynamic modeling of 5.5 hr of NIRISS and NIRSpec observations of six TRAPPIST-1 flares of 2.2–8.7 × 1030 erg. The flare lines and continua are characterized using grid searches of RADYN beam-heating models spanning 104 times in electron beam parameters. Best-fit models indicate these flares result from moderate-intensity beams with emergent electron fluxes of Fe = 1012 erg s−1 cm−2 and energies ≤37 keV, although all models overpredict the Paschen jump. These models predict X-ray and extreme UV (XUV), far-UV, and near-UV counterparts to the IR peak fluxes of 8.9–28.9 × 1027, 4.3–13.9 × 1026, and 3.4–11.4 × 1027 erg s−1, respectively. Scaling the flare rate into the XUV suggests flaring contributes 1.35 −0.15+2.0× quiescence yr−1. We bin integrations of similar flare effective temperature to construct fiducial flare spectra from 2000 to 4500 K, in order to develop separate empirical and RADYN-based mitigation pipelines. Both pipelines are applied to all 5.5 hr of R = 10 data, resulting in maximum residuals from 1 to 2.8 μm of 100–140 ppm and typical residuals of 54 ± 14 and 65 ± 17 ppm for the empirical and RADYN-based pipelines, respectively. Injection testing supports a 3σ detection capability for CO2 atmospheres with features of 150–250 ppm, with weak evidence (Bayes factor ≈ 3) still obtained at 130 ppm. Our results motivate multiwavelength observations to improve model fidelity and test high-energy predictions.

Abstract:

LTT-9779,b is an inhabitant of the hot-Neptune desert and one of only a few planets with a measured high albedo. Characterising the atmosphere of this world is the key to understanding the processes that dominate in reducing the number of short-period intermediate-mass planets that create the hot-Neptune desert. We aim to characterise the reflected light of LTT-9779,b at high spectral resolution to break the degeneracy between clouds and atmospheric metallicity. This is key to interpreting its mass-loss history, which might illuminate how it kept its place in the desert. We used the high-resolution cross-correlation spectroscopy technique on four half-nights of ESPRESSO observations in 4-UT mode (16.4 m effective mirror) to constrain the reflected-light spectrum of łttb. We did not detect the reflected-light spectrum of łttb, although these data had the expected sensitivity at the level 100 ppm. Injection tests of the post-eclipse data indicated that TiO should have been detected for a range of different equilibrium chemistry models. Therefore, this non-detection suggests TiO depletion in the western hemisphere, but this conclusion is sensitive to temperature, which affects the chemistry in the upper atmosphere and the reliability of the line list. Additionally, we were able to constrain the top of the western cloud deck to P_ top, western bar and the top of the eastern cloud deck to P_ top, eastern bar, which is consistent with the predicted altitude of MgSiO_3 and Mg_2SiO_4 clouds from JWST NIRISS/SOSS. While we did not detect the reflected-light spectrum of łttb, we verified that this technique can be used in practice to characterise the reflected light of exoplanets at high spectral resolution when their spectra contain a sufficient number of deep spectral lines. Therefore, this technique may become an important cornerstone of exoplanet characterisation with the ELT and beyond.