Almost all planetary atmospheres are affected by disequilibrium chemical processes. In this paper, we introduce our recently developed chemical kinetic model (ChemKM). We show that the results of our ...HD 189733b model are in good agreement with previously published results, except at the bar regime, where molecular diffusion and photochemistry are the dominant processes. We thus recommend careful consideration of these processes when abundances at the top of the atmosphere are desired. We also propose a new metric for a quantitative measure of quenching levels. By applying this metric, we find that quenching pressure decreases with the effective temperature of planets, but it also varies significantly with other atmospheric parameters such as Fe/H, log(g), and C/O. In addition, we find that the "methane valley," a region between 800 and 1500 K where above a certain C/O threshold value a greater chance of CH4 detection is expected, still exists after including the vertical mixing. The first robust CH4 detection on an irradiated planet (HD 102195b) places this object within this region, supporting our prediction. We also investigate the detectability of disequilibrium spectral fingerprints by the James Webb Space Telescope and suggest focusing on the targets with Teff between 1000 and 1800 K, orbiting around M dwarfs, and having low surface gravity but high metallicity and a C/O ratio value around unity. Finally, constructing Spitzer color maps suggests that the main two color populations are largely insensitive to the vertical mixing. Therefore, any deviation of observational points from these populations is likely due to the presence of clouds and not disequilibrium processes. However, some cold planets (Teff < 900 K) with very low C/O ratios (<0.25) show significant deviations, making these planets interesting cases for further investigation.
Observations suggest an abundance of water and a paucity of methane in the majority of observed exoplanetary atmospheres. We isolate the effect of atmospheric processes to investigate possible ...causes. Previously, we studied the effect of effective temperature, surface gravity, metallicity, carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical equilibrium and disequilibrium chemistry. However, under these assumptions, methane remains a persisting spectral feature in the transmission spectra of exoplanets over a certain parameter space, the Methane Valley. In this work, we investigate the role of clouds on this domain and we find that clouds change the spectral appearance of methane in two direct ways: (1) by heating up the photosphere of colder planets and (2) by obscuring molecular features. The presence of clouds also affects methane features indirectly: (1) cloud heating results in more evaporation of condensates and hence releases additional oxygen, causing water-dominated spectra of colder carbon-poor exoplanets, and (2) HCN/CO production results in a suppression of depleted methane features by these molecules. The presence of HCN/CO and a lack of methane could be an indication of cloud formation on hot exoplanets. Cloud heating can also deplete ammonia. Therefore, a simultaneous depletion of methane and ammonia is not unique to photochemical processes. We propose that the best targets for methane detection are likely to be massive but smaller planets with a temperature around 1450 K orbiting colder stars. We also construct Spitzer synthetic color maps and find that clouds can explain some of the high-contrast observations by IRAC's channel 1 and 2.
A carbon-to-oxygen ratio (C/O) of around unity is believed to act as a natural separator of water- and methane-dominated spectra when characterizing exoplanet atmospheres. In this paper, we quantify ...the C/O ratios at which this separation occurs by calculating a large self-consistent grid of cloud-free atmospheric models in chemical equilibrium using the latest version of petitCODE. Our study covers a broad range of parameter space: 400 K < Teff < 2600 K, 2.0 < log(g) < 5.0, −1.0 < Fe/H < 2.0, 0.25 < C/O < 1.25, and stellar types from M to F. We make the synthetic transmission and emission spectra, as well as the temperature structures, publicly available. We find that the transition C/O ratio depends on many parameters, such as effective temperature, surface gravity, metallicity, and spectral type of the host star, and could have values less than, equal to, or higher than unity. By mapping all of the transition C/O ratios, we propose a "four-class" classification scheme for irradiated planets in this temperature range. We find a parameter space where methane always remains the cause of dominant spectral features. Detection of CH4 in this region, or the lack of it, provides a diagnostic tool to identify the prevalence of cloud formation and nonequilibrium chemistry. As another diagnostic tool, we construct synthetic Spitzer Infrared Array Camera color diagrams showing two distinguishable populations of planets. Since most of the exoplanet atmospheres appear cloudy when studied in transmission, we regard this study as a starting point of how such a C/O-sensitive observation-based classification scheme should be constructed. This preparatory work will have to be refined by future cloudy and nonequilibrium modeling to further investigate the existence and exact location of the classes, as well as the color-diagram analysis.
Abstract
Constraining planet formation based on the atmospheric composition of exoplanets is a fundamental goal of the exoplanet community. Existing studies commonly try to constrain atmospheric ...abundances, or to analyze what abundance patterns a given description of planet formation predicts. However, there is also a pressing need to develop methodologies that investigate how to transform atmospheric compositions into planetary formation inferences. In this study we summarize the complexities and uncertainties of state-of-the-art planet formation models and how they influence planetary atmospheric compositions. We introduce a methodology that explores the effect of different formation model assumptions when interpreting atmospheric compositions. We apply this framework to the directly imaged planet HR 8799e. Based on its atmospheric composition, this planet may have migrated significantly during its formation. We show that including the chemical evolution of the protoplanetary disk leads to a reduced need for migration. Moreover, we find that pebble accretion can reproduce the planet’s composition, but some of our tested setups lead to too low atmospheric metallicities, even when considering that evaporating pebbles may enrich the disk gas. We conclude that the definitive inversion from atmospheric abundances to planet formation for a given planet may be challenging, but a qualitative understanding of the effects of different formation models is possible, opening up pathways for new investigations.
Given the forthcoming launch of the James Webb Space Telescope (JWST), which will allow observing exoplanet atmospheres with unprecedented signal-to-noise ratio, spectral coverage, and spatial ...resolution, the uncertainties in the atmosphere modeling used to interpret the data need to be assessed. As the first step, we compare three independent 1D radiative-convective models: ATMO, Exo-REM, and petitCODE. We identify differences in physical and chemical processes that are taken into account thanks to a benchmark protocol we have developed. We study the impact of these differences on the analysis of observable spectra. We show the importance of selecting carefully relevant molecular linelists to compute the atmospheric opacity. Indeed, differences between spectra calculated with Hitran and ExoMol exceed the expected uncertainties of future JWST observations. We also show the limits of the precision of the models due to uncertainties on alkali and molecule lineshape, which induce spectral effects that are also larger than the expected JWST uncertainties. We compare two chemical models, Exo-REM and Venot Chemical Code, which do not lead to significant differences in the emission or transmission spectra. We discuss the observational consequences of using equilibrium or out-of-equilibrium chemistry and the major impact of phosphine, detectable with the JWST. Each of the models has benefited from the benchmarking activity and has been updated. The protocol developed in this paper and the online results can constitute a test case for other models.
Abstract
Ground-based high-resolution spectra provide a powerful tool for characterizing exoplanet atmospheres. However, they are greatly hampered by the dominating telluric and stellar lines, which ...need to be removed prior to any analysis. Such removal techniques (“preparing pipelines”) deform the spectrum; hence, a key point is to account for this process in the forward models used in retrievals. We develop a formal derivation on how to prepare froward models for retrievals, in the case where the telluric and instrumental deformations can be represented as a matrix multiplied element-wise with the data. We also introduce the notion of a “bias pipeline metric,” which can be used to compare the bias potential of preparing pipelines. We use the resulting framework to retrieve simulated observations of 1D and 3D exoplanet atmospheres and to reanalyze high-resolution (
R
≈
80,400
) near-infrared (0.96–1.71
μ
m) CARMENES transit data of HD 189733 b. We compare these results with those obtained from a cross-correlation function analysis. With our fiducial retrieval, we find a blueshift of the absorption features of
−
5.51
−
0.53
+
0.66
km s
−1
. In addition, we retrieve a H
2
O log
10
(VMR) of
−
2.39
−
0.16
+
0.12
and a temperature of
660
−
11
+
6
K. We are also able to put upper limits for the abundances of CH
4
, CO, H
2
S, HCN, and NH
3
, consistent with a subsolar-metallicity atmosphere enriched in H
2
O. We retrieve a broadened line shape, consistent with rotation- and wind-induced line broadening. Finally, we find a lower limit for the pressure of an opaque cloud consistent with a clear atmosphere, and we find no evidence for hazes.
ABSTRACT
We explore the chemistry and observability of nitrogen-dominated atmospheres for ultra-short-period super-Earths. We base the assumption that super-Earths could have nitrogen-filled ...atmospheres on observations of 55 Cancri e that favour a scenario with a high-mean-molecular-weight atmosphere. We take Titan’s elemental budget as our starting point and using chemical kinetics compute a large range of possible compositions for a hot super-Earth. We use analytical temperature profiles and explore a parameter space spanning orders of magnitude in C/O and N/O ratios, while always keeping nitrogen the dominant component. We generate synthetic transmission and emission spectra and assess their potential observability with the future James Webb Space Telescope (JWST) and ARIEL. Our results suggest that HCN is a strong indicator of a high C/O ratio, which is similar to what is found for H-dominated atmospheres. We find that these worlds are likely to possess C/O > 1.0, and that HCN, CN, and CO should be the primary molecules to be searched for in thermal emission. For lower temperatures (T < 1500 K), we additionally find NH3 in high N/O ratio cases, and C2H4 and CH4 in low N/O ratio cases to be strong absorbers. Depletion of hydrogen in such atmospheres would make CN, CO, and NO exceptionally prominent molecules to look for in the 0.6–5.0 $\rm{\mu m}$ range. Our models show that the upcoming JWST and ARIEL missions will be able to distinguish atmospheric compositions of ultra-short-period super-Earths with unprecedented confidence.
Here we present a publicly available database of opacities for molecules of astrophysical interest named ExoMolOP that has been compiled for over 80 species, and is based on the latest line list data ...from the ExoMol, HITEMP, and MoLLIST databases. These data are generally suitable for characterising high-temperature exoplanet or cool stellar and substellar atmospheres, and have been computed at a variety of pressures and temperatures, with a few molecules included at room temperature only from the HITRAN database. The data are formatted in different ways for four different exoplanet atmosphere retrieval codes; ARCiS, TauREx, NEMESIS, and petitRADTRANS, and include both cross sections (at R = λ /Δ λ = 15000) and k -tables (at R = λ /Δ λ = 1000) for the 0.3–50 μ m wavelength region. Opacity files can be downloaded and used directly for these codes. Atomic data for alkali metals Na and K are also included, using data from the NIST database and the latest line shapes for the resonance lines. Broadening parameters have been taken from the literature where available, or have been estimated from the parameters of a known molecule with similar molecular properties where no broadening data are available.
ABSTRACT
The atmospheres of synchronously rotating exoplanets are intrinsically 3D, and fast vertical and horizontal winds are expected to mix the atmosphere, driving the chemical composition out of ...equilibrium. Due to the longer computation times associated with multidimensional forward models, horizontal mixing has only been investigated for a few case studies. In this paper, we aim to generalize the impact of horizontal and vertical mixing on the chemistry of exoplanet atmospheres over a large parameter space. We do this by applying a sequence of post-processed forward models for a large grid of synchronously rotating gaseous exoplanets, where we vary the effective temperature (between 400 and 2600 K), surface gravity, and rotation rate. We find that there is a dichotomy in the horizontal homogeneity of the chemical abundances. Planets with effective temperatures below 1400 K tend to have horizontally homogeneous, vertically quenched chemical compositions, while planets hotter than 1400 K exhibit large compositional day-night differences for molecules such as CH4. Furthermore, we find that the planet’s rotation rate impacts the planetary climate, and thus also the molecular abundances and transmission spectrum. By employing a hierarchical modelling approach, we assess the relative importance of disequilibrium chemistry on the exoplanet transmission spectrum, and conclude that the temperature has the most profound impact. Temperature differences are also the main cause of limb asymmetries, which we estimate could be observable with the James Webb Space Telescope. This work highlights the value of applying a consistent modelling setup to a broad parameter space in exploratory theoretical research.
Recent observations of the potentially habitable planets TRAPPIST-1 e, f, and g suggest that they possess large water mass fractions of possibly several tens of wt% of water, even though the host ...star’s activity should drive rapid atmospheric escape. These processes can photolyze water, generating free oxygen and possibly desiccating the planet. After the planets formed, their mantles were likely completely molten with volatiles dissolving and exsolving from the melt. In order to understand these planets and prepare for future observations, the magma ocean phase of these worlds must be understood. To simulate these planets, we have combined existing models of stellar evolution, atmospheric escape, tidal heating, radiogenic heating, magma ocean cooling, planetary radiation, and water-oxygen-iron geochemistry. We present Magm Oc, a versatile magma ocean evolution model, validated against the rocky Super-Earth GJ 1132b and early Earth. We simulate the coupled magma ocean-atmospheric evolution of TRAPPIST-1 e, f, and g for a range of tidal and radiogenic heating rates, as well as initial water contents between 1 and 100 Earth oceans. We also reanalyze the structures of these planets and find they have water mass fractions of 0–0.23, 0.01–0.21, and 0.11–0.24 for planets e, f, and g, respectively. Our model does not make a strong prediction about the water and oxygen content of the atmosphere of TRAPPIST-1 e at the time of mantle solidification. In contrast, the model predicts that TRAPPIST-1 f and g would have a thick steam atmosphere with a small amount of oxygen at that stage. For all planets that we investigated, we find that only 3 ́5% of the initial water will be locked in the mantle after the magma ocean solidified.
PDF
