The M3V dwarf star L 98-59 hosts three small (R < 1.6 Rꚛ) planets. The host star is bright (K = 7.1) and nearby (10.6 pc), making the system a prime target for follow-up characterization with the ...Hubble Space Telescope (HST) and the upcoming James Webb Space Telescope (JWST). Herein, we use simulated transmission spectroscopy to evaluate the detectability of spectral features with HST and JWST assuming diverse atmospheric scenarios (e.g., atmospheres dominated by H2, H2O, CO2, or O2). We find that H2O and CH4 present in a low mean molecular weight atmosphere could be detected with HST in one transit for the two outermost planets, while H2O in a clear steam atmosphere could be detected in six transits or fewer with HST for all three planets. We predict that observations using JWST/NIRISS would be capable of detecting a clear steam atmosphere in one transit for each planet and H2O absorption in a hazy steam atmosphere in two transits or less. In a clear, desiccated atmosphere, O2 absorption may be detectable for all three planets with NIRISS. If the L 98-59 planets possess a clear, Venus-like atmosphere, NIRSpec could detect CO2 within 26 transits for each planet, but the presence of H2SO4 clouds would significantly suppress CO2 absorption. The L 98-59 system is an excellent laboratory for comparative planetary studies of transiting multiplanet systems, and observations of the system via HST and JWST would present a unique opportunity to test the accuracy of the models presented in this study.
Abstract
With the increasing number of planets discovered by the Transit Exoplanet Survey Satellite, the atmospheric characterization of small exoplanets is accelerating. L98-59 is an M-dwarf hosting ...a multiplanet system, and so far, four small planets have been confirmed. The innermost planet b is ∼15% smaller and ∼60% lighter than Earth, and should thus have a predominantly rocky composition. The Hubble Space Telescope observed five primary transits of L98-59 b in 1.1–1.7
μ
m, and here we report the data analysis and the resulting transmission spectrum of the planet. We measure the transit depths for each of the five transits and, by combination, we obtain a transmission spectrum with an overall precision of ∼20 ppm in for each of the 18 spectrophotometric channels. With this level of precision, the transmission spectrum does not show significant modulation, and is thus consistent with a planet without any atmosphere or a planet having an atmosphere and high-altitude clouds or haze. The scenarios involving an aerosol-free, H
2
-dominated atmosphere with H
2
O or CH
4
are inconsistent with the data. The transmission spectrum also disfavors, but does not rule out, an H
2
O-dominated atmosphere without clouds. A spectral retrieval process suggests that an H
2
-dominated atmosphere with HCN and clouds or haze may be the preferred solution, but this indication is nonconclusive. Future James Webb Space Telescope observations may find out the nature of the planet among the remaining viable scenarios.
Abstract
The era of atmospheric characterization of terrestrial exoplanets is just around the corner. Modeling prior to observations is crucial in order to predict the observational challenges and to ...prepare for the data interpretation. This paper presents the report of the TRAPPIST Habitable Atmosphere Intercomparison workshop (2020 September 14–16). A review of the climate models and parameterizations of the atmospheric processes on terrestrial exoplanets, model advancements, and limitations, as well as direction for future model development, was discussed. We hope that this report will be used as a roadmap for future numerical simulations of exoplanet atmospheres and maintaining strong connections to the astronomical community.
We used one-dimensional photochemical and radiative transfer models to study the potential of organic sulfur compounds (CS(2), OCS, CH(3)SH, CH(3)SCH(3), and CH(3)S(2)CH(3)) to act as remotely ...detectable biosignatures in anoxic exoplanetary atmospheres. Concentrations of organic sulfur gases were predicted for various biogenic sulfur fluxes into anoxic atmospheres and were found to increase with decreasing UV fluxes. Dimethyl sulfide (CH(3)SCH(3), or DMS) and dimethyl disulfide (CH(3)S(2)CH(3), or DMDS) concentrations could increase to remotely detectable levels, but only in cases of extremely low UV fluxes, which may occur in the habitable zone of an inactive M dwarf. The most detectable feature of organic sulfur gases is an indirect one that results from an increase in ethane (C(2)H(6)) over that which would be predicted based on the planet's methane (CH(4)) concentration. Thus, a characterization mission could detect these organic sulfur gases-and therefore the life that produces them-if it could sufficiently quantify the ethane and methane in the exoplanet's atmosphere.
Abstract
To identify promising exoplanets for atmospheric characterization and to make the best use of observational data, a thorough understanding of their atmospheres is needed. Three-dimensional ...general circulation models (GCMs) are one of the most comprehensive tools available for this task and will be used to interpret observations of temperate rocky exoplanets. Due to parameterization choices made in GCMs, they can produce different results, even for the same planet. Employing four widely used exoplanetary GCMs—ExoCAM, LMD-G, ROCKE-3D, and the UM—we continue the TRAPPIST-1 Habitable Atmosphere Intercomparison by modeling aquaplanet climates of TRAPPIST-1e with a moist atmosphere dominated by either nitrogen or carbon dioxide. Although the GCMs disagree on the details of the simulated regimes, they all predict a temperate climate with neither of the two cases pushed out of the habitable state. Nevertheless, the intermodel spread in the global mean surface temperature is nonnegligible: 14 K and 24 K in the nitrogen- and carbon dioxide-dominated case, respectively. We find substantial intermodel differences in moist variables, with the smallest amount of clouds in LMD-Generic and the largest in ROCKE-3D. ExoCAM predicts the warmest climate for both cases and thus has the highest water vapor content and the largest amount and variability of cloud condensate. The UM tends to produce colder conditions, especially in the nitrogen-dominated case due to a strong negative cloud radiative effect on the day side of TRAPPIST-1e. Our study highlights various biases of GCMs and emphasizes the importance of not relying solely on one model to understand exoplanet climates.
Saturn's Moon Titan receives volatiles into the top of its atmosphere—including atomic oxygen—sourced from cryovolcanoes on Enceladus. Similar types of atmosphere exchange from one body to another, ...such as O2 and O3 sourced from TRAPPIST‐1 d, could be introduced into the upper atmosphere of TRAPPIST‐1 e and might be interpreted as biosignatures. We simulate this potential false‐positive for life on TRAPPIST‐1 e, by applying an external influx of water and oxygen into the top of the atmosphere using a coupled 1‐D photochemical‐climate model (Atmos), to predict atmospheric composition. In addition, synthetic spectral observations are produced using the Planetary Spectrum Generator for the James Webb Space Telescope, Origins Space Telescope, Habitable Exoplanet Observatory and Large Ultra‐violet/Optical/Infrared Surveyor to test the detectability of abiotic‐generated O2 and O3 in the presence of abiotic and biotic surface fluxes of CH4. We determine that the incoming flux of material needed to trigger detection of abiotic O2/O3 by any of these observatories is more than two orders of magnitude (1 × 1012 molecules/cm2/s) above what is physically plausible.
Plain Language Summary
In the Saturnian system, Enceladus' icy volcanoes spew liquid and gassy material—including water—into outer space, and some of that material ends up in Titan's atmosphere. This is a prominent phenomenon within our solar system that has been observed in great detail and shows proof of foreign matter exchange being possible between worlds. The simultaneous presence of detectable amounts of methane and oxygen or ozone in an atmosphere is considered strong evidence for the presence of life because in a methane rich atmosphere, oxygen or ozone will be destroyed to the point of being undetectable unless life is present to replenish it and vice versa. We explore here whether a similar type of matter exchange, with methane present and occurring in the TRAPPIST‐1 system, could increase oxygen and water abundances to the point of creating atmospheric signatures that may be mistaken for signs of life on another planet. To probe this question we use computer models of atmospheres and current and next‐generation space telescopes. We conclude that when looking for the simultaneous presence of methane and oxygen/ozone, this matter exchange will not be mistaken for signs of life.
Key Points
The transfer of volatiles through atmospheric loss processes as seen in the Titan‐Enceladus system may occur amongst close‐in exoplanets
We simulate the atmosphere and spectra of TRAPPIST‐1 e if it was receiving an external flux of water and oxygen
Our results are important for upcoming and future observatories, which must prepare for false‐positive biosignatures while searching for life
We present molecular orbital/density functional theory (MO/DFT) calculations that predict a greater isotopic fractionation in redox reactions than in reactions involving ligand exchange. The ...predicted fractionation factors, reported as 1000·ln(
56–54
α), associated with equilibrium between Fe–organic and Fe–H
2O species were <1.6‰ in vacuo and <1.2‰ in solution when the oxidation state of the system was held constant. These fractionation factors were significantly smaller than those predicted for equilibrium between different oxidation states of Fe, for which 1000·ln(
56–54
α) was >2.7‰ in vacuo and >2.2‰ in solution when the bound ligands were unchanged. The predicted
56Fe/
54Fe ratio was greater in complexes containing Fe
3+ and in complexes with shorter Fe–O bond lengths; both of these trends follow previous theoretical results. Our predictions also agree with previous experimental measurements that suggest that the largest biological fractionations will be associated with processes that change the oxidation state of Fe, and that identification of biologically controlled Fe isotope fractionation may be difficult when abiotic redox fractionations are present in the system. The models studied here also have important implications for future theoretical isotope calculations, because we have discovered the necessity of using vibrational frequencies instead of reduced masses when predicting reduced partition functions in aqueous-phase species.
With the commissioning of powerful, new-generation telescopes such as the James Webb Space Telescope (JWST) and the ground-based Extremely Large Telescopes, the first characterization of a high ...molecular weight atmosphere around a temperate rocky exoplanet is imminent. Atmospheric simulations and synthetic observables of target exoplanets are essential to prepare and interpret these observations. Here we report the results of the first part of the TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project, which compares 3D numerical simulations performed with four state-of-the-art global climate models (ExoCAM, LMD-Generic, ROCKE-3D, Unified Model) for the potentially habitable target TRAPPIST-1e. In this first part, we present the results of dry atmospheric simulations. These simulations serve as a benchmark to test how radiative transfer, subgrid-scale mixing (dry turbulence and convection), and large-scale dynamics impact the climate of TRAPPIST-1e and consequently the transit spectroscopy signature as seen by JWST. To first order, the four models give results in good agreement. The intermodel spread in the global mean surface temperature amounts to 7 K (6 K) for the N2-dominated (CO2-dominated) atmosphere. The radiative fluxes are also remarkably similar (intermodel variations less than 5%), from the surface (1 bar) up to atmospheric pressures ∼5 mbar. Moderate differences between the models appear in the atmospheric circulation pattern (winds) and the (stratospheric) thermal structure. These differences arise between the models from (1) large-scale dynamics, because TRAPPIST-1e lies at the tipping point between two different circulation regimes (fast and Rhines rotators) in which the models can be alternatively trapped, and (2) parameterizations used in the upper atmosphere such as numerical damping.
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