[en] The James Webb Space Telescope ( JWST ) is nearing its launch date of 2018, and is expected to revolutionize our knowledge of exoplanet atmospheres. In order to specifically identify which observing modes will be most useful for characterizing a diverse range of exoplanetary atmospheres, we use an information content (IC) based approach commonly used in the studies of solar system atmospheres. We develop a system based upon these IC methods to trace the instrumental and atmospheric model phase space in order to identify which observing modes are best suited for particular classes of planets, focusing on transmission spectra. Specifically, the atmospheric parameter space we cover is T  = 600–1800 K, C/O = 0.55–1, [M/H] = 1–100 × Solar for an R  = 1.39 R_J, M  = 0.59 M_J planet orbiting a WASP-62-like star. We also explore the influence of a simplified opaque gray cloud on the IC. We find that obtaining broader wavelength coverage over multiple modes is preferred over higher precision in a single mode given the same amount of observing time. Regardless of the planet temperature and composition, the best modes for constraining terminator temperatures, C/O ratios, and metallicity are NIRISS SOSS+NIRSpec G395. If the target’s host star is dim enough such that the NIRSpec prism is applicable, then it can be used instead of NIRISS SOSS+NIRSpec G395. Lastly, observations that use more than two modes should be carefully analyzed because sometimes the addition of a third mode results in no gain of information. In these cases, higher precision in the original two modes is favorable.

[en] Future space-based direct imaging missions will perform low-resolution (R < 100) optical (0.3–1 μm) spectroscopy of planets, thus enabling reflected spectroscopy of cool giants. Reflected light spectroscopy is encoded with rich information about the scattering and absorbing properties of planet atmospheres. Given the diversity of clouds and hazes expected in exoplanets, it is imperative that we solidify the methodology to accurately and precisely retrieve these scattering and absorbing properties that are agnostic to cloud species. In particular, we focus on determining how different cloud parameterizations affect resultant inferences of both cloud and atmospheric composition. We simulate mock observations of the reflected spectra from three top-priority direct imaging cool giant targets with different effective temperatures, ranging from 135 to 533 K. We perform retrievals of cloud structure and molecular abundances on these three planets using four different parameterizations, each with an increasing level of cloud complexity. We find that the retrieved atmospheric and scattering properties depend strongly on the choice of cloud parameterization. For example, parameterizations that are too simplistic tend to overestimate the abundances. Overall, we are unable to retrieve precise/accurate gravity beyond ±50%. Lastly, we find that even reflected light spectroscopy with a low signal-to-noise ratio of 5 and low R = 40 gives cursory zeroth-order insights into the position of the cloud deck relative to the molecular and Rayleigh optical depth level.

[en] Stellar, substellar, and planetary atmosphere models are all highly sensitive to the input opacities. Generational differences between various state-of-the-art stellar/planetary models arise primarily because of incomplete and outdated atomic/molecular line lists. Here we present a database of precomputed absorption cross sections for all isotopologues of key atmospheric molecules relevant to late-type stellar, brown dwarf, and planetary atmospheres: MgH, AlH, CaH, TiH, CrH, FeH, SiO, TiO, VO, and H₂O. The pressure and temperature ranges of the computed opacities are 10⁻⁶–3000 bar and 75–4000 K, and their spectral ranges are 0.25–330 μm for many cases where possible. For cases with no pressure-broadening data, we use collision theory to bridge the gap. We also probe the effect of absorption cross sections calculated from different line lists in the context of ultrahot Jupiter and M-dwarf atmospheres. Using 1D self-consistent radiative–convective thermochemical equilibrium models, we report significant variations in the theoretical spectra and thermal profiles of substellar atmospheres. With a 2000 K representative ultrahot Jupiter, we report variations of up to 320 and 80 ppm in transmission and thermal emission spectra, respectively. For a 3000 K M-dwarf, we find differences of up to 125% in the spectra. We find that the most significant differences arise as a result of the choice of TiO line lists, primarily below 1 μm. In summary, (1) we present a database of precomputed molecular absorption cross sections, and (2) we quantify biases that arise when characterizing substellar/exoplanet atmospheres as a result of differences in the line lists, therefore highlighting the importance of correct and complete opacities for eventual applications to high-precision spectroscopy and photometry.

[en] Lithium is an important element for the understanding of ultracool dwarfs because it is lost to fusion at masses above ∼68 M _J. Hence, the presence of atomic Li has served as an indicator of the nearby H-burning boundary at about 75 M _J between brown dwarfs and very low mass stars. Historically, the “lithium test,” a search for the presence of the Li line at 670.8 nm, has been a marker if an object has a substellar mass. While the Li test could, in principle, be used to distinguish masses of later-type L–T dwarfs, Li is predominantly no longer found as an atomic gas but rather a molecular species such as LiH, LiF, LiOH, and LiCl in cooler atmospheres. The L- and T-type dwarfs are quite faint at 670 nm and thus challenging targets for high-resolution spectroscopy. But only recently have experimental molecular line lists become available for the molecular Li species, allowing molecular Li mass discrimination. Here we generated the latest opacity of these Li-bearing molecules and performed a thermochemical equilibrium atmospheric composition calculation of their abundances. Finally, we computed thermal emission spectra for a series of radiative–convective equilibrium models of cloudy and cloudless brown dwarf atmospheres (with T _eff = 500–2400 K and $\mathrm{l}\mathrm{o}\mathrm{g}g=4.0\text{--}5.0$) to understand where the presence of atmospheric lithium-bearing species is most easily detected as a function of brown dwarf mass and age. After atomic Li, the best spectral signatures were found to be LiF at 10.5–12.5 μm and LiCl at 14.5–18.5 μm. Also, LiH shows a narrow feature at ∼9.38 μm.

[en] MassSpec , a method for determining the mass of a transiting exoplanet from its transmission spectrum alone, was proposed by de Wit and Seager. The premise of this method relies on the planet’s surface gravity being extracted from the transmission spectrum via its effect on the atmospheric scale height, which in turn determines the strength of absorption features. Here, we further explore the applicability of MassSpec to low-mass exoplanets—specifically those in the super-Earth size range for which radial velocity determinations of the planetary mass can be extremely challenging and resource intensive. Determining the masses of these planets is of the utmost importance because their nature is otherwise highly unconstrained. Without knowledge of the mass, these planets could be rocky, icy, or gas-dominated. To investigate the effects of planetary mass on transmission spectra, we present simulated observations of super-Earths with atmospheres made up of mixtures of H_2O and H_2, both with and without clouds. We model their transmission spectra and run simulations of each planet as it would be observed with James Webb Space Telescope using the NIRISS, NIRSpec, and MIRI instruments. We find that significant degeneracies exist between transmission spectra of planets with different masses and compositions, making it impossible to unambiguously determine the planet’s mass in many cases.

[en] Terrestrial planets in the habitable zones (HZs) of low-mass stars and cool dwarfs have received significant scrutiny recently. Transit spectroscopy of such planets with the James Webb Space Telescope (JWST) represents our best shot at obtaining the spectrum of a habitable planet within the next decade. As these planets are likely tidally locked, improved 3D numerical simulations of such planetary atmospheres are needed to guide target selection. Here we use a 3D climate system model, updated with new water-vapor absorption coefficients derived from the HITRAN 2012 database, to study ocean-covered planets at the inner edge of the HZ around late M to mid-K stars ($2600\mathrm{K}⩽<!--\; ???\; -->T_{\mathrm{e}\mathrm{f}\mathrm{f}}⩽<!--\; ???\; -->4500\mathrm{K}$). Our results indicate that these updated water-vapor coefficients result in significant warming compared to previous studies, so the inner HZ around M dwarfs is not as close as suggested by earlier work. Assuming synchronously rotating Earth-sized and Earth-mass planets with background 1 bar ${\mathrm{N}}_2$ atmospheres, we find that planets at the inner HZ of stars with $T_{\mathrm{e}\mathrm{f}\mathrm{f}}><!--\; >\; -->3000\mathrm{K}$ undergo the classical “moist greenhouse” (${\mathrm{H}}_2\mathrm{O}$ mixing ratio $><!--\; >\; -->{10}^{-<!--\; ???\; -->3}$ in the stratosphere) at significantly lower surface temperature (∼280 K) in our 3D model compared with 1D climate models (∼340 K). This implies that some planets around low-mass stars can simultaneously undergo water loss and remain habitable. However, for stars with $T_{\mathrm{e}\mathrm{f}\mathrm{f}}⩽<!--\; ???\; -->3000\mathrm{K}$, planets at the inner HZ may directly transition to a runaway state, while bypassing the moist greenhouse water loss entirely. We analyze transmission spectra of planets in a moist greenhouse regime and find that there are several prominent ${\mathrm{H}}_2\mathrm{O}$ features, including a broad feature between 5 and 8 μm, within JWST MIRI instrument range. Thus, relying only on standard Earth-analog spectra with 24 hr rotation period around M dwarfs for habitability studies will miss the strong ${\mathrm{H}}_2\mathrm{O}$ features that one would expect to see on synchronously rotating planets around M dwarf stars, with JWST.

[en] The majority of exoplanets found to date have been discovered via the transit method, and transmission spectroscopy represents the primary method of studying these distant worlds. Currently, in-depth atmospheric characterization of transiting exoplanets entails the use of spectrographs on large telescopes, requiring significant observing time to study each planet. Previous studies have demonstrated trends for solar system worlds using color–color photometry of reflectance spectra, as well as trends within transmission spectra for hot Jupiters. Building on these concepts, we have investigated the use of transmission color photometric analysis for efficient, coarse categorization of exoplanets and for assessing the nature of these worlds, with a focus on resolving the bulk composition degeneracy to aid in discriminating super-Earths and sub-Neptunes. We present our methodology and first results, including spectrum models, model comparison frameworks, and wave band selection criteria. We present our results for different transmission “color” metrics, filter selection methods, and numbers of filters. Assuming noise-free spectra of isothermal atmospheres in chemical equilibrium, with our pipeline, we are able to constrain atmospheric mean molecular weight in order to distinguish between super-Earth and sub-Neptune atmospheres with >90% overall accuracy using specific low-resolution filter combinations, . We also found that increasing the number of filters does not substantially impact this performance. This method could allow for broad characterization of large numbers of planets much more efficiently than current methods permit, enabling population- and system-level studies. Additionally, data collected via this method could inform follow-up observing time by large telescopes for more detailed studies of worlds of interest.