Abstract
Transmission spectroscopy is one of the premier methods used to probe the temperature, composition, and cloud properties of exoplanet atmospheres. Recent studies have demonstrated that the ...multidimensional nature of exoplanet atmospheres—due to nonuniformities across the day–night transition and between the morning and evening terminators—can strongly influence transmission spectra. However, the computational demands of 3D radiative-transfer techniques have precluded their usage within atmospheric retrievals. Here we introduce TRIDENT, a new 3D radiative-transfer model which rapidly computes transmission spectra of exoplanet atmospheres with day–night, morning–evening, and vertical variations in temperature, chemical abundances, and cloud properties. We also derive a general equation for transmission spectra, accounting for 3D atmospheres, refraction, multiple scattering, ingress/egress, grazing transits, stellar heterogeneities, and nightside thermal emission. After introducing TRIDENT’s linear-algebra-based approach to 3D radiative transfer, we propose new parametric prescriptions for 3D temperature and abundance profiles and 3D clouds. We show that multidimensional transmission spectra exhibit two significant observational signatures: (i) day–night composition gradients alter the relative amplitudes of absorption features; and (ii) morning–evening composition gradients distort the peak-to-wing contrast of absorption features. Finally, we demonstrate that these signatures of multidimensional atmospheres incur residuals >100 ppm compared to 1D models, rendering them potentially detectable with the James Webb Space Telescope. TRIDENT’s rapid radiative transfer, coupled with parametric multidimensional atmospheres, unlocks the final barrier to 3D atmospheric retrievals.
ABSTRACT Efforts to characterize extrasolar giant planet (EGP) atmospheres have so far emphasized planets within 0.05 AU of their stars. Despite this focus, known EGPs populate a continuum of orbital ...separations from canonical hot Jupiter values (0.03-0.05 AU) out to 1 AU and beyond. Unlike typical hot Jupiters, these more distant EGPs will not generally be synchronously rotating. In anticipation of observations of this population, we here present three-dimensional atmospheric circulation models exploring the dynamics that emerge over a broad range of rotation rates and incident stellar fluxes appropriate for warm and hot Jupiters. We find that the circulation resides in one of two basic regimes. On typical hot Jupiters, the strong day-night heating contrast leads to a broad, fast superrotating (eastward) equatorial jet and large day-night temperature differences. At faster rotation rates and lower incident fluxes, however, the day-night heating gradient becomes less important, and baroclinic instabilities emerge as a dominant player, leading to eastward jets in the midlatitudes, minimal temperature variations in longitude, and, often, weak winds at the equator. Our most rapidly rotating and least irradiated models exhibit similarities to Jupiter and Saturn, illuminating the dynamical continuum between hot Jupiters and the weakly irradiated giant planets of our own solar system. We present infrared (IR) light curves and spectra of these models, which depend significantly on incident flux and rotation rate. This provides a way to identify the regime transition in future observations. In some cases, IR light curves can provide constraints on the rotation rate of nonsynchronously rotating planets.
Transmission spectroscopy is a powerful technique widely used to probe exoplanet terminators. Atmospheric retrievals of transmission spectra are enabling comparative studies of exoplanet atmospheres. ...However, the atmospheric properties inferred by retrieval techniques display a significant anomaly: most retrieved temperatures are far colder than expected. In some cases, retrieved temperatures are colder than . Here, we provide an explanation for this conundrum. We demonstrate that erroneously cold temperatures result when 1D atmospheric models are applied to spectra of planets with differing morning-evening terminator compositions. Despite providing an acceptable fit, 1D retrieval techniques artificially tune atmospheric parameters away from terminator-averaged properties. Retrieved temperature profiles are hundreds of degrees cooler and have weaker temperature gradients than reality. Retrieved abundances are mostly biased by and sometimes by , with the most extreme biases for ultra-hot Jupiters. When morning-evening compositional differences manifest for prominent opacity sources, H2O abundances retrieved by 1D models can be biased by over an order of magnitude. Finally, we demonstrate that these biases provide an explanation for the cold retrieved temperatures reported for WASP-17b and WASP-12b. To overcome biases associated with 1D atmospheric models, there is an urgent need to develop multidimensional retrieval techniques.
Space-based high-contrast imaging mission concepts for studying rocky exoplanets in reflected light are currently under community study. We develop an inverse modeling framework to estimate the ...science return of such missions given different instrument design considerations. By combining an exoplanet albedo model, instrument noise model, and ensemble Markov chain Monte Carlo sampler, we explore retrievals of atmospheric and planetary properties for Earth twins as a function of signal-to-noise ratio (S/N) and resolution (R). Our forward model includes Rayleigh-scattering, single-layer water clouds with patchy coverage, and pressure-dependent absorption due to water vapor, oxygen, and ozone. We simulate data at R = 70 and 140 from 0.4 to 1.0 m with S/N = 5, 10, 15, and 20 at 550 nm (i.e., for HabEx/LUVOIR-type instruments). At these same S/Ns, we simulate data for WFIRST paired with a starshade, which includes two photometric points between 0.48 and 0.6 m and R = 50 spectroscopy from 0.6 to 0.97 m. Given our noise model for WFIRST-type detectors, we find that weak detections of water vapor, ozone, and oxygen can be achieved with observations with at least R = 70/S/N = 15 or R = 140/S/N = 10 for improved detections. Meaningful constraints are only achieved with R = 140/S/N = 20 data. The WFIRST data offer limited diagnostic information, needing at least S/N = 20 to weakly detect gases. Most scenarios place limits on planetary radius but cannot constrain surface gravity and, thus, planetary mass.
ABSTRACT We present results from an atmospheric circulation study of nine hot Jupiters that compose a large transmission spectral survey using the Hubble and Spitzer Space Telescopes. These ...observations exhibit a range of spectral behavior over optical and infrared wavelengths, suggesting diverse cloud and haze properties in their atmospheres. By utilizing the specific system parameters for each planet, we naturally probe a wide phase space in planet radius, gravity, orbital period, and equilibrium temperature. First, we show that our model "grid" recovers trends shown in traditional parametric studies of hot Jupiters, particularly equatorial superrotation and increased day-night temperature contrast with increasing equilibrium temperature. We show how spatial temperature variations, particularly between the dayside and nightside and west and east terminators, can vary by hundreds of kelvin, which could imply large variations in Na, K, CO and CH 4 abundances in those regions. These chemical variations can be large enough to be observed in transmission with high-resolution spectrographs, such as ESPRESSO on VLT, METIS on the E-ELT, or MIRI and NIRSpec aboard JWST. We also compare theoretical emission spectra generated from our models to available Spitzer eclipse depths for each planet and find that the outputs from our solar-metallicity, cloud-free models generally provide a good match to many of the data sets, even without additional model tuning. Although these models are cloud-free, we can use their results to understand the chemistry and dynamics that drive cloud formation in their atmospheres.
Two of Transiting Exoplanet Survey Satellite's major science goals are to measure masses for 50 planets smaller than 4 Earth radii and to discover high-quality targets for atmospheric ...characterization efforts. It is important that these two goals are linked by quantifying what precision of mass constraint is required to yield robust atmospheric properties of planets. Here, we address this by conducting retrievals on simulated James Webb Space Telescope transmission spectra under various assumptions for the degree of uncertainty in the planets mass for a representative population of seven planets ranging from terrestrials to warm Neptunes to hot Jupiters. Only for the cloud-free, low-metallicity gas giants are we able to infer exoplanet mass from transmission spectroscopy alone, to ∼10% accuracy. For low-metallicity cases (<4× solar) we are able to accurately constrain atmospheric properties without prior knowledge of the planet's mass. For all other cases (including terrestrial-like planets), atmospheric properties can only be inferred with a mass precision of better than 50%. At this level, though, the widths of the posterior distributions of the atmospheric properties are dominated by the uncertainties in mass. With a precision of 20%, the widths of the posterior distributions are dominated by the spectroscopic data quality. Therefore, as a rule of thumb, we recommend a 50% mass precision for initial atmospheric characterization and a 20% mass precision for more detailed atmospheric analyses.
ABSTRACT
We present four new secondary eclipse observations for the ultrahot Jupiter WASP-121b acquired using the Hubble Space Telescope Wide Field Camera 3. The eclipse depth is measured to a median ...precision of 60 ppm across 28 spectroscopic channels spanning the 1.12–$1.64\, \mu {\rm m}$ wavelength range. This is a considerable improvement to the 90 ppm precision we achieved previously for a single eclipse observation using the same observing set-up. Combining these data with those reported at other wavelengths, a blackbody spectrum for WASP-121b is ruled out at >6σ confidence and we confirm the interpretation of previous retrieval analyses that found the data are best explained by a dayside thermal inversion. The updated spectrum clearly resolves the water emission band at 1.3–$1.6\, \mu {\rm m}$, with higher signal-to-noise than before. It also fails to reproduce a bump in the spectrum at $1.25\, \mu {\rm m}$ derived from the first eclipse observation, which had tentatively been attributed to VO emission. We conclude that the latter was either a statistical fluctuation or a systematic artefact specific to the first eclipse data set.
We present Hubble Space Telescope (HST) near-ultraviolet (NUV) transits of the hot Jupiter WASP-121b, acquired as part of the PanCET program. Time-series spectra during two transit events were used ...to measure the transmission spectra between 2280 and 3070 at a resolution of 30,000. Using HST data from 61 Space Telescope Imaging Spectrograph visits, we show that data from HST's Pointing Control System can be used to decorrelate the instrument systematic errors (jitter decorrelation), which we used to fit the WASP-121b light curves. The NUV spectra show very strong absorption features, with the NUV white light curve found to be larger than the average optical and near-infrared value at 6 confidence. We identify and spectrally resolve absorption from the Mg ii doublet in the planetary exosphere at a 5.9 confidence level. The Mg ii doublet is observed to reach altitudes of Rpl/Rstar = 0.284 0.037 for the 2796 line and 0.242 0.0431 for the 2804 line, which exceeds the Roche lobe size as viewed in transit geometry (ReqRL/Rstar = 0.158). We also detect and resolve strong features of the Fe ii UV1 and UV2 multiplets, and observe the lines reaching altitudes of Rpl/Rstar 0.3. At these high altitudes, the atmospheric Mg ii and Fe ii gas is not gravitationally bound to the planet, and these ionized species may be hydrodynamically escaping or could be magnetically confined. Refractory Mg and Fe atoms at high altitudes also indicate that these species are not trapped into condensate clouds at depth, which places constraints on the deep interior temperature.
Recent work has shown that sulfur hazes may arise in the atmospheres of some giant exoplanets, due to the photolysis of H2S. We investigate the impact such a haze would have on an exoplanet's ...geometric albedo spectrum and how it may affect the direct imaging results of the Wide Field Infrared Survey Telescope (WFIRST), a planned NASA space telescope. For temperate (250 K < Teq < 700 K) Jupiter-mass planets, photochemical destruction of H2S results in the production of ∼1 ppmv of S8 between 100 and 0.1 mbar, which, if cool enough, will condense to form a haze. Nominal haze masses are found to drastically alter a planet's geometric albedo spectrum: whereas a clear atmosphere is dark at wavelengths between 0.5 and 1 m, due to molecular absorption, the addition of a sulfur haze boosts the albedo there to ∼0.7, due to scattering. Strong absorption by the haze shortward of 0.4 m results in albedos <0.1, in contrast to the high albedos produced by Rayleigh scattering in a clear atmosphere. As a result, the color of the planet shifts from blue to orange. The existence of a sulfur haze masks the molecular signatures of methane and water, thereby complicating the characterization of atmospheric composition. Detection of such a haze by WFIRST is possible, though discriminating between a sulfur haze and any other highly reflective, high-altitude scatterer will require observations shortward of 0.4 m, which is currently beyond WFIRST's design.
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