We introduce PLanetary Atmospheric Transmission for Observer Noobs (PLATON), a Python package that calculates transmission spectra for exoplanets and retrieves atmospheric characteristics based on ...observed spectra. PLATON is easy to install and use, with common use cases taking no more than a few lines of code. It is also fast, with the forward model taking much less than one second, and a typical retrieval finishing in minutes on an ordinary desktop. PLATON supports the most common atmospheric parameters, such as temperature, metallicity, C/O ratio, cloud-top pressure, and scattering slope. It also has less commonly included features, such as a Mie scattering cloud model and unocculted starspot corrections. The code is available online at https://github.com/ideasrule/platon under the open-source GPL-3.0 license.
We present Exo-Transmit, a software package to calculate exoplanet transmission spectra for planets of varied composition. The code is designed to generate spectra of planets with a wide range of ...atmospheric composition, temperature, surface gravity, and size, and is therefore applicable to exoplanets ranging in mass and size from hot Jupiters down to rocky super-Earths. Spectra can be generated with or without clouds or hazes with options to (1) include an optically thick cloud deck at a user-specified atmospheric pressure or (2) to augment the nominal Rayleigh scattering by a user-specified factor. The Exo-Transmit code is written in C and is extremely easy to use. Typically the user will only need to edit parameters in a single user input file in order to run the code for a planet of their choosing. Exo-Transmit is available publicly on Github with open-source licensing at https://github.com/elizakempton/Exo_Transmit.
Recently, we introduced PLanetary Atmospheric Tool for Observer Noobs (PLATON), a Python package that calculates model transmission spectra for exoplanets and retrieves atmospheric characteristics ...based on observed spectra. We now expand its capabilities to include the ability to compute secondary eclipse depths. We have also added the option to calculate models using the correlated-k method for radiative transfer, which improves accuracy without sacrificing speed. Additionally, we update the opacities in PLATON-many of which were generated using old or proprietary line lists-using the most recent and complete public line lists. These opacities are made available at R = 1000 and R = 10,000 over the 0.3-30 m range, and at R = 375,000 in select near-IR bands, making it possible to utilize PLATON for ground-based high-resolution cross-correlation studies. To demonstrate PLATON's new capabilities, we perform a retrieval on published Hubble Space Telescope (HST) and Spitzer transmission and emission spectra of the archetypal hot Jupiter HD 189733b. This is the first joint transit and secondary eclipse retrieval for this planet in the literature, as well as the most comprehensive set of both transit and eclipse data assembled for a retrieval to date. We find that these high signal-to-noise data are well matched by atmosphere models with a C/O ratio of and a metallicity of times solar where the terminator is dominated by extended nanometer-sized haze particles at optical wavelengths. These are among the smallest uncertainties reported to date for an exoplanet, demonstrating both the power and the limitations of HST and Spitzer exoplanet observations.
The search for habitable exoplanets and life beyond the solar system is one of the most compelling scientific opportunities of our time. Nevertheless, the high cost of building facilities that can ...address this topic and the keen public interest in the results of such research requires rigorous development of experiments that can deliver a definitive advancement in our understanding. Most work to date in this area has focused on a "systems science" approach of obtaining and interpreting comprehensive data for individual planets to make statements about their habitability and the possibility that they harbor life. This strategy is challenging because of the diversity of exoplanets, both observed and expected, and the limited information that can be obtained with astronomical instruments. Here, we propose a complementary approach that is based on performing surveys of key planetary characteristics and using statistical marginalization to answer broader questions than can be addressed with a small sample of objects. The fundamental principle of this comparative planetology approach is maximizing what can be learned from each type of measurement by applying it widely rather than requiring that multiple kinds of observations be brought to bear on a single object. As a proof of concept, we outline a survey of terrestrial exoplanet atmospheric water and carbon dioxide abundances that would test the habitable zone hypothesis and lead to a deeper understanding of the frequency of habitable planets. We also discuss ideas for additional surveys that could be developed to test other foundational hypotheses in this area.
Abstract
The upcoming deployment of the James Webb Space Telescope will dramatically advance our ability to characterize exoplanet atmospheres, both in terms of precision and sensitivity to smaller ...and cooler planets. Disequilibrium chemical processes dominate these cooler atmospheres, requiring accurate photochemical modeling of such environments. The host star’s UV spectrum is a critical input to these models, but most exoplanet hosts lack UV observations. For cases in which the host UV spectrum is unavailable, a reconstructed or proxy spectrum will need to be used in its place. In this study, we use the MUSCLES catalog and UV line scaling relations to understand how well reconstructed host star spectra reproduce photochemically modeled atmospheres using real UV observations. We focus on two cases: a modern Earth-like atmosphere and an Archean Earth-like atmosphere that forms copious hydrocarbon hazes. We find that modern Earth-like environments are well-reproduced with UV reconstructions, whereas hazy (Archean Earth) atmospheres suffer from changes at the observable level. Specifically, both the stellar UV emission lines and the UV continuum significantly influence the chemical state and haze production in our modeled Archean atmospheres, resulting in observable differences in their transmission spectra. Our modeling results indicate that UV observations of individual exoplanet host stars are needed to accurately characterize and predict the transmission spectra of hazy terrestrial atmospheres. In the absence of UV data, reconstructed spectra that account for both UV emission lines and continuum are the next best option, albeit at the cost of modeling accuracy.
Results from the Kepler mission indicate that the occurrence rate of small planets (<3 R⊕) in the habitable zone of nearby low-mass stars may be as high as 80%. Despite this abundance, probing the ...conditions and atmospheric properties on any habitable-zone planet is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid and icy water clouds in the atmosphere of the 2.6 R⊕ habitable-zone planet K2-18b. The simultaneous detection of water vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing because K2-18b receives virtually the same amount of total insolation from its host star ( 1368 − 107 + 114 W m−2) as the Earth receives from the Sun (1361 W m−2), resulting in the right conditions for water vapor to condense and explain the detected clouds. In this study we observed nine transits of K2-18b using Hubble Space Telescope/WFC3 in order to achieve the necessary sensitivity to detect the water vapor, and we supplement this data set with Spitzer and K2 observations to obtain a broader wavelength coverage. While the thick hydrogen-dominated envelope we detect on K2-18b means that the planet is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.
The signature of wind patterns caused by the interplay of rotation and energy redistribution in hot Jupiters is detectable at high spectral resolution, yet no direct comparison has been attempted ...between predictions from general circulation models (GCMs) and observed high-resolution spectra. We present the first such comparison on near-infrared transmission spectra of the hot Jupiter HD 189733b. Exploring 12 rotation rates and two chemical regimes, we have created model spectra from 3D GCMs and cross-correlated them with the observed spectra. Comparing our models against those of HD 189733b, we obtain three key results: (1) we confirm CO and H2O in the planet's atmosphere at a detection significance of 8.2 ; (2) we recover the signature of day-to-night winds with speeds of several km s−1 at pressures of several millibars; and (3) we constrain the rotation period of the planet to between 1.2 and 4.69 days (synchronous rotation (2.2 days) remains consistent with existing observations). Our results do not suffer from the shortcomings of 1D models as cross-correlation templates-these models mainly tend to overconstrain the slower rotation rates and show evidence for anomalous blueshifts. Our 3D models instead match the observed line-of-sight velocity of this planet by self-consistently including the effects of high-altitude day-to-night winds. Overall, we find a high degree of consistency between observations of HD 189733b and our GCM-based spectra, implying that the physics and chemistry are adequately described in current 3D forward models for the purpose of interpreting observations at high spectral resolution.
Abstract
High-resolution spectroscopy has opened the way for new, detailed study of exoplanet atmospheres. There is evidence that this technique can be sensitive to the complex, three-dimensional ...(3D) atmospheric structure of these planets. In this work, we perform cross-correlation analysis of high-resolution (
R
∼ 100,000) CRIRES/VLT emission spectra of the hot Jupiter HD 209458b. We generate template emission spectra from a 3D atmospheric circulation model of the planet, accounting for temperature structure and atmospheric motions—winds and planetary rotation—missed by spectra calculated from one-dimensional models. In this first-of-its-kind analysis, we find that using template spectra generated from a 3D model produces a more significant detection (6.9
σ
) of the planet’s signal than any of the hundreds of one-dimensional models we tested (maximum of 5.1
σ
). We recover the planet’s thermal emission, its orbital motion, and the presence of CO in its atmosphere at high significance. Additionally, we analyzed the relative influences of 3D temperature and chemical structures in this improved detection, including the contributions from CO and H
2
O, as well as the role of atmospheric Doppler signatures from winds and rotation. This work shows that the hot Jupiter’s 3D atmospheric structure has a first-order influence on its emission spectra at high resolution and motivates the use of multidimensional atmospheric models in high-resolution spectral analysis.
Abstract
Recently, the first JWST measurement of thermal emission from a rocky exoplanet was reported. The inferred dayside brightness temperature of TRAPPIST-1 b at 15
μ
m is consistent with the ...planet having no atmosphere and therefore no mechanism by which to circulate heat to its nightside. In this Letter, we compare TRAPPIST-1 b's measured secondary eclipse depth to predictions from a suite of self-consistent radiative-convective equilibrium models in order to quantify the maximum atmospheric thickness consistent with the observation. We find that plausible atmospheres (i.e., those that contain at least 100 ppm CO
2
) with surface pressures greater than 0.3 bar are ruled out at 3
σ
, regardless of the choice of background atmosphere, and a Mars-like thin atmosphere with surface pressure 6.5 mbar composed entirely of CO
2
is also ruled out at 3
σ
. Thicker atmospheres of up to 10 bar (100 bar) are consistent with the data at 1
σ
(3
σ
) only if the atmosphere lacks
any
strong absorbers across the mid-IR wavelength range—a scenario that we deem unlikely. We additionally model the emission spectra for bare-rock planets of various compositions. We find that a basaltic, metal-rich, and Fe-oxidized surface best matches the measured eclipse depth to within 1
σ
, and the best-fit gray albedo is 0.02 ± 0.11. We conclude that planned secondary eclipse observations at 12.8
μ
m will serve to validate TRAPPIST-1 b's high observed brightness temperature, but are unlikely to further distinguish among the consistent atmospheric and bare-rock scenarios.
We perform modeling investigations to aid in understanding the atmospheres and composition of small planets of ~2-4 Earth radii, which are now known to be common in our Galaxy. GJ 1214b is a ...well-studied example whose atmospheric transmission spectrum has been observed by many investigators. Here we take a step back from GJ 1214b to investigate the role that planetary mass, composition, and temperature play in impacting the transmission spectra of these low-mass low-density (LMLD) planets. Under the assumption that these planets accrete modest hydrogen-dominated atmospheres and planetesimals, we use population synthesis models to show that predicted metal enrichments of the H/He envelope are high, with metal mass fraction Z sub(env) values commonly 0.6-0.9, or ~100-400+ times solar. The high mean molecular weight of such atmospheres ( mu approx = 5-12) would naturally help to flatten the transmission spectrum of most LMLD planets. The high metal abundance would also provide significant condensible material for cloud formation. It is known that the H/He abundance in Uranus and Neptune decreases with depth, and we show that atmospheric evaporation of LMLD planets could expose atmospheric layers with gradually higher Z sub(env). However, values of Z sub(env) close to solar composition can also arise, so diversity should be expected. Photochemically produced hazes, potentially due to methane photolysis, are another possibility for obscuring transmission spectra. Such hazes may not form above T sub(eq) of ~800-1100 K, which is testable if such warm, otherwise low mean molecular weight atmospheres are stable against atmospheric evaporation. We find that available transmission data are consistent with relatively high mean molecular weight atmospheres for GJ 1214b and "warm Neptune" GJ 436b. We examine future prospects for characterizing GJ 1214b with Hubble and the James Webb Space Telescope.
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