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The James Webb Space Telescope will revolutionize transiting exoplanetatmospheric science due to its capability for continuous, long-durationobservations and its larger collecting area, spectral coverage, and spectralresolution compared to existing space-based facilities. However, it is unclearprecisely how well JWST will perform and which of its myriad instruments andobserving modes will be best suited for transiting exoplanet studies. In thisarticle, we describe a prefatory JWST Early Release Science (ERS) program thatfocuses on testing specific observing modes to quickly give the community thedata and experience it needs to plan more efficient and successful futuretransiting exoplanet characterization programs. We propose a multi-prongedapproach wherein one aspect of the program focuses on observing transits of asingle target with all of the recommended observing modes to identify andunderstand potential systematics, compare transmission spectra at overlappingand neighboring wavelength regions, confirm throughputs, and determine overallperformances. In our search for transiting exoplanets that are well suited toachieving these goals, we identify 12 objects (dubbed "community targets") thatmeet our defined criteria. Currently, the most favorable target is WASP-62bbecause of its large predicted signal size, relatively bright host star, andlocation in JWST's continuous viewing zone. Since most of the community targetsdo not have well-characterized atmospheres, we recommend initiating preparatoryobserving programs to determine the presence of obscuring clouds/hazes withintheir atmospheres. Measurable spectroscopic features are needed to establishthe optimal resolution and wavelength regions for exoplanet characterization.Other initiatives from our proposed ERS program include testing the instrumentbrightness limits and performing phase-curve observations.(Abridged)

The JWST Early Release Observations (ERO) included a NIRISS/SOSS (0.6–2.8  μ m) transit of the ∼ 850 K Saturn-mass exoplanet HAT-P-18 b. Initial analysis of these data reported detections of water, escaping helium and haze. However, active K dwarfs like HAT-P-18 possess surface heterogeneities – star-spots and faculae – that can complicate the interpretation of transmission spectra, and indeed, a spot-crossing event is present in HAT-P-18 b's NIRISS/SOSS light curves. Here, we present an extensive reanalysis and interpretation of the JWST ERO transmission spectrum of HAT-P-18 b, as well as HST /WFC3 and Spitzer /IRAC transit observations. We detect H2O (12.5σ), CO2 (7.3σ), a cloud deck (7.4σ), and unocculted star-spots (5.8σ), alongside hints of Na (2.7σ). We do not detect the previously reported CH4 (log  CH4 < −6 to 2σ). We obtain excellent agreement between three independent retrieval codes, which find a sub-solar H2O abundance (log  H2O ≈ −4.4 ± 0.3). However, the inferred CO2 abundance (log  CO2 ≈ −4.8 ± 0.4) is significantly super-solar and requires further investigation into its origin. We also introduce new stellar heterogeneity considerations by fitting for the active regions' surface gravities – a proxy for the effects of magnetic pressure. Finally, we compare our JWST inferences to those from HST /WFC3 and Spitzer /IRAC. Our results highlight the exceptional promise of simultaneous planetary atmosphere and stellar heterogeneity constraints in the era of JWST and demonstrate that JWST transmission spectra may warrant more complex treatments of the transit light source effect. [ABSTRACT FROM AUTHOR]

Earth-like planets orbiting M-dwarf stars, M-Earths, are currently the best targets to search for signatures of life. Life as we know it requires water. The habitability of M-Earths is jeopardized by water loss to space: high flux from young M-dwarf stars can drive the loss of 3–20 Earth oceans from otherwise habitable planets. We develop a 0-D box model for Earth-mass terrestrial exoplanets, orbiting within the habitable zone, which tracks water loss to space and exchange between reservoirs during an early surface magma ocean phase and the longer deep-water cycling phase. A key feature is the duration of the surface magma ocean, assumed concurrent with the runaway greenhouse. This time-scale can discriminate between desiccated planets, planets with desiccated mantles but substantial surface water, and planets with significant water sequestered in the mantle. A longer-lived surface magma ocean helps M-Earths retain water: dissolution of water in the magma provides a barrier against significant loss to space during the earliest, most active stage of the host M-dwarf, depending on the water saturation limit of the magma. Although a short-lived basal magma ocean can be beneficial to surface habitability, a long-lived basal magma ocean may sequester significant water in the mantle at the detriment of surface habitability. We find that magma oceans and deep-water cycling can maintain or recover habitable surface conditions on Earth-like planets at the inner edge of the habitable zone around late M-dwarf stars – these planets would otherwise be desiccated if they form with less than ∼10 terrestrial oceans of water. [ABSTRACT FROM AUTHOR]

NASA is engaged in planning for a Habitable Worlds Observatory (HabWorlds ), a coronagraphic space mission to detect rocky planets in habitable zones and establish their habitability. Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase curves of starlight reflected by an exoplanet can reveal ocean glint, rainbows, and other phenomena caused by scattering by clouds or atmospheric gas. Direct imaging missions are optimized for planets near quadrature, but HabWorlds ' coronagraph may obscure the phase angles where such optical features are strongest. The range of accessible phase angles for a given exoplanet will depend on the planet's orbital inclination and/or the coronagraph's inner working angle (IWA). We use a recently created catalog relevant to HabWorlds of 164 stars to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects due to Rayleigh scattering. We find that the polarimetric Rayleigh scattering peak is accessible in most of the exo-Earth planetary systems. The rainbow due to water clouds at phase angles of ∼20○ − 60○ would be accessible with HabWorlds  for a planet with an Earth equivalent instellation in ∼46 systems, while the ocean glint signature at phase angles of ∼130○ − 170○ would be accessible in ∼16 systems, assuming an IWA = 62 mas (3λ/ D). Improving the IWA = 41 mas (2λ/ D) increases accessibility to rainbows and glints by factors of approximately 2 and 3, respectively. By observing these scattering features, HabWorlds could detect a surface ocean and water cycle, key indicators of habitability. [ABSTRACT FROM AUTHOR]

The future is now – after its long-awaited launch in 2021 December, JWST began science operations in 2022 July and is already revolutionizing exoplanet astronomy. The Early Release Observations (ERO) programme was designed to provide the first images and spectra from JWST , covering a multitude of science cases and using multiple modes of each on-board instrument. Here, we present transmission spectroscopy observations of the hot-Saturn WASP-96 b with the Single Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph, observed as part of the ERO programme. As the SOSS mode presents some unique data reduction challenges, we provide an in-depth walk-through of the major steps necessary for the reduction of SOSS data: including background subtraction, correction of 1/ f noise, and treatment of the trace order overlap. We furthermore offer potential routes to correct for field star contamination, which can occur due to the SOSS mode's slitless nature. By comparing our extracted transmission spectrum with grids of atmosphere models, we find an atmosphere metallicity between 1× and 5× solar, and a solar carbon-to-oxygen ratio. Moreover, our models indicate that no grey cloud deck is required to fit WASP-96 b's transmission spectrum, but find evidence for a slope shortward of 0.9 μm, which could either be caused by enhanced Rayleigh scattering or the red wing of a pressure-broadened Na feature. Our work demonstrates the unique capabilities of the SOSS mode for exoplanet transmission spectroscopy and presents a step-by-step reduction guide for this new and exciting instrument. [ABSTRACT FROM AUTHOR]

Precise measurements of chemical abundances in planetary atmospheres are necessary to constrain the formation histories of exoplanets. A recent study of WASP-127 b, a close-in puffy sub-Saturn orbiting its solar-type host star in 4.2 d, using Hubble Space Telescope (HST) and Spitzer revealed a feature-rich transmission spectrum with strong excess absorption at 4.5 μm. However, the limited spectral resolution and coverage of these instruments could not distinguish between CO and/or CO2 absorption causing this signal, with both low and high C/O ratio scenarios being possible. Here we present near-infrared (0.9–2.5 μm) transit observations of WASP-127 b using the high-resolution SPIRou spectrograph, with the goal to disentangle CO from CO2 through the 2.3 μm CO band. With SPIRou, we detect H2O at a t -test significance of 5.3 σ and observe a tentative (3σ) signal consistent with OH absorption. From a joint SPIRou + HST + Spitzer retrieval analysis, we rule out a CO-rich scenario by placing an upper limit on the CO abundance of log10[CO] <−4.0, and estimate a log10[CO2] of −3.7 |$^{+0.8}_{-0.6}$| , which is the level needed to match the excess absorption seen at 4.5 μm. We also set abundance constraints on other major C-, O-, and N-bearing molecules, with our results favouring low C/O (0.10 |$^{+0.10}_{-0.06}$|), disequilibrium chemistry scenarios. We further discuss the implications of our results in the context of planet formation. Additional observations at high and low resolution will be needed to confirm these results and better our understanding of this unusual world. [ABSTRACT FROM AUTHOR]

The James Webb Space Telescope (JWST) will provide an opportunity to investigate the atmospheres of potentially habitable planets. Aerosols significantly mute molecular features in transit spectra because they prevent light from probing the deeper layers of the atmosphere. Earth occasionally has stratospheric/high tropospheric clouds at 15–20 km that could substantially limit the observable depth of the underlying atmosphere. We use solar occultations of Earth's atmosphere to create synthetic JWST transit spectra of Earth analogues orbiting dwarf stars. Unlike previous investigations, we consider both clear and cloudy sightlines from the SCISAT satellite. We find that the maximum difference in effective thickness of the atmosphere between a clear and globally cloudy atmosphere is 8.5 km at 2.28   |$\mu$| m, with a resolution of 0.02  |$\mu$| m. After incorporating the effects of refraction and Pandexo's noise modelling, we find that JWST would not be able to detect Earth-like stratospheric clouds if an exo-Earth was present in the TRAPPIST-1 system, as the cloud spectrum differs from the clear spectrum by a maximum of 10 ppm. These stratospheric clouds are also not robustly detected by TauREx when performing spectral retrieval for a cloudy TRAPPIST-1 planet. However, if an Earth-sized planet were to orbit in a white dwarf's habitable zone, then we predict that JWST 's NIRSpec would be able to detect its stratospheric clouds after only four transits. We conclude that stratospheric clouds would not impede JWST transit spectroscopy or the detection of biosignatures for Earth-like atmospheres. [ABSTRACT FROM AUTHOR]

Lava planets have non-global, condensible atmospheres similar to icy bodies within the Solar system. Because they depend on interior dynamics, studying the atmospheres of lava planets can lead to understanding unique geological processes driven by their extreme environment. Models of lava planet atmospheres have thus far focused on either radiative transfer or hydrodynamics. In this study, we couple the two processes by introducing ultraviolet (UV) and infrared (IR) radiation to a turbulent boundary layer model. We also test the effect of different vertical temperature profiles on atmospheric dynamics. Results from the model show that UV radiation affects the atmosphere much more than IR. UV heating and cooling work together to produce a horizontally isothermal atmosphere away from the substellar point regardless of the vertical temperature profile. We also find that stronger temperature inversions induce stronger winds and hence cool the atmosphere. Our simulated transmission spectra of the bound atmosphere show a strong SiO feature in the UV that would be challenging to observe in the planet's transit spectrum due to the precision required. Our simulated emission spectra are more promising, with significant SiO spectral features at 4.5 and 9 |$\mu$| m that can be observed with the James Webb Space Telescope. Different vertical temperature profiles produce discernible dayside emission spectra, but not in the way one would expect. [ABSTRACT FROM AUTHOR]

Reflected light photometry of terrestrial exoplanets could reveal the presence of oceans and continents, hence placing direct constraints on the current and long-term habitability of these worlds. Inferring the albedo map of a planet from its observed light curve is challenging because different maps may yield indistinguishable light curves. This degeneracy is aggravated by changing clouds. It has previously been suggested that disc-integrated photometry spanning multiple days could be combined to obtain a cloud-free surface map of an exoplanet. We demonstrate this technique as part of a Bayesian retrieval by simultaneously fitting for the fixed surface map of a planet and the time-variable overlying clouds. We test this approach on synthetic data and then apply it to real disc-integrated observations of the Earth. We find that 8 d of continuous synthetic observations are sufficient to reconstruct a faithful low-resolution surface albedo map, without needing to make assumptions about cloud physics. For light curves with negligible photometric uncertainties, the minimal top-of-atmosphere albedo at a location is a good estimate of its surface albedo. When applied to observations from the Earth Polychromating Imaging Camera aboard the Deep Space Climate Observatory spacecraft, our approach removes only a small fraction of clouds. We attribute this difficulty to the full-phase geometry of observations combined with the short correlation length for Earth clouds. For exoplanets with Earth-like climatology, it may be hard to do much better than a cloud-averaged map. We surmise that cloud removal will be most successful for exoplanets imaged near quarter phase that harbour large cloud systems. [ABSTRACT FROM AUTHOR]

Transit searches have uncovered Earth-size planets orbiting so close to their host star that their surface should be molten, so-called lava planets. We present idealized simulations of the atmosphere of lava planet K2-141b and calculate the return flow of material via circulation in the magma ocean. We then compare how pure Na, SiO, or SiO2 atmospheres would impact future observations. The more volatile Na atmosphere is thickest followed by SiO and SiO2, as expected. Despite its low vapour pressure, we find that a SiO2 atmosphere is easier to observe via transit spectroscopy due to its greater scale height near the day–night terminator and the planetary radial velocity and acceleration are very high, facilitating high dispersion spectroscopy. The special geometry that arises from very small orbits allows for a wide range of limb observations for K2-141b. After determining the magma ocean depth, we infer that the ocean circulation required for SiO steady-state flow is only 10−4 m s−1, while the equivalent return flow for Na is several orders of magnitude greater. This suggests that a steady-state Na atmosphere cannot be sustained and that the surface will evolve over time. [ABSTRACT FROM AUTHOR]

Water cycling between Earth's mantle and surface has previously been modelled and extrapolated to rocky exoplanets, but these studies neglected the host star. M-dwarf stars are more common than Sun-like stars and at least as likely to host temperate rocky planets (M-Earths). However, M dwarfs are active throughout their lifetimes; specifically, X-ray and extreme ultraviolet (XUV) radiation during their early evolution can cause rapid atmospheric loss on orbiting planets. The increased bolometric flux reaching M-Earths leads to warmer, moister upper atmospheres, while XUV radiation can photodissociate water molecules and drive hydrogen and oxygen escape to space. Here, we present a coupled model of deep-water cycling and water loss to space on M-Earths to explore whether these planets can remain habitable despite their volatile evolution. We use a cycling parametrization accounting for the dependence of mantle degassing on seafloor pressure, the dependence of regassing on mantle temperature, and the effect of water on mantle viscosity and thermal evolution. We assume the M dwarf's XUV radiation decreases exponentially with time, and energy-limited water loss with 30 per cent efficiency. We explore the effects of cycling and loss to space on planetary water inventories and water partitioning. Planet surfaces desiccated by loss can be rehydrated, provided there is sufficient water sequestered in the mantle to degas once loss rates diminish at later times. For a given water loss rate, the key parameter is the mantle overturn time-scale at early times: if the mantle overturn time-scale is longer than the loss time-scale, then the planet is likely to keep some of its water. [ABSTRACT FROM AUTHOR]

In this work, we outline a new method for estimating dayside effective temperatures of exoplanets and associated uncertainties using Gaussian process (GP) regression. By applying our method to simulated observations, we show that the GP method estimates uncertainty more robustly than other model-independent approaches. We find that unbiased estimates of effective temperatures can be made using as few as three broad-band measurements (white-light HST WFC3 and the two warm Spitzer IRAC channels), although we caution that estimates made using only IRAC can be significantly biased. We then apply our GP method to the 12 hot Jupiters in the literature whose secondary eclipse depths have been measured by WFC3 and IRAC channels 1 and 2: CoRoT-2 b; HAT-P-7 b; HD 189733 b; HD 209458 b; Kepler-13A b; TrES-3 b; WASP-4 b; WASP-12 b; WASP-18 b; WASP-33 b; WASP-43 b; and WASP-103 b. We present model-independent dayside effective temperatures for these planets, with uncertainty estimates that range from ±66 to ±136 K. [ABSTRACT FROM AUTHOR]

The Atmospheric Chemistry Experiment Fourier Transform Spectrometer on the SCISAT satellite has been measuring infrared (IR) transmission spectra of Earth during Solar occultations since 2004. We use these data to build an IR transit spectrum of Earth. Regions of low atmospheric opacity, known as windows, are of particular interest, as they permit observations of the planet's lower atmosphere. Even in the absence of clouds or refraction, imperfect transmittance leads to a minimum effective thickness of h min ≈ 4 km in the 10–12  |$\mu \mathrm{m}$| opacity window at a spectral resolution of R  = 103. None the less, at R  = 105, the maximum transmittance at the surface is around |${70}{{{\ \rm per\ cent}}}$|⁠. In principle, one can probe the troposphere of an Earth-like planet via high-dispersion transit spectroscopy in the mid-IR; in practice aerosols and/or refraction likely make this impossible. We simulate the transit spectrum of an Earth-like planet in the TRAPPIST-1 system. We find that a long-term near-IR (NIR) campaign with the James Webb Space Telescope (JWST) could readily detect CO2 and H2O, establishing the presence of an atmosphere. A mid-IR campaign or longer NIR campaign would be more challenging, but in principle could detect the biosignatures O3 and CH4. [ABSTRACT FROM AUTHOR]

The disc-integrated reflected brightness of an exoplanet changes as a function of time due to orbital and rotational motions coupled with an inhomogeneous albedo map. We have previously derived analytic reflected light curves for spherical harmonic albedo maps in the special case of a synchronously rotating planet on an edge-on orbit (Cowan, Fuentes & Haggard). In this paper, we present analytic reflected light curves for the general case of a planet on an inclined orbit, with arbitrary spin period and non-zero obliquity. We do so for two different albedo basis maps: bright points (δ-maps), and spherical harmonics ( $Y_\mathrm{ l}^m$ -maps). In particular, we use Wigner D-matrices to express an harmonic light curve for an arbitrary viewing geometry as a non-linear combination of harmonic light curves for the simpler edge-on, synchronously rotating geometry. These solutions will enable future exploration of the degeneracies and information content of reflected light curves, as well as fast calculation of light curves for mapping exoplanets based on time-resolved photometry. To these ends, we make available Exoplanet Analytic Reflected Lightcurves, a simple open-source code that allows rapid computation of reflected light curves. [ABSTRACT FROM AUTHOR]

In addition to the transits of a planet in front of its star and the eclipses of the planet by its star, researchers have reported flux variations at the orbital frequency and its harmonics: planetary reflection and/or emission and Doppler beaming of starlight produce one peak per orbit, while ellipsoidal variations of a tidally distorted star and/or planet produce two maxima per orbit. Researchers have also reported significant photometric variability at three times the orbital frequency, as yet unexplained. Reflected phase variations of homogeneous planets only contain power at the orbital frequency and its even harmonics. We show that odd harmonics can, however, be produced by an edge-on planet with a time-variable map, or an inclined planet with a north-south (N-S) asymmetric map. For tidally locked planets with thick atmospheres, either of these scenarios entail weather: planets with zero obliquity experience N-S symmetric stellar forcing. North-south asymmetry would therefore suggest stochastic localized features, i.e. weather. However, we find that previous claims of large-amplitude odd modes in Kepler photometry are artefacts of removing planetary transits rather than modelling them. The only reliable claims of odd harmonics remain HAT-P-7b and Kepler-13Ab, for which the third mode amplitude is 6-8 per cent of the planetary flux. Although time-variable albedo maps could in principle explain these odd harmonics, upper-limits on the infrared variability of other hot Jupiters make this scenario unlikely. We recommend further studying the tidal effects of close-in planets on their host stars, as this remains the only plausible hypothesis. [ABSTRACT FROM AUTHOR]

A planet's emission spectrum contains information about atmospheric composition and structure. We compare the Bayesian Information Criterion (BIC) of blackbody fits and idealized spectral retrieval fits for the 44 planets with published eclipse measurements in multiple thermal wavebands, mostly obtained with the Spitzer Space Telescope. The evidence for spectral features depends on eclipse depth uncertainties. Spitzer has proven capable of eclipse precisions better than 10−4 when multiple eclipses are analysed simultaneously, but this feat has only been performed four times. It is harder to self-calibrate photometry when a single occultation is reduced and analysed in isolation; we find that such measurements have not passed the test of repeatability. Single-eclipse measurements either have an uncertainty floor of 5 × 10−4, or their uncertainties have been underestimated by a factor of 3. If one adopts these empirical uncertainties for single-eclipse measurements, then the evidence for molecular features all but disappears: blackbodies have better BIC than spectral retrieval for all planets, save HD 189733b, and the few planets poorly fit by blackbodies are also poorly fit by self-consistent radiative transfer models. This suggests that the features in extant broad-band emission spectra are due to astrophysical and instrumental noise rather than molecular bands. Claims of stratospheric inversions, disequilibrium chemistry, and high C/O ratios based solely on photometry are premature. We recommend that observers be cautious of error estimates from self-calibration of small data sets, and that modellers compare the evidence for spectral models to that of simpler models such as blackbodies. [ABSTRACT FROM AUTHOR]

We report Warm Spitzer full-orbit phase observations of WASP-12b at 3.6 and 4.5 µm. This extremely inflated hot Jupiter is thought to be overflowing its Roche lobe, undergoing mass loss and accretion onto its host star, and has been claimed to have a C/O ratio in excess of unity. We are able to measure the transit depths, eclipse depths, thermal and ellipsoidal phase variations at both wavelengths. The large-amplitude phase variations, combined with the planet's previously measured dayside spectral energy distribution, are indicative of non-zero Bond albedo and very poor day--night heat redistribution. The transit depths in the mid-infrared--(Rp/R*)² = 0.0123(3) and 0.0111(3) at 3.6 and 4.5 µm, respectively--indicate that the atmospheric opacity is greater at 3.6 than at 4.5 µm, in disagreement with model predictions, irrespective of C/O ratio. The secondary eclipse depths are consistent with previous studies: Fday/F* = 0.0038(4) and 0.0039(3) at 3.6 and 4.5 µm, respectively. We do not detect ellipsoidal variations at 3.6 µm, but our parameter uncertainties--estimated via prayer-bead Monte Carlo--keep this non-detection consistent with model predictions. At 4.5 µm, on the other hand, we detect ellipsoidal variations that are much stronger than predicted. If interpreted as a geometric effect due to the planet's elongated shape, these variations imply a 3:2 ratio for the planet's longest:shortest axes and a relatively bright day-night terminator. If we instead presume that the 4.5 µm ellipsoidal variations are due to uncorrected systematic noise and we fix the amplitude of the variations to zero, the best-fit 4.5 µm transit depth becomes commensurate with the 3.6 µm depth, within the uncertainties. The relative transit depths are then consistent with a solar composition and short scale height at the terminator. Assuming zero ellipsoidal variations also yields a much deeper 4.5 µm eclipse depth, consistent with a solar composition and modest temperature inversion. We suggest future observations that could distinguish between these two scenarios. [ABSTRACT FROM AUTHOR]