Exoplanet Transmission Spectra Research Articles

A Framework for Characterizing Transmission Spectra of Exoplanets with Circumplanetary Rings

Recent observations revealed that several extremely low-density exoplanets show featureless transmission spectra. While atmospheric aerosols are a promising explanation for both the low-density and featureless spectra, there is another attractive possibility: the presence of circumplanetary rings. Previous studies suggested that rings cause anomalously large transit radii. However, it remains poorly understood how rings affect the transmission spectrum. Here, we provide a framework to characterize the transmission spectra of ringed exoplanets. We develop an analytical prescription to include rings in the transmission spectra for arbitrarily viewing geometries. We also establish a simple postprocessing model that can include the ring’s effects on precomputed ring-free spectra. The ring flattens the transmission spectrum for a wide range of viewing geometries, consistent with the featureless spectra of extremely low-density exoplanets. Near-future observations by the James Webb Space Telescope at longer wavelengths would be able to distinguish the aerosol and ring scenarios. We also find that rocky rings might cause a silicate feature at ∼10 μm if the ring’s optical depth is around unity. Thus, the ring’s spectral features, if detected, would provide tight constrains on the physical properties of exoplanetary rings. We also discuss the ring’s stability and suggest that thick rings are sustainable only at the equilibrium temperature of ≲300 K for the ring’s age comparable to Kepler planets. This might indicate the intrinsic deficit of thick rings in the Kepler samples, unless rings are much younger than the planets as suggested for Saturn.

CaRM: Exploring the chromatic Rossiter-McLaughlin effect

Aims. In this paper we introduce CaRM, a semi-automatic code for the retrieval of broadband transmission spectra of transiting planets through the chromatic Rossiter-McLaughlin method. We applied it to HARPS and ESPRESSO observations of two exoplanets to retrieve the transmission spectrum and we analyze its fitting transmission models. Methods. We used the strong radius dependence of the Rossiter-McLaughlin (RM) effect amplitude, caused by planetary companions, to measure the apparent radius change caused by the exoplanet atmosphere. In order to retrieve the transmission spectrum, the radial velocities, which were computed over wavelength bins that encompass several spectral orders, were used to simultaneously fit the Keplerian motion and the RM effect. From this, the radius ratio was computed as a function of the wavelength, which allows one to retrieve the low-resolution broadband transmission spectrum of a given exoplanet. CaRM offers the possibility to use two Rossiter-McLaughlin models taken from ARoME and PyAstronomy, associated with a Keplerian function to fit radial velocities during transit observations automatically. Furthermore it offers the possibility to use some methods that could, in theory, mitigate the effect of perturbation in the radial velocities during transits. Results. We applied CaRM to recover the transmission spectrum of HD 189733b and WASP-127b, with HARPS and ESPRESSO data, respectively. Our results for HD 189733b suggest that the blue part of the spectrum is dominated by Rayleigh scattering, which is compatible with former studies. The analysis of WASP-127b shows a flat transmission spectrum. Conclusions. The CaRM code allows one to retrieve the transmission spectrum of a given exoplanet using minimal user interaction. We demonstrate that it allows one to compute the low-resolution broadband transmission spectra of exoplanets observed using high-resolution spectrographs such as HARPS and ESPRESSO.

Implementation of disequilibrium chemistry to spectral retrieval code ARCiS and application to 16 exoplanet transmission spectra

Context. The retrieval approach is currently a standard method for deriving atmospheric properties from observed spectra of exoplanets. However, the approach ignores disequilibrium chemistry in most current retrieval codes, which can lead to a misinterpretation of the metallicity or elemental abundance ratios of the atmosphere. Aims. We have implemented the disequilibrium effect of vertical mixing or quenching for the major species in hydrogen and helium-dominated atmospheres, namely CH4, CO, H2O, NH3, N2, and CO2, for the spectral retrieval code ARCiS with a physical basis. Methods. We used the chemical relaxation method and developed a module to compute the profiles of molecular abundances, taking the disequilibrium effect into account. Then, using ARCiS updated with this module, we performed retrievals of the observed transmission spectra of 16 exoplanets with sizes ranging from that of Jupiter to mini-Neptunes. Results. We find indications of disequilibrium chemistry for HD 209458b (≥4.1σ) and WASP-39b (≥2.7σ). The retrieved spectrum of HD 209458b exhibits a strong NH3 absorption feature at 10.5 μm that is accessible by JWST owing to an enhanced abundance of NH3 due to the quenching effect. This feature is absent in the spectrum retrieved assuming equilibrium chemistry, which makes HD 209458b an ideal target for studying disequilibrium chemistry in exoplanet atmospheres. Moreover, for HAT-P-11b and GJ 436b, we obtain relatively different results compared to the retrieval with the equilibrium assumption, such as a 2.9σ difference for the C/O ratio. We have also examined the retrieved eddy diffusion coefficient but could not identify a trend over the equilibrium temperature, possibly due to the limits of the current observational precision. Conclusions. We have demonstrated that the assumption of equilibrium chemistry can lead to a misinterpretation of the observed data, showing that spectral retrieval with a consideration of disequilibrium chemistry is essential in the era of JWST and Ariel.

Transmission Spectroscopy of the Earth–Sun System to Inform the Search for Extrasolar Life

Abstract Upcoming NASA astrophysics missions such as the James Webb Space Telescope will search for signs of life on planets transiting nearby stars. Doing so will require coadding dozens of transmission spectra to build up sufficient signal to noise while simultaneously accounting for challenging systematic effects such as surface/weather variability, atmospheric refraction, and stellar activity. To determine the magnitude and impacts of both stellar and planet variability on measured transmission spectra, we must assess the feasibility of stacking multiple transmission spectra of exo-Earths around their host stars. Using our own solar system, we can determine if current methodologies are sufficient to detect signs of life in Earth’s atmosphere and measure the abundance of habitability indicators, such as H2O and CO2, and biosignature pairs, such as O2 and CH4. We assess the impact on transmission spectra of Earth transiting across the Sun from solar and planetary variability and identify remaining unknowns for understanding exoplanet transmission spectra. We conclude that a satellite observing Earth transits across the Sun from beyond L2 is necessary to address these long-standing concerns about the reliability of coadding planet spectra at UV, optical, and infrared wavelengths from multiple transits in the face of relatively large astrophysical systematics.

ACCESS and LRG-BEASTS: A Precise New Optical Transmission Spectrum of the Ultrahot Jupiter WASP-103b

Abstract We present a new ground-based optical transmission spectrum of the ultrahot Jupiter WASP-103b ( T eq = 2484 K). Our transmission spectrum is the result of combining five new transits from the ACCESS survey and two new transits from the LRG-BEASTS survey with a reanalysis of three archival Gemini/GMOS transits and one VLT/FORS2 transit. Our combined 11-transit transmission spectrum covers a wavelength range of 3900–9450 Å with a median uncertainty in the transit depth of 148 parts per million, which is less than one atmospheric scale height of the planet. In our retrieval analysis of WASP-103b’s combined optical and infrared transmission spectrum, we find strong evidence for unocculted bright regions (4.3σ) and weak evidence for H2O ( 1.9 σ ), HCN ( 1.7 σ ), and TiO ( 2.1 σ ), which could be responsible for WASP-103b’s observed temperature inversion. Our optical transmission spectrum shows significant structure that is in excellent agreement with the extensively studied ultrahot Jupiter WASP-121b, for which the presence of VO has been inferred. For WASP-103b, we find that VO can only provide a reasonable fit to the data if its abundance is implausibly high and we do not account for stellar activity. Our results highlight the precision that can be achieved by ground-based observations and the impacts that stellar activity from F-type stars can have on the interpretation of exoplanet transmission spectra.

On the Compatibility of Ground-based and Space-based Data: WASP-96 b, an Example*

Abstract The study of exoplanetary atmospheres relies on detecting minute changes in the transit depth at different wavelengths. To date, a number of ground- and space-based instruments have been used to obtain transmission spectra of exoplanets in different spectral bands. One common practice is to combine observations from different instruments in order to achieve a broader wavelength coverage. We present here two inconsistent observations of WASP-96 b, one by the Hubble Space Telescope (HST) and the other by the Very Large Telescope (VLT). We present two key findings in our investigation: (1) a strong water signature is detected via the HST WFC3 observations and (2) a notable offset in transit depth (>1100 ppm) can be seen when the ground-based and space-based observations are combined. The discrepancy raises the question of whether observations from different instruments could indeed be combined. We attempt to align the observations by including an additional parameter in our retrieval studies but are unable to definitively ascertain that the aligned observations are indeed compatible. The case of WASP-96 b signals that compatibility of instruments should not be assumed. While wavelength overlaps between instruments can help, it should be noted that combining data sets remains risky business. The difficulty of combining observations also strengthens the need for next-generation instruments that possess broader spectral coverage.

Broadband transmission spectroscopy of HD 209458b with ESPRESSO: evidence for Na, TiO, or both

Context. The detection and characterization of exoplanet atmospheres is currently one of the main drivers pushing the development of new observing facilities. In this context, high-resolution spectrographs are proving their potential and showing that high-resolution spectroscopy will be paramount in this field. Aims. We aim to make use of ESPRESSO high-resolution spectra, which cover two transits of HD 209458b, to probe the broadband transmission optical spectrum of the planet. Methods. We applied the chromatic Rossiter–McLaughin method to derive the transmission spectrum of HD 209458b. We compared the results with previous HST observations and with synthetic spectra. Results. We recover a transmission spectrum of HD 209458b similar to the one obtained with HST data. The models suggest that the observed signal can be explained by only Na, only TiO, or both Na and TiO, even though none is fully capable of explaining our observed transmission spectrum. Extra absorbers may be needed to explain the full dataset, though modeling approximations and observational errors can also be responsible for the observed mismatch. Conclusions. Using the chromatic Rossiter–McLaughlin technique, ESPRESSO is able to provide broadband transmission spectra of exoplanets from the ground, in conjunction with space-based facilities, opening good perspectives for similar studies of other planets.

The ARCiS framework for exoplanet atmospheres

Aims.We present ARCiS, a novel code for the analysis of exoplanet transmission and emission spectra. The aim of the modelling framework is to provide a tool able to link observations to physical models of exoplanet atmospheres.Methods.The modelling philosophy chosen in this paper is to use physical and chemical models to constrain certain parameters while leaving certain parts of the model, where our physical understanding remains limited, free to vary. This approach, in between full physical modelling and full parameterisation, allows us to use the processes we understand well and parameterise those less understood. We implemented a Bayesian retrieval framework and applied it to the transit spectra of a set of ten hot Jupiters. The code contains chemistry and cloud formation and has the option for self-consistent temperature structure computations.Results.The code presented is fast and flexible enough to be used for retrieval and for target list simulations for JWST or the ESA Ariel missions for example. We present results for the retrieval of elemental abundance ratios using the physical retrieval framework and compare this to results obtained using a parameterised retrieval setup.Conclusions.We conclude that for most of the targets considered, their elemental abundance ratios cannot be reliably constrained based on the current dataset. We find no significant correlations between different physical parameters. We confirm that planets in our sample with a strong slope in the optical transmission spectrum are those for which we find cloud formation to be most active. Finally, we conclude that with ARCiS we have a computationally efficient tool to analyse exoplanet observations in the context of physical and chemical models.

The Role of Clouds on the Depletion of Methane and Water Dominance in the Transmission Spectra of Irradiated Exoplanets

Abstract Observations suggest an abundance of water and a paucity of methane in the majority of observed exoplanetary atmospheres. We isolate the effect of atmospheric processes to investigate possible causes. Previously, we studied the effect of effective temperature, surface gravity, metallicity, carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical equilibrium and disequilibrium chemistry. However, under these assumptions, methane remains a persisting spectral feature in the transmission spectra of exoplanets over a certain parameter space, the Methane Valley. In this work, we investigate the role of clouds on this domain and we find that clouds change the spectral appearance of methane in two direct ways: (1) by heating up the photosphere of colder planets and (2) by obscuring molecular features. The presence of clouds also affects methane features indirectly: (1) cloud heating results in more evaporation of condensates and hence releases additional oxygen, causing water-dominated spectra of colder carbon-poor exoplanets, and (2) HCN/CO production results in a suppression of depleted methane features by these molecules. The presence of HCN/CO and a lack of methane could be an indication of cloud formation on hot exoplanets. Cloud heating can also deplete ammonia. Therefore, a simultaneous depletion of methane and ammonia is not unique to photochemical processes. We propose that the best targets for methane detection are likely to be massive but smaller planets with a temperature around 1450 K orbiting colder stars. We also construct Spitzer synthetic color maps and find that clouds can explain some of the high-contrast observations by IRAC’s channel 1 and 2.

Alkaline exospheres of exoplanet systems: evaporative transmission spectra

ABSTRACT Hydrostatic equilibrium is an excellent approximation for the dense layers of planetary atmospheres, where it has been canonically used to interpret transmission spectra of exoplanets. Here, we exploit the ability of high-resolution spectrographs to probe tenuous layers of sodium and potassium gas due to their formidable absorption cross-sections. We present an atmosphere–exosphere degeneracy between optically thick and optically thin mediums, raising the question of whether hydrostatic equilibrium is appropriate for Na i lines observed at exoplanets. To this end we simulate three non-hydrostatic, evaporative, density profiles: (i) escaping, (ii) exomoon, and (iii) torus to examine their imprint on an alkaline exosphere in transmission. By analysing an evaporative curve of growth, we find that equivalent widths of $W_{\mathrm{Na D2}} \sim 1{\!-\!} 10\, \mathrm{m\mathring{\rm A}}$ are naturally driven by evaporation rates ∼103−105 kg s−1 of pure atomic Na. To break the degeneracy between atmospheric and exospheric absorption, we find that if the line ratio is D2/D1 ≳ 1.2 the gas is optically thin on average roughly indicating a non-hydrostatic structure of the atmosphere/exosphere. We show this is the case for Na i observations at hot Jupiters WASP-49b and HD189733b and also simulate their K i spectra. Lastly, motivated by the slew of metal detections at ultra-hot Jupiters, we suggest a toroidal atmosphere at WASP-76b and WASP-121b is consistent with the Na i data at present.

Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes

Aerosols are common in the atmospheres of exoplanets across a wide swath of temperatures, masses, and ages. These aerosols strongly impact observations of transmitted, reflected, and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition. Knowing the dominant aerosol composition would facilitate interpretations of exoplanet observations and theoretical understanding of their atmospheres. A variety of compositions have been proposed, including metal oxides and sulphides, iron, chromium, sulphur, and hydrocarbons. However, the relative contributions of these species to exoplanet aerosol opacity is unknown. Here we show that the aerosol composition of giant exoplanets observed in transmission is dominated by silicates and hydrocarbons. By constraining an aerosol microphysics model with trends in giant exoplanet transmission spectra, we find that silicates dominate aerosol opacity above planetary equilibrium temperatures of 950 K due to low nucleation energy barriers and high elemental abundances, while hydrocarbon aerosols dominate below 950 K due to an increase in methane abundance. Our results are robust to variations in planet gravity and atmospheric metallicity within the range of most giant transiting exoplanets. We predict that spectral signatures of condensed silicates in the mid-infrared are most prominent for hot (>1600 K), low-gravity (<10 m s$^{-2}$) objects.

Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets

Abstract Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the solar system, Io, we describe how sodium (Na i) and potassium (K i) gas could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal . This tidal energy is also focused into the satellite driving an ∼105±2 higher mass-loss rate than Io’s supply to Jupiter’s Na exosphere based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are, on average, more than sufficient to provide the ∼1010±1 Na cm−2 required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter’s extended (∼1000 R J) Na exosphere and Jupiter’s atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the “high-altitude” Na at WASP-49b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.

Planet and star synergy at high-spectral resolution. A rationale for the characterization of exoplanet atmospheres

Context. Spectroscopy of exoplanet atmospheres at high-resolving powers is rapidly gaining popularity for measuring the presence of atomic and molecular species. While this technique is particularly robust against contaminant absorption in the Earth’s atmosphere, the non-stationary stellar spectrum, in the form of either Doppler shift or distortion of the line profile during planetary transits, creates a non-negligible source of noise that can alter or even prevent detection. Aims. Our aim was to use state-of-the art three-dimensional stellar simulations to directly remove the signature of the star from observations prior to cross correlation with templates for the planet’s atmosphere, which are commonly used to extract the faint exoplanet signal from noisy data. Methods. We computed synthetic spectra from 3D simulations of stellar convection resolved both spatially and temporally, and we coupled them with an analytical model reproducing the correct geometry of a transiting exoplanet. We applied the method to the early K-dwarf, HD 189733, and re-analyzed transmission and emission spectroscopy of its hosted exoplanet. In addition, we also analyzed emission spectroscopy of the non transiting exoplanet 51 Pegasi b, orbiting a solar-type star. Results. We find a significant improvement in planet detectability when removing the stellar spectrum with our method. In all cases, we show that the method is superior to a simple parametrisation of the stellar line profile or to the use of 1D stellar models. We show that this is due to the intrinsic treatment of convection in 3D simulations, which allows us to correctly reproduce asymmetric and blue-shifted spectral lines, and intrinsically model center-to-limb variation and Rossiter-McLaughlin effect potentially altering the interpretation of exoplanet transmission spectra. In the case of 51 Pegasi b, we succeed in confirming a previous tentative detection of the planet’s K-band spectrum due to the improved suppression of stellar residuals. Conclusions. Future high-resolution observations will benefit from the synergy with stellar spectroscopy and can be used to test the correct modeling of physical processes in stellar atmospheres. We highlight key improvements in modeling techniques and knowledge of opacity sources to extend this work to shorter wavelengths and later-type stars.

On physical interpretations of the reference transit radius of gas-giant exoplanets

ABSTRACT Two theoretical quandaries involving transmission spectra of gas-giant exoplanets are elucidated. When computing the transit radius as a function of wavelength, one needs to specify a reference transit radius corresponding to a reference pressure. Mathematically, the reference transit radius is a constant of integration that originates from evaluating an integral for the transit depth. Physically, its interpretation has been debated in the literature. Jordán &amp; Espinoza suggested that the optical depth is discontinuous across, and infinite below, the reference transit radius. Bétrémieux &amp; Swain interpreted the spherical surface located at the reference transit radius to represent the boundary associated with an opaque cloud deck. It is demonstrated that continuous functions for the optical depth may be found. The optical depth below and at the reference transit radius need not take on special or divergent values. In the limit of a spatially uniform grey cloud with constant opacity, the transit chord with optical depth of the order of unity mimics the presence of a ‘cloud top’. While the surface located at the reference pressure may mimic the presence of grey clouds, it is more natural to include the effects of these clouds as part of the opacity function because the cloud opacity may be computed from first principles. It is unclear how this mimicry extends to non-grey clouds comprising small particles.

Helium Absorption at 1083 nm from Extended Exoplanet Atmospheres: Dependence on Stellar Radiation

Strong absorption signatures in the helium line at 1083 nm have recently been detected in transmission spectra of several close-in exoplanets. This absorption line originates from neutral helium atoms in an excited, metastable 2$^3$S state. The population of helium atoms in this excited state is governed by the spectral shape and intensity of the incident stellar radiation field. We investigate what kind of stellar environments are most favorable for populating the metastable helium state in extended planetary atmospheres. Our results suggest that planets orbiting at close separations from late-type stars, particularly K stars, are the most promising candidates for transit absorption signals at 1083 nm. This result is supported by observations, as all four exoplanets with currently reported helium detections orbit K-type stars. In general, conditions for exciting helium atoms become more favorable at closer orbital separations, and around stars with higher levels of extreme-ultraviolet (EUV) flux, which ionizes the helium ground state, and lower levels of mid-ultraviolet (mid-UV) flux, which ionizes the helium metastable state.

Aggregate Hazes in Exoplanet Atmospheres

Abstract Photochemical hazes have frequently been used to interpret exoplanet transmission spectra that show an upward slope toward shorter wavelengths and weak molecular features. While previous studies have only considered spherical haze particles, photochemical hazes composed of hydrocarbon aggregate particles are common throughout the solar system. We use an aerosol microphysics model to investigate the effect of aggregate photochemical haze particles on the transmission spectra of warm exoplanets. We find that the wavelength dependence of the optical depth of aggregate particle hazes is flatter than for spheres because aggregates grow to larger radii. Consequently, while spherical haze opacity displays a scattering slope toward shorter wavelengths, aggregate haze opacity can be gray in the optical and near-infrared, similar to those assumed for condensate cloud decks. We further find that haze opacity increases with increasing production rate, decreasing eddy diffusivity, and increasing monomer size, although the magnitude of the latter effect is dependent on production rate and the atmospheric pressure levels probed. We generate synthetic exoplanet transmission spectra to investigate the effect of these hazes on spectral features. For high haze opacity cases, aggregate hazes lead to flat, nearly featureless spectra, while spherical hazes produce sloped spectra with clear spectral features at long wavelengths. Finally, we generate synthetic transmission spectra of GJ 1214b for aggregate and spherical hazes and compare them to space-based observations. We find that aggregate hazes can reproduce the data significantly better than spherical hazes, assuming a production rate that is limited by delivery of methane to the upper atmosphere.

Effects of a fully 3D atmospheric structure on exoplanet transmission spectra: retrieval biases due to day–night temperature gradients

Transmission spectroscopy provides us with information on the atmospheric properties at the limb, which is often intuitively assumed to be a narrow annulus around the planet. Consequently, studies have focused on the effect of atmospheric horizontal heterogeneities along the limb. Here we demonstrate that the region probed in transmission – the limb – actually extends significantly towards the day and night sides of the planet. We show that the strong day–night thermal and compositional gradients expected on synchronous exoplanets create sufficient heterogeneities across the limb that result in important systematic effects on the spectrum and bias its interpretation. To quantify these effects, we developed a 3D radiative-transfer model able to generate transmission spectra of atmospheres based on 3D atmospheric structures. We first apply this tool to a simulation of the atmosphere of GJ 1214 b to produce synthetic JWST observations and show that producing a spectrum using only atmospheric columns at the terminator results in errors greater than expected noise. This demonstrates the necessity for a real 3D approach to model data for such precise observatories. Secondly, we investigate how day–night temperature gradients cause a systematic bias in retrieval analysis performed with 1D forward models. For that purpose we synthesise a large set of forward spectra for prototypical HD 209458 b- and GJ 1214 b-type planets varying the temperatures of the day and night sides as well as the width of the transition region. We then perform typical retrieval analyses and compare the retrieved parameters to the ground truth of the input model. This study reveals systematic biases on the retrieved temperature (found to be higher than the terminator temperature) and abundances. This is due to the fact that the hotter dayside is more extended vertically and screens the nightside – a result of the non-linear properties of atmospheric transmission. These biases will be difficult to detect as the 1D profiles used in the retrieval procedure are found to provide an excellent match to the observed spectra based on standard fitting criteria. This must be kept in mind when interpreting current and future data.

Photochemistry in Hot H2-dominated Exoplanet Atmospheres

Abstract Photochemistry has the potential to substantially impact the atmospheric composition of exoplanets with consequences on the radiative transfer, thermal structure, and dynamics of the atmospheres, particularly in UV-rich stellar environments. Here, we present the results of a first laboratory experimental simulation of photochemistry in carbon-rich exoplanet atmospheres at elevated temperatures. The evolution of gas-phase molecular composition was quantitatively monitored with infrared spectroscopy and mass spectrometry. We found that H2/CO gas compositions can change significantly from thermal equilibria compositions when irradiated with Lyα photons at temperatures ranging from 600 to 1500 K. Carbon dioxide and water were found to be the main products caused by photolysis, while the formation of methane was also observed to a lesser extent. We find that photochemistry efficiency is strongly correlated with increasing temperature. Our finding that water is efficiently produced by photochemistry in a supersolar C/O = 1 environment, representing C enhancement relative to solar values C/O ratio = 0.54, has significant implications for the interpretation of many exoplanet transmission spectra. We also find the formation of an organic solid condensate at 1500 K and under Lyα UV radiation, confirming the possibility of forming photochemical hazes in hot-Jupiter exoplanet atmospheres with an enhanced C/O ratio compared to solar.

Exoplanet transits with next-generation radio telescopes

Nearly everything we know about extrasolar planets to date comes from optical astronomy. While exoplanetary aurorae are predicted to be bright at low radio frequencies (< 1 GHz), we consider the effect of an exoplanet transit on radio emission from the host star. As radio emission from solar-like stars is concentrated in active regions, a planet occulting a starspot can cause a disproportionately deep transit which should be detectable with major radio arrays currently under development, such as the Square Kilometre Array (SKA). We calculate the radiometric sensitivity of the SKA stages and components, finding that SKA2-Mid can expect to detect transits around the very nearest solar-like stars and many cool dwarfs. The shape of this radiometric light curve will be affected by scintillation and lensing from the planet's magnetosphere and thereby encode magnetospheric parameters. Furthermore, these transits will also probe the distribution of stellar activity across a star's surface, and will help scrub out contamination from stellar activity on exoplanet transmission spectra and radial velocity spectra. This radio window on exoplanets and their host stars is therefore a valuable complement to existing optical tools.

Exoplanet Atmosphere Forecast: Observers Should Expect Spectroscopic Transmission Features to be Muted to 33%

To ensure robust constraints are placed on exoplanet atmospheric transmission spectra, future observations need to obtain high signal-to-noise ratio (SNR) measurements assuming smaller amplitude molecular signatures than those of clear solar metallicity atmospheres. Analyzing 37 exoplanet transmission spectra we find clear solar molecular features are measured in <7% of cases.