Jayesh Goyal

An ultrahot gas-giant exoplanet with a stratosphere

Infrared radiation emitted from a planet contains information about the chemical composition and vertical temperature profile of its atmosphere. If upper layers are cooler than lower layers, molecular gases will produce absorption features in the planetary thermal spectrum. Conversely, if there is a stratosphere—where temperature increases with altitude—these molecular features will be observed in emission. It has been suggested that stratospheres could form in highly irradiated exoplanets, but the extent to which this occurs is unresolved both theoretically and observationally. A previous claim for the presence of a stratosphere remains open to question, owing to the challenges posed by the highly variable host star and the low spectral resolution of the measurements. Here we report a near-infrared thermal spectrum for the ultrahot gas giant WASP-121b, which has an equilibrium temperature of approximately 2,500 kelvin. Water is resolved in emission, providing a detection of an exoplanet stratosphere at 5σ confidence. These observations imply that a substantial fraction of incident stellar radiation is retained at high altitudes in the atmosphere, possibly by absorbing chemical species such as gaseous vanadium oxide and titanium oxide.

The effects of consistent chemical kinetics calculations on the pressure-temperature profiles and emission spectra of hot Jupiters

In this work we investigate the impact of calculating non-equilibrium chemical abundances consistently with the temperature structure for the atmospheres of highly-irradiated, close-in gas giant exoplanets. Chemical kinetics models have been widely used in the literature to investigate the chemical compositions of hot Jupiter atmospheres which are expected to be driven away from chemical equilibrium via processes such as vertical mixing and photochemistry. All of these models have so far used pressure--temperature (P-T) profiles as fixed model input. This results in a decoupling of the chemistry from the radiative and thermal properties of the atmosphere, despite the fact that in nature they are intricately linked. We use a one-dimensional radiative-convective equilibrium model, ATMO, which includes a sophisticated chemistry scheme to calculate P-T profiles which are fully consistent with non-equilibrium chemical abundances, including vertical mixing and photochemistry. Our primary conclusion is that, in cases of strong chemical disequilibrium, consistent calculations can lead to differences in the P-T profile of up to 100 K compared to the P-T profile derived assuming chemical equilibrium. This temperature change can, in turn, have important consequences for the chemical abundances themselves as well as for the simulated emission spectra. In particular, we find that performing the chemical kinetics calculation consistently can reduce the overall impact of non-equilibrium chemistry on the observable emission spectrum of hot Jupiters. Simulated observations derived from non-consistent models could thus yield the wrong interpretation. We show that this behaviour is due to the non-consistent models violating the energy budget balance of the atmosphere.

Exploring the climate of Proxima B with the Met Office Unified Model

We present results of simulations of the climate of the newly discovered planet Proxima Centauri B, performed using the Met Office Unified Model (UM). We examine the responses of both an “Earth-like” atmosphere and simplified nitrogen and trace carbon dioxide atmosphere to the radiation likely received by Proxima Centauri B. Additionally, we explore the effects of orbital eccentricity on the planetary conditions using a range of eccentricities guided by the observational constraints. Overall, our results are in agreement with previous studies in suggesting Proxima Centauri B may well have surface temperatures conducive to the presence of liquid water. Moreover, we have expanded the parameter regime over which the planet may support liquid water to higher values of eccentricity (≳0.1) and lower incident fluxes (881.7 W m -2 ) than previous work. This increased parameter space arises because of the low sensitivity of the planet to changes in stellar flux, a consequence of the stellar spectrum and orbital configuration. However, we also find interesting differences from previous simulations, such as cooler mean surface temperatures for the tidally-locked case. Finally, we have produced high-resolution planetary emission and reflectance spectra, and highlight signatures of gases vital to the evolution of complex life on Earth (oxygen, ozone and carbon dioxide).

Helium in the eroding atmosphere of an exoplanet

Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres1. Searches for helium, however, have hitherto been unsuccessful2. Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near-infrared transmission spectrum of the warm gas giant3 WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 ± 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 1010 to 3 × 1011 grams per second (0.1–4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure.A detection of helium absorption at 10,833 Å on the exoplanet WASP-107b reveals that its atmosphere is extended and eroding, and demonstrates a new way to study upper exoplanetary atmospheres.

The Complete Transmission Spectrum of WASP-39b with a Precise Water Constraint

WASP-39b is a hot Saturn-mass exoplanet with a predicted clear atmosphere based on observations in the optical and infrared. Here we complete the transmission spectrum of the atmosphere with observations in the near-infrared (NIR) over three water absorption features with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) G102 (0.8-1.1 microns) and G141 (1.1-1.7 microns) spectroscopic grisms. We measure the predicted high amplitude H2O feature centered at 1.4 microns, and the smaller amplitude features at 0.95 and 1.2 microns, with a maximum water absorption amplitude of 2.4 planetary scale heights. We incorporate these new NIR measurements into previously published observational measurements to complete the transmission spectrum from 0.3-5 microns. From these observed water features, combined with features in the optical and IR, we retrieve a well constrained temperature Teq = 1030(+30,-20) K, and atmospheric metallicity 151 (+48,-46)x solar which is relatively high with respect to the currently established mass-metallicity trends. This new measurement in the Saturn-mass range hints at further diversity in the planet formation process relative to our solar system giants.

A Library of ATMO Forward Model Transmission Spectra for Hot Jupiter Exoplanets

J.M.G and N.M are part funded by a Leverhulme Trust Research Project Grant, and in part by a University of Exeter College of Engineering, Mathematics and Physical Sciences PhD studentship. D.K.S, T.E, N.N acknowledges support from the European Research Council under the European Unions Seventh Framework Programme (FP7/2007- 2013)/ ERC grant agreement number 336792. B.D. thanks the University of Exeter for support through a Ph.D. studentship. D.S.A. acknowledges support from the NASA Astrobiology Program through the Nexus for Exoplanet System Science.This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility. This work also used the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter.

Simulating the cloudy atmospheres of HD 209458 b and HD 189733 b with the 3D Met Office Unified Model

S.L is funded by and thankful to support from the Leverhulme Trust. The calculations for this paper were performed on the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS, and the University of Exeter. Material produced using Met Office Software. BD acknowledgesfunding from the European Research Council (ERC) under the European Unions Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no. 336792. D.S.A. acknowledges support from the NASA Astrobiology Program through the Nexus for Exoplanet System Science. GKHL acknowledges support from the Universities of Oxford and Bern through the Bernoulli fellowship program.

The effect of metallicity on the atmospheres of exoplanets with fully coupled 3D hydrodynamics, equilibrium chemistry, and radiative transfer (article)

This work is partly supported by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 Grant Agreement No. 247060-PEPS and grant No. 320478-TOFU). BD acknowledges funding from the European Research Council (ERC) under the European Unions Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no. 336792 and thanks the University of Exeter for support through a PhD studentship. DSA acknowledges support from the NASA Astrobiology Program through the Nexus for Exoplanet System Science. NJM and JG’s contributions were in part funded by a Leverhulme Trust Research Project Grant, and in part by a University of Exeter College of Engineering, Mathematics and Physical Sciences studentship. This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility. This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure. This work also used the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter. Material produced using Met Office Software.

Exonephology: transmission spectra from a 3D simulated cloudy atmosphere of HD 209458b

S. Lines and J. Goyal are funded by and thankful to the Leverhulme Trust. N. J. Mayne is part funded by a Leverhulme Trust Research Project Grant. J. Manners and I. A. Boutle acknowledge the support of a Met Office Academic Partnership secondment. B. Drummond acknowledges funding from the European Research Council (ERC) under the European Unions Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no. 336792. G. K. H. Lee acknowledges support from the Universities of Oxford and Bern through the Bernoulli fellowship program. A. L. Carter is funded by a Science and Technology Facilities Council (STFC) studentship. The calculations for this paper were performed on the University of Exeter supercomputer, a Distributed Research using Advanced Computing (DiRAC) facility jointly funded by STFC, the Large Facilities Capital Fund of the Department for Business, Innovation and Skills (BIS), and the University of Exeter.