Exoplanet phase curves: observations and theory

Phase curves are the best technique to probe the three dimensional structure of exoplanets' atmospheres. In this chapter we first review current exoplanets phase curve observations and the particular challenges they face. We then describe the different physical mechanisms shaping the atmospheric phase curves of highly irradiated tidally locked exoplanets. Finally, we discuss the potential for future missions to further advance our understanding of these new worlds.Comment: Fig.5 has been updated. Table 1 and corresponding figures have been updated with new values for WASP-103b and WASP-18b. Contains a table sumarizing phase curve observation

Identification of carbon dioxide in an exoplanet atmosphere

Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of the metal enrichment (that is, elements heavier than helium, also called ‘metallicity’)1–3, and thus the formation processes of the primary atmospheres of hot gas giants4–6. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets7–9. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO2, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification10–12. Here we present the detection of CO2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme13,14. The data used in this study span 3.0–5.5 micrometres in wavelength and show a prominent CO2 absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative–convective–thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO2, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models

Atmospheric circulation of hot Jupiters: dayside–nightside temperature differences. II. Comparison with observations

The full-phase infrared light curves of low-eccentricity hot Jupiters show a trend of increasing fractional dayside–nightside brightness temperature difference with increasing incident stellar flux, both averaged across the infrared and in each individual wavelength band. The analytic theory of Komacek & Showman shows that this trend is due to the decreasing ability with increasing incident stellar flux of waves to propagate from day to night and erase temperature differences. Here, we compare the predictions of this theory with observations, showing that it explains well the shape of the trend of increasing dayside–nightside temperature difference with increasing equilibrium temperature. Applied to individual planets, the theory matches well with observations at high equilibrium temperatures but, for a fixed photosphere pressure of $100\ \mathrm{mbar}$, systematically underpredicts the dayside–nightside brightness temperature differences at equilibrium temperatures less than $2000\ {\rm{K}}$. We interpret this as being due to the effects of a process that moves the infrared photospheres of these cooler hot Jupiters to lower pressures. We also utilize general circulation modeling with double-gray radiative transfer to explore how the circulation changes with equilibrium temperature and drag strengths. As expected from our theory, the dayside–nightside temperature differences from our numerical simulations increase with increasing incident stellar flux and drag strengths. We calculate model phase curves using our general circulation models, from which we compare the broadband infrared offset from the substellar point and dayside–nightside brightness temperature differences against observations, finding that strong drag or additional effects (e.g., clouds and/or supersolar metallicities) are necessary to explain many observed phase curves

Vertical tracer mixing in hot Jupiter atmospheres

Aerosols appear to be ubiquitous in close-in gas giant atmospheres, and disequilibrium chemistry likely impacts the emergent spectra of these planets. Lofted aerosols and disequilibrium chemistry are caused by vigorous vertical transport in these heavily irradiated atmospheres. Here we numerically and analytically investigate how vertical transport should change over the parameter space of spin-synchronized gas giants. In order to understand how tracer transport depends on planetary parameters, we develop an analytic theory to predict vertical velocities and mixing rates (K zz) and compare the results to our numerical experiments. We find that both our theory and numerical simulations predict that if the vertical mixing rate is described by an eddy diffusivity, then this eddy diffusivity K zz should increase with increasing equilibrium temperature, decreasing frictional drag strength, and increasing chemical loss timescales. We find that the transition in our numerical simulations between circulation dominated by a superrotating jet and that with solely day-to-night flow causes a marked change in the vertical velocity structure and tracer distribution. The mixing ratio of passive tracers is greatest for intermediate drag strengths that correspond to this transition between a superrotating jet with columnar vertical velocity structure and day-to-night flow with upwelling on the dayside and downwelling on the nightside. Finally, we present analytic solutions for K zz as a function of planetary effective temperature, chemical loss timescales, and other parameters, for use as input to 1D chemistry models of spin-synchronized gas giant atmospheres

Constraining exoplanet metallicities and aerosols with the contribution to ARIEL spectroscopy of exoplanets (CASE)

Launching in 2028, ESA’s 0.64 m2 Atmospheric Remote-sensing Exoplanet Large-survey (ARIEL) survey of ∼1000 transiting exoplanets will build on the legacies of NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS), and complement the James Webb Space Telescope (JWST) by placing its high-precision exoplanet observations into a large, statistically significant planetary population context. With continuous 0.5–7.8 μm coverage from both FGS (0.5–0.6, 0.6–0.81, and 0.81–1.1 μm photometry; 1.1–1.95 μm spectroscopy) and AIRS (1.95–7.80 μm spectroscopy), ARIEL will determine atmospheric compositions and probe planetary formation histories during its 3.5 yr mission. NASA’s proposed Contribution to ARIEL Spectroscopy of Exoplanets (CASE) would be a subsystem of ARIEL’s Fine Guidance Sensor (FGS) instrument consisting of two visible-to-infrared detectors, associated readout electronics, and thermal control hardware. FGS, to be built by the Polish Academy of Sciences Space Research Centre, will provide both fine guiding and visible to near-infrared photometry and spectroscopy, providing powerful diagnostics of atmospheric aerosol contribution and planetary albedo, which play a crucial role in establishing planetary energy balance. The CASE team presents here an independent study of the capabilities of ARIEL to measure exoplanetary metallicities, which probe the conditions of planet formation, and FGS to measure scattering spectral slopes, which indicate if an exoplanet has atmospheric aerosols (clouds and hazes), and geometric albedos, which help establish planetary climate. Our simulations assume that ARIEL’s performance will be 1.3×the photon-noise limit. This value is motivated by current transiting exoplanet observations: Spitzer/IRAC and Hubble/WFC3 have empirically achieved 1.15×the photon-noise limit. One could expect similar performance from ARIEL, JWST, and other proposed future missions such as HabEx, LUVOIR, and Origins. Our design reference mission simulations show that ARIEL could measure the mass– metallicity relationship of its 1000-planet single-visit sample to >7.5σ and that FGS could distinguish between clear, cloudy, and hazy skies and constrain an exoplanet’s atmospheric aerosol composition to ≳5σ for hundreds of targets, providing statistically transformative science for exoplanet atmospheres