Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b

Abstract: The temperature profile of a planetary atmosphere is a key diagnostic of radiative and dynamical processes governing the absorption, redistribution, and emission of energy. Observations have revealed dayside stratospheres that either cool or warm with altitude for a small number of gas giant exoplanets, while other dayside stratospheres are consistent with constant temperatures. Here we report spectroscopic phase curve measurements for the gas giant WASP-121b, which constrain stratospheric temperatures throughout the diurnal cycle. Variations measured for a water vapour spectral feature reveal a temperature profile that transitions from warming with altitude on the dayside hemisphere to cooling with altitude on the nightside hemisphere. The data are well explained by models assuming chemical equilibrium, with water molecules thermally dissociating at low pressures on the dayside and recombining on the nightside. Nightside temperatures are low enough for perovskite (CaTiO3) to condense, which could deplete titanium from the gas phase and explain recent non-detections at the day-night terminator. Nightside temperatures are also consistent with the condensation of refractory species such as magnesium, iron, and vanadium. Detections of these metals at the day-night terminator suggest, however, that if they do form nightside clouds, cold trapping does not efficiently remove them from the upper atmosphere. Horizontal winds and vertical mixing could keep these refractory condensates aloft in the upper atmosphere of the nightside hemisphere until they are recirculated to the hotter dayside hemisphere and vaporised.

• The paper demonstrates that spectroscopic phase curve measurements reveal an asymmetric thermal profile between the warming dayside and cooling nightside of WASP-121b.
• It employs general circulation models to capture the interplay of radiation, chemistry, and dynamics, highlighting the role of water thermal dissociation and metal absorption.
• The findings suggest vigorous vertical mixing in ultrahot Jupiter atmospheres and emphasize the need for further spectroscopic studies with instruments like the JWST.

Insights on the Diurnal Variations in the Stratosphere of the Ultrahot Giant Exoplanet WASP-121b

The study on the diurnal variations in the stratosphere of WASP-121b offers a comprehensive examination of the thermal and chemical dynamics occurring in the atmosphere of this ultrahot gas giant exoplanet. WASP-121b, with temperatures exceeding 2,000 Kelvin, exists in close proximity to its host F6V star, completing an orbit every 30.6 hours. This paper presents spectroscopic phase curve measurements that reveal a dynamic temperature profile and intricate chemical processes.

Key Findings

The primary finding of this study is the asymmetric thermal profile across the exoplanet's atmosphere, characterized by a shift from a warming trend with altitude on the dayside to a cooling trend on the nightside. Such variations are attributed to water molecules that undergo thermal dissociation on the dayside, transitioning to recombination on the nightside due to cooler temperatures. This distinction is evidenced by spectroscopic data that targets a water vapor spectral feature.

A noteworthy observation is the presence of a dayside thermal inversion, where temperature increases with altitude, driven by the absorption of stellar radiation by gaseous species such as Fe, Mg, Cr, and VO. This phenomenon has been substantiated by the detection of these metals in the planetary atmosphere during transit observations. Crucially, despite the presence of thermal inversion constituents, the nightside shows a substantial cooling to temperatures that promote condensation of refractory species like magnesium, iron, and vanadium.

Implications for Atmospheric Dynamics

This study's results underscore the complexity of atmospheric dynamics on ultrahot Jupiters. For WASP-121b, the contrasting temperature profiles between the hemispheres suggest vigorous vertical mixing processes. These processes could prevent the efficient cold trapping of refractory species on the nightside, allowing them to be retained in the gas phase and recirculated to the hotter dayside.

The research further demonstrates the importance of general circulation models (GCMs) in replicating atmospheric conditions. While the GCMs used in this study satisfactorily capture the interplay of radiation, chemistry, and dynamics on WASP-121b, they acknowledge the limitations posed by absent metal opacities and cloud formation, particularly clouds that may influence emission observations at terminator regions.

Future Research Directions

Ongoing and future observations, particularly with instruments like the James Webb Space Telescope, will enhance our understanding of such atmospheric processes. By extending spectroscopic coverage across broader wavelengths, researchers aim to break down the opacity interplay of different species and refine the compositional and thermal models of ultrahot Jupiters.

Overall, the paper contributes significantly to our understanding of the atmospheric behaviors of ultrahot Jupiters, providing a framework for observing and interpreting similar exoplanetary systems. The study highlights the intricate balance of radiative and dynamic processes shaping atmospheres vastly different from those within our solar system and sets the stage for further explorations into the exotic environments of closely orbiting exoplanets.