(MIT / Patricia Klein and MPIA)
The Hubble Space Telescope has helped astronomers build global maps of the temperature distribution in the stratosphere of the ultra-hot Jupiter WASP-121b.
At the same time, scientists described the processes of circulation of water vapor and refractory metals in the stratosphere.
The dependence of temperature on altitude in the atmospheres of exoplanets plays an important role in understanding the radiative and dynamic processes that take place in atmospheres and govern the distribution of energy.
Hot gas giants located close to their stars and exposed to powerful radiation fluxes are of great interest to scientists, which makes them natural laboratories for studying atmospheres under extreme conditions.
WASP-121b is an ultra-hot Jupiter tidally locked to an F6V star, 850 light-years from the Sun. The year on it lasts 30.6 hours, and the effective temperature is more than two thousand kelvins.
This exoplanet is notable for having the first known stratosphere containing water vapor in 2017.
Observations have shown that a thermal inversion is observed on the dayside of WASP-121b, which can be caused by the presence of substances that absorb radiation from the star, such as Fe, Mg, Cr, V, and VO vapors.
A team of astronomers led by Thomas Mikal-Evans of the Kavli Institute for Astrophysics and Space at MIT has published the results of an analysis of 2018-2019 Hubble Space Telescope infrared spectrograph observations of WASP-121b.
Scientists wanted to study in more detail the structure of the atmosphere of the ultra-hot giant, in particular its stratosphere.
The work resulted in global maps of the stratospheric temperature distribution WASP-121b, both depending on latitude and longitude, and on height.
In the daytime hemisphere, the temperature of the upper atmosphere ranges from 2500 to 3500 kelvins, and in the coldest regions of the night hemisphere it drops below 1500 kelvins with a maximum of 1800 kelvins.
The hottest region of the planet is shifted to the east of the substellar point. Thermal inversion on the dayside is observed at pressures below 30 mbar, which is consistent with the results of earlier studies.
On the dayside of WASP-121b, the water vapor content drops sharply with decreasing pressure due to thermal dissociation of molecules.
Thermal ionization also increases the number of free electrons that bond with atomic hydrogen to form the hydride ion (H-).
As the temperature decreases on the night side, the H2O molecules recombine at low pressures.
Winds are responsible for the circulation of water vapor on the planet, the speed of which reaches five kilometers per second.
Stratospheric temperatures on the nightside of WASP-121b are cold enough to condense refractory particles such as magnesium, iron, and vanadium and condense perovskite (CaTiO3), which can lead to depletion of titanium in the gas phase.
The detection of refractory metals near the exoplanet's terminator suggests that if they do form clouds on the night side, then such a cold trap cannot effectively remove them from the upper atmosphere.
Horizontal winds and vertical mixing can keep these condensed particles in the upper atmosphere of the night hemisphere until they return to the day hemisphere and evaporate.
It is expected that later this year WASP-121b will become a target for observations using the James Webb Space Telescope, which will allow us to determine in more detail the distribution of various substances in its atmosphere.
Source:
Nature Astronomy: https://www.nature.com/articles/s41550-021-01592-w
MIT news: https://news.mit.edu/2022/hot-jupiter-dark-side-
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