Researchers using NASA’s James Webb Space Telescope have finally confirmed what models had previously predicted: An exoplanet has differences between its eternal morning and eternal evening atmospheres.
WASP-39 b, a giant planet 1.3 times the diameter of Jupiter but with a mass similar to that of Saturn, orbiting a star about 700 light-years from Earth, is tidally locked by its parent star. This means that it has a constant day side and a constant night side – one side of the planet is always exposed to its star, while the other is always shrouded in darkness.
Using Webb’s NIRSpec (near-infrared spectrograph), astronomers confirmed a temperature difference between eternal morning and evening on WASP-39 b, with the evening appearing warmer by about 200 degrees Celsius. They also found evidence for a different cloud cover, with the eternal morning portion of the planet likely cloudier than the evening.
Astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39 b, a technique that studies the exoplanet terminator, the boundary that separates the planet’s dayside from nightside.
A transmission spectrum is made by comparing the starlight filtered through a planet’s atmosphere as it moves in front of the star with the unfiltered starlight detected when the planet is near the star. By making this comparison, researchers can gain information about the temperature, composition, and other properties of the planet’s atmosphere.
Differences between the exoplanet’s morning and evening atmosphere
“WASP-39 has become something of a benchmark planet in the study of exoplanet atmospheres with Webb,” said Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author of the study. “It has a puffy, puffy atmosphere, so the signal from starlight filtering through the planet’s atmosphere is quite strong.”
Webb’s previously published spectra of WASP-39b’s atmosphere, which revealed the presence of carbon dioxide, sulfur dioxide, water vapor, and sodium, represent the entire day/night boundary—there was no detailed attempt to differentiate between a one side and the other, write EurekAlert.
Now, the new analysis constructs two different spectra from the terminator region, effectively dividing the boundary between day and night into two semicircles, one evening and the other morning. The data shows that the evening is significantly hotter with a temperature of 800 degrees Celsius and the morning is relatively cooler with a temperature of 600 degrees Celsius.
Extensive modeling of the obtained data also allows researchers to investigate the structure of WASP-39 b’s atmosphere, cloud cover and why the evening is warmer. While the team’s future work will study how cloud cover can affect temperature and vice versa, astronomers have confirmed the circulation of gases around the planet as the main culprit for the temperature difference on WASP-39 b.
A reference planet
On a heavily irradiated exoplanet like WASP-39 b that orbits relatively close to its star, researchers generally expect the gas to move as the planet orbits its star: the hotter dayside gas should to move towards the night side by means of a strong equatorial jet. Because the temperature difference is so large, the air pressure difference should also be significant, which in turn would cause high wind speeds.
Using general circulation models, three-dimensional models similar to those used to predict weather patterns on Earth, the researchers found that on WASP-39 b the prevailing winds likely shift from the nightside through the morning terminator around the dayside, through the evening terminator and then around the night part.
As a result, the morning part is cooler than the evening part. In other words, the morning side is hit by winds of air that have been cooled on the night side, while the evening side is hit by winds of warmed air on the day side. Research suggests that wind speeds on WASP-39 b can reach thousands of kilometers per hour!
The team’s results were published in the journal Nature.
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Source: www.descopera.ro
