UMD Astronomers Investigate Eternal Darkness and Daylight on Distant World
Researchers found atmospheric differences between the morning and evening sides of exoplanet WASP-39 b.
Researchers using NASA’s James Webb Space Telescope confirmed predictions that an exoplanet with eternal daylight on one side and eternal darkness on the other will have atmospheric differences between the two halves.
WASP-39 b, a giant gas planet that orbits a star about 700 light-years from Earth, is tidally locked to its parent star. This means that one side of the planet is always exposed to its star, resulting in constant daylight, while the other remains shrouded in darkness. Models predicted that one side would be cooler and cloudier than the other, but there was no observational evidence—until now.
A team of researchers that included University of Maryland astronomers revealed that the evening side of WASP-39 b is hotter than the morning side by about 300 degrees Fahrenheit. Their findings, published in the journal Nature on July 15, 2024, are thanks in large part to a new technique that study co-author and UMD Astronomy Professor Eliza Kempton refers to as “transit mapping.” This process involves measuring the light filtered through an exoplanet’s atmosphere as it moves in front of it its star, then beside it.
By mapping WASP-39 b’s entire journey in relation to its star, the researchers could view the planet as a three-dimensional object rather than just a “dot in the sky,” Kempton explained. They then used this data to learn how heat moves from one side of the planet to the other.
“The atmosphere is the only mechanism to transport direct heat from the star on the day side of the planet around to the night side,” Kempton explained. “We had lots of models for how that heat transport happens and how much heat is transferred, but those are models. You always want to make measurements that test those predictions.”
The advent of the Webb telescope in 2022 also enabled the researchers to take these detailed measurements.
“Researchers had attempted to extract versions of transit mapping signals from certain Hubble Space Telescope datasets, but it had never been done successfully before,” Kempton said. “We really needed a bigger telescope to capture more light and make more sensitive measurements.”
WASP-39 b is similar in mass to Saturn but has a diameter 1.3 times greater than Jupiter, with its “puffy,” extended atmosphere accounting for its large size. This unique feature made WASP-39 b one of the first targets to be analyzed by the Webb telescope in 2022.
“WASP-39 b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb,” said the Nature study’s lead author, Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute. “It has an inflated, puffy atmosphere, so the signal coming from starlight filtered through the planet’s atmosphere is quite strong.”
WASP-39 b was the subject of earlier research, also co-authored by Kempton, which provided the first evidence of carbon dioxide in an exoplanet’s atmosphere in August 2022. In their new paper, the researchers applied the technique of transit mapping to that 2022 dataset to glean new information about WASP-39 b’s atmosphere.
The researchers not only determined that one side of WASP-39 b is much hotter than the other—they also found evidence of different cloud cover, with the morning side of the planet appearing cloudier than the evening side. It is unclear how exactly cloud cover affects temperature and vice versa, but the researchers confirmed that gas circulation around the planet is the main culprit of the temperature difference on WASP-39 b.
Because the temperature difference between the two sides of WASP-39 b is so extreme, the air pressure difference produces high winds. Using three-dimensional models similar to the ones used to predict weather patterns on Earth, the researchers found that the morning side of WASP-39 b gets slammed with winds that have been cooled on the night side, while the evening side is hit by winds heated on the day side. Research suggests the wind speeds on WASP-39 b can reach thousands of miles an hour.
While transit mapping succeeded at casting WASP-39 b’s atmosphere in a new light, Kempton explained that this method is still in its infancy. She hopes this technique will continue to be improved and applied to other exoplanets that are tidally locked to their stars. By studying the diverse array of atmospheres in the universe, exoplanet researchers hope to determine whether certain types of planets—especially “hot Jupiter” gas giants that closely orbit their host stars—share any similarities.
“This is one of the very first attempts to extract a transit mapping signal, so there's still more work to be done to really optimize how we make this measurement,” Kempton said. “But it looks like it's delivering on its promise, and hopefully we'll be able to refine our techniques and apply this to a lot more planets in the future.”
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This article was adapted from text provided by the Space Telescope Science Institute.
In addition to Kempton, co-authors of this study from UMD’s Department of Astronomy include Ph.D. student Arjun Savel (M.S. ’22, astronomy) and former faculty assistant Kenneth Arnold.
Their study, “Inhomogeneous terminators on the exoplanet WASP-39 b,” was published in Nature on July 15, 2024.
This research is supported by NASA (Contract No. NAS 5-03127) and the Heising-Simons Foundation. This article does not necessarily reflect the views of these organizations.
