For the first time, “morning” and “evening” side atmospherics have been measured on a tidally locked exoplanet, WASP-39 b, 700 light years from Earth.
Unlike in our solar system, some distant planets complete an orbit around their sun and a daily rotation in the same amount of time, creating eternal night and day halves of the planet. Now, scientists have analyzed the atmosphere of WASP-39 b to investigate what happens where these halves converge.
Scientists Look at the Atmosphere of WASP-39 b
Launched on Christmas Day 2021, the James Webb Space Telescope (JWST) has been providing scientists with new data from further out in space than humans were previously able to measure. Astronomers have long speculated about the conditions that may exist far out in deep space on exoplanets, which are planets beyond Earth’s solar system.
WASP-39 b is a case where scientists’ predictions about this distant exoplanet turned out to be remarkably accurate. The journal Nature posted an early manuscript version of a new study, led by the Space Telescope Science Institute, providing a synthesis of various techniques used by different team members. For example, one looked at the light curve of each side of the planet in isolation, while the lead author modeled the entire planet as a sphere with two hemispheres of slightly different radii.
All Eyes on a “Hot Jupiter”
While it was only discovered in 2011, scientists have investigated the exoplanet for years. This new study builds upon atmospheric analysis performed in 2022 after it became the first exoplanet to be studied by the JSWT. Even that previous work was described as “the most detailed analysis of an exoplanet atmosphere ever” at that time. The 2022 study identified the presence of sulfur dioxide, leading the International Astronomical Union to give the planet the designation “Bocaprins,” after a beach in Aruba.
To put the planet into better perspective, it’s a gas giant 1.27 times the size of Jupiter but with only 0.28 times the mass. Since WASP-39’s rotation is synched to its orbit in a 1:1 ratio, one side is perpetually dark, while the other always faces the sun. Part of what enables this is the planet’s proximity to its sun, leaving the dayside with a blistering 1000-degree surface temperature.
This isn’t merely an astronomical quirk exclusive to Earth’s moon and WASP-39b; scientists have observed many exoplanets with very close solar orbits like WASP-39b. In fact, Jupiter-like planets possessing a tight near-solar orbit are common enough to have their own designation as “hot Jupiters.”
Measuring Morning and Evening on an Exoplanet
Where these sides meet, distinct “morning” and “evening” zones were identified. A major finding of this study was the differences between the zones’ atmospheres, as previous studies of exoplanets have assumed a homogenous atmosphere. The stability of the temperature divergence between the light and dark sides of the planets results in a unidirectional wind blowing along the equator. This wind pushes cold air from the planet’s night side into the morning zone and hot air from the day side into the evening zone. As a result, the morning zone is substantially cooler, at 600 C, than the evening zone, which is at 800 C. The morning also has much higher cloud cover than the evening due to the impact of temperature on cloud formation, as the team expected.
The methodology used in the study involved novel approaches for measuring light. When a planet moves across its star, the James Webb Space Telescope measures the light coming off it. The star producing this light is very similar to the Earth’s sun, although a bit cooler. This light comes from the edges of the planet’s disc as it would appear from the telescope’s point of view. In the case of a planet like WASP-39 b, the edges would be from the morning and evening zones, as they continually hold the light-filtering edges of the disc.
With Hypotheses Confirmed, Scientists Look Toward Expanding Research
When asked if the team would study other exoplanets to confirm the results, Dr. James Kirk of the Imperial College London Physics Department said that they were more interested in remaining focused on WASP-39 b but viewing other wavelengths of light. Between the exciting results of this study and the untapped potential of other highly precise instruments on board the JWST, the team anticipates further expansions of their understanding of the planet’s atmosphere.
“Now we’ve demonstrated the feasibility of this method with JWST, and the precision of JWST is so immense, it really opens up a new avenue into understanding and measuring atmospheric circulation for exoplanets that we were previously largely insensitive to,” said Dr. Kirk.
Despite the team’s initial focus on WASP-39 b, once the exoplanet is well understood, the process the team has developed will applied to other candidate planets, and the team will investigate their different temperatures, masses, and radii.
The team’s recent paper, “Inhomogeneous terminators on the exoplanet WASP-39 b,” appeared in the journal Nature on July 15, 2024.
Ryan Whalen is a writer based in New York. He has served in the Army National Guard and holds a BA in History and a Master of Library and Information Science with a certificate in Data Science. He is currently finishing an MA in Public History and working with the Harbor Defense Museum at Fort Hamilton, Brooklyn.