Courtesy of NASA, ESA, CSA, Joseph Olmsted (STScI)

A depiction of what exoplanet WASP-39b is expected to look like based off of models and simulations.

March 22, 2023

Cornell Astronomers Reveal Chemistry of Exoplanet Atmospheres, James Webb Telescope 

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Cornell astronomers Jake Turner grad, Yu-Cian Hong grad and Research Associate Laura Flagg published a paper on Jan. 9 revealing a broad-wavelength atmospheric transmission spectrum of gas giant exoplanet WASP-39b. The paper also revealed the James Webb Space Telescope’s sensitivity to a diversity of exoplanet compositions and chemistry — meaning astronomers can now detect active chemical processes happening in an exoplanet’s atmosphere. 

JWST conducts infrared astronomy through high-resolution and high-sensitivity instruments, which capture and relay data to be interpreted via mediums such as the transmission spectrum, a graph of a planet’s apparent change in size as a function of wavelength of light from the host star. Transmission spectrums can provide information on gaseous particles, haze and clouds in atmospheres of exoplanets, which are planets that orbit outside of the solar system. 

“We’ve been asking questions about exoplanet atmospheres for a long time and JWST is now the best tool we have to answer them,” Turner, a postdoctoral fellow at the Cornell Center for Astrophysics and Planetary Sciences, said.

WASP-39b’s wide radius, low surface gravity and relatively clear skies make its atmosphere ideal for analysis. In 2018, the Hubble and Spitzer Space Telescopes revealed water on WASP-39b, while JWST detected carbon dioxide in its atmosphere in 2022. The new observations from JWST offer a more accurate understanding of the planet’s atmosphere — such as the presence of carbon monoxide, sodium, potassium and sulfur dioxide.

Flagg explained that the team expected to just detect water and carbon dioxide but also found sulfur dioxide.

The presence of sulfur dioxide also revealed photochemistry happening in the exoplanet’s atmosphere. Photochemistry refers to chemical processes that occur in the presence of light, as sulfur dioxide forms from a series of chemical reactions when hydrogen sulfide interacts with high-energy ultraviolet light particles and hydrogen atoms. 

“Everything we’re doing now is really cool, and it’s leading to some of the most interesting questions you can ask,” Flagg said. “But we’re also building to the bigger picture by making sure we refine our techniques and really understand the data before we get down to planets with weaker signals, which are harder to detect.”  

The data from JWST allows astronomers to look at the light of a star as it passes through gasses in the atmosphere of the planet, accessing a wide range of colors of light — including the infrared, which are low energy electromagnetic radiations. 

When a planet crosses directly between its star and Earth, it may block a portion of the light from its star, causing it to dim. In order to measure the percentage of the star’s light that the planet blocked and the time it took for the planet to cross the star’s disk, the astronomers plotted a graph of the brightness of the respective star as the time passed. The types of gasses released on the planet are then identified by measuring absorbed wavelengths of a star’s transmitted light.

“By looking at the different types of gasses that are present in the atmosphere, we get clues as to where the planet formed,” said Emily Deibert, a postdoctoral science fellow at the Gemini South Observatory in Chile.

If a planet is formed away from its star, it has a different composition of gasses compared to if it develops closer to the star. Planets and their atmospheres come from the same material as their parent star, which forms from a giant cloud of dust and gas. 

A planet’s atmosphere also obstructs more light at some wavelengths and less light at other wavelengths. Studying the differences between them can reveal information about the composition, size and density of the planet’s atmosphere. 

The images taken by JWST are expected to clarify these differences, allowing astronomers to better understand terrestrial planets beyond the solar system. 

“There is going to be so much more coming from James Webb,” Deibert said. “We are expecting to learn a lot more from spectral signatures of other exoplanets.”