Webb discover methane in an exoplanet’s atmosphere

The James Webb Space Telescope conducted observations of the exoplanet WASP-80 b as it transited both in front of and behind its host star. These observations unveiled spectra indicating the presence of an atmosphere containing methane gas and water vapor. Although water vapor has been identified in over a dozen planets, the detection of methane, a molecule abundant in the atmospheres of Jupiter, Saturn, Uranus, and Neptune within our solar system, had proven elusive in the atmospheres of transiting exoplanets when examined through space-based spectroscopy, until recently.

An artist’s rendering of the warm exoplanet WASP-80 b whose color may appear bluish to human eyes due to the lack of high-altitude clouds and the presence of atmospheric methane identified by NASA’s James Webb Space Telescope, similar to the planets Uranus and Neptune in our own solar system. Image credit: NASA.

Taylor Bell, affiliated with the Bay Area Environmental Research Institute (BAERI) and based at NASA’s Ames Research Center in California’s Silicon Valley, along with Luis Welbanks from Arizona State University, delve into the significance of discovering methane in exoplanet atmospheres. They discuss how observations made by the Webb telescope played a crucial role in identifying this long-sought-after molecule. These noteworthy findings were recently published in Nature.

“WASP-80 b, classified as a ‘warm Jupiter’ with a temperature of approximately 825 Kelvin (about 1,025 degrees Fahrenheit), shares similarities in size and mass with Jupiter in our solar system. However, it exhibits a temperature range that falls between that of hot Jupiters, such as the 1,450-K (2,150-F) HD 209458 b (the first transiting exoplanet discovered), and cold Jupiters, like our own, which is about 125 K (235 F).”

“WASP-80 b completes an orbit around its red dwarf star every three days and is located 163 light-years away from us in the constellation Aquila. Due to the proximity of the planet to its star and the considerable distance of both from Earth, direct observation of the planet, even with advanced telescopes like Webb, is not possible. Instead, researchers employ the transit method (widely used in discovering most known exoplanets) and the eclipse method, studying the combined light from the star and the planet.”

“Using the transit method, we observed the system as the planet passed in front of its star from our viewpoint, causing a slight dimming of the starlight, akin to someone passing in front of a lamp and dimming the light.”

“During this phase, a narrow band of the planet’s atmosphere along the day/night boundary is illuminated by the star. At specific wavelengths where molecules in the planet’s atmosphere absorb light, the atmosphere appears denser, leading to a more pronounced dimming compared to wavelengths where the atmosphere is transparent. This method aids scientists in understanding the composition of the planet’s atmosphere by identifying the blocked colors of light.”

“Simultaneously, using the eclipse method, we observed the system as the planet moved behind its star from our perspective, causing another minor dip in the total light received. All objects emit thermal radiation, with the intensity and color depending on the object’s temperature.”

“Before and after the eclipse, the planet’s hot dayside faces us, and by measuring the dip in light during the eclipse, we could gauge the infrared light emitted by the planet. In eclipse spectra, the absorption by molecules in the planet’s atmosphere typically manifests as a reduction in the planet’s emitted light at specific wavelengths. Moreover, since the planet is smaller and cooler than its host star, the eclipse’s depth is much smaller than that of a transit.”

The measured transit spectrum (top) and eclipse spectrum (bottom) of WASP-80 b from NIRCam’s slitless spectroscopy mode on NASA’s James Webb Space Telescope. In both spectra, there is clear evidence for absorption from water and methane whose contributions are indicated with colored contours. During a transit, the planet passes in front of the star, and in a transit spectrum, the presence of molecules makes the planet’s atmosphere block more light at certain colors, causing a deeper dimming at those wavelengths. During an eclipse, the planet passes behind the star, and in this eclipse spectrum, molecules absorb some of the planet’s emitted light at specific colors, leading to a smaller dip in brightness during the eclipse compared to a transit. Image Credit: BAERI/NASA/Taylor Bell.

“The initial observations we conducted required transformation into a spectrum, essentially a measurement depicting the extent to which the planet’s atmosphere either obstructs or emits light across different colors (or wavelengths). Various tools are available to convert raw observations into meaningful spectra, and to ensure the robustness of our findings, we employed two distinct approaches.”

“Subsequently, we interpreted this spectrum using two types of models to simulate the appearance of a planet’s atmosphere under extreme conditions. The first model, characterized as entirely flexible, explored millions of combinations of methane and water abundances and temperatures to identify the combination aligning best with our data. The second type, termed ‘self-consistent models,’ also explored millions of combinations but utilized existing knowledge of physics and chemistry to predict methane and water levels.”

“Both model types led to the same conclusion: a conclusive detection of methane.”

“To validate our results, we applied rigorous statistical methods to assess the likelihood of our detection being random noise. In our field, achieving a ‘5-sigma detection’ is considered the ‘gold standard,’ where the chances of detection resulting from random noise are 1 in 1.7 million. In contrast, our methane detection reached 6.1-sigma in both transit and eclipse spectra, establishing the odds of a spurious detection in each observation at 1 in 942 million. This surpasses the 5-sigma ‘gold standard,’ reinforcing our confidence in both detections.”

“With such a confident detection, not only did we identify a highly elusive molecule, but we can now delve into what this chemical composition reveals about the planet’s origin, development, and evolution. For instance, by gauging the methane and water content, we can infer the carbon-to-oxygen ratio, expected to vary depending on a planet’s formation location and timeline.”

“This discovery also excites us due to the opportunity to compare planets beyond our solar system with those within it. NASA has historically sent spacecraft to the gas giants in our solar system to measure methane and other molecules in their atmospheres. Now, with measurements of the same gas in an exoplanet, we can conduct a direct comparison, allowing us to see if our solar system’s expectations align with observations outside of it.”

“As we anticipate future discoveries with the Webb telescope, this outcome suggests that more exciting findings are on the horizon. Further observations of WASP-80 b with Webb’s MIRI and NIRCam instruments will enable us to explore the atmosphere’s properties across different light wavelengths. Our findings lead us to anticipate the observation of other carbon-rich molecules, such as carbon monoxide and carbon dioxide, contributing to a more comprehensive understanding of this planet’s atmospheric conditions.”

“Moreover, as we identify methane and other gases in exoplanets, our knowledge of chemistry and physics under conditions distinct from Earth will continue to expand. Perhaps soon, we’ll gain insights into other planets that resemble our own home. One thing is evident—the journey of discovery with the James Webb Space Telescope holds the promise of potential surprises.”

More information: Taylor Bell, Methane throughout the atmosphere of the warm exoplanet WASP-80b, Nature (2023). DOI: 10.1038/s41586-023-06687-0. www.nature.com/articles/s41586-023-06687-0

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