JWST peers into the atmosphere of an exoplanet bombarded by stellar radiation

This week, astronomers announced that they’d found evidence of chemical reactions in the atmosphere of an exoplanet 700 light years away from Earth. Researchers using the James Webb Space Telescope created a detailed chemical portrait of the scorching gases swirling around exoplanet WASP-39b. This “hot Saturn” planet orbits extremely close to its host star, meaning it has high temperatures of up to 1,600 degrees Fahrenheit or 900 degrees Celsius. It is also puffy, with around one quarter the mass of Jupiter but 1.3 times its size. 

Early data about WASP-39b was shared this summer when JWST detected carbon dioxide in its atmosphere — the first time this gas had been detected on a planet outside our solar system. Now, a more detailed picture of its atmosphere has been painted in a series of papers posted recently on arXiv, three of which have been accepted for publication in Nature and two of which are under review, as part of a program designed to quickly release observations and data made by the telescope to scientists around the world. The researchers used three of Webb’s instruments, NIRSpec, NIRCam, and NIRISS, to collect spectroscopy information about the planet’s atmosphere.

“We observed the exoplanet with several instruments that together cover a broad swath of the infrared spectrum and a panoply of chemical fingerprints inaccessible until JWST,” said one of the researchers, Natalie Batalha of the University of California, Santa Cruz, in a statement. “Data like these are a game changer.”

In the last decade, astronomy researchers have discovered a plethora of exoplanets, or planets outside our solar system. With more than 5,000 exoplanets confirmed to date, the challenge now is to understand these planets in more depth. More than just knowing an exoplanet’s size or mass, cutting-edge research now focuses on learning about their atmospheres. And tools like JWST are making it possible to see into these far-off atmospheres in more detail than ever before.

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Composition of the atmosphere of WASP-39b
NASA, ESA, CSA, J. Olmsted (STScI)

JWST’s instruments are used to perform a technique called transit spectroscopy. They observe light coming from the host star as it passes through the planet’s atmosphere. This light is split into different wavelengths, and from this, researchers can see which wavelengths have been absorbed. Different chemicals absorb different wavelengths of light, allowing researchers to determine the composition of the planet’s atmosphere.

The research found that there was sodium, potassium, carbon monoxide, and water vapor in the atmosphere, which confirms previous findings that WASP-39b has water vapor in its atmosphere. But it also found sulfur dioxide, the first time this molecule has been detected in an exoplanet atmosphere. Finding these molecules hints at a process similar to that found in Earth’s ozone layer, as sulfur dioxide is formed from chemical reactions in the upper atmosphere caused by light from the host star. 

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“This is the first time we see concrete evidence of photochemistry – chemical reactions initiated by energetic stellar light – on exoplanets,” said another of the researchers, Shang-Min Tsai of the University of Oxford. “I see this as a really promising outlook for advancing our understanding of exoplanet atmospheres with [this mission].”

With WASP-39 b orbiting so close to its host star, at one-eighth the distance between Mercury and the Sun, studying it can show how radiation from stars interacts with planetary atmospheres. While radiation can be harmful to life (Earth is protected from the Sun’s radiation by its magnetosphere, without which the planet could have been uninhabitable), it can also play an important role in chemical reactions creating molecules needed to sustain a habitable atmosphere.

“Planets are sculpted and transformed by orbiting within the radiation bath of the host star,” Batalha said. “On Earth, those transformations allow life to thrive.”

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