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Space telescope gives scientists most detailed look yet at an exoplanet’s atmosphere

New data collected by the James Webb Space Telescope provides the first hard evidence of planetary photochemistry outside our solar system.

(CN) — NASA announced Tuesday that an international team of scientists has compiled the most thorough chemical profile of an exoplanet's atmosphere to date, thanks to readings captured by the James Webb Space Telescope.

The exoplanet in question, WASP-39b, is a "hot Saturn" gas giant orbiting a star slightly smaller than the sun, about 700 light years from Earth. Even before Webb began collecting atmospheric data on it, the planet's strange characteristics - to Earthling eyes, at least - have made it a particularly alluring target of study since its 2011 discovery.

It has a mass only about a quarter that of Jupiter, despite being over 25% wider, and orbits its parent star at an average distance of 7.2 million kilometers. That's five times closer than the average orbit of Mercury, the innermost planet in our own solar system. And while it takes Mercury 88 Earth days to circle the sun, a year on WASP-39b lasts only four days, with its gaseous surface soaring to temperatures above 900 degrees Celsius (1,652 degrees Fahrenheit).

Now the distant gas giant is providing even more valuable data: the readings captured by Webb show evidence of sulfur dioxide in WASP-39b's clouds, the first time that chemical has been detected in an exoplanet's atmosphere. The presence of sulfur dioxide also indicates that the surface of WASP-39b is host to complex photochemistry, another first in exoplanet research.

“This is the first time we see concrete evidence of photochemistry – chemical reactions initiated by energetic stellar light – on exoplanets,” said Shang-Min Tsai, an astrochemistry researcher at the University of Oxford in the United Kingdom.

Scientists detected evidence of other chemicals in WASP-39b's atmosphere, including carbon dioxide, carbon monoxide and water vapor. To pinpoint these materials, they used a process known as spectroscopy: Webb tracked WASP-39b as it transited in front of its parent star and captured the light of that star as it passed through the gas giant's atmosphere. As on Earth, the chemicals in WASP-39b's atmosphere refract certain wavelengths of light and absorb others. Analyzing the wavelengths of light Webb's infrared cameras detected - or more precisely, those they didn't detect - allowed scientists to reconstruct WASP-39b's atmospheric composition.

"Different types of chemicals in the atmosphere absorb different colors of the starlight spectrum, so the colors that are missing tell astronomers which molecules are present," NASA stated in a Tuesday press release. "By viewing the universe in infrared light, Webb can pick up chemical fingerprints that can’t be detected in visible light."

Spectroscopic analysis of WASP-39b's atmosphere shows which chemicals are present - and which aren't - in its scorching clouds. (Joseph Olmsted/NASA)

The range of chemicals detected indicates that WASP-39b was likely formed by the collision of numerous smaller planetoids, and possibly further out from its star than its current orbit, said University of California Santa Cruz exoplanet researcher Kazumasa Ohno.

“The abundance of sulfur [relative to] hydrogen indicated that the planet presumably experienced significant accretion of planetesimals... The data also indicates that the oxygen is a lot more abundant than the carbon in the atmosphere," Ohno said. "This potentially indicates that WASP-39b originally formed far away from the central star.”

Webb's readings also suggest that the outer clouds of WASP-39b cover the planet unevenly, NASA said.

"The latest data... gives a hint of how these clouds might look up close: broken up rather than a single, uniform blanket over the planet," NASA said.

Tsai, Ohno and other scientists are still combing Webb's data to see what other conclusions can be drawn about WASP-39b, and about exoplanet formation in general. The discoveries made by observing WASP-39b already fill five new scientific papers, NASA said Tuesday, and the next step may be to point Webb at smaller, rockier exoplanets more similar to Earth.

“We observed the exoplanet with multiple instruments that, together, provide a broad swath of the infrared spectrum and a panoply of chemical fingerprints inaccessible until [now],” said Natalie Batalha, another astronomer at the University of California, Santa Cruz. “Data like these are a game changer.”

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