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6 molecules seen on exoplanet

Gas giant likely formed far from its star before wandering closer

by Sam Lemonick
April 17, 2021 | A version of this story appeared in Volume 99, Issue 14

Illustration of the exoplanet HD 209458b passing in front of its star.
Credit: University of Warwick/Mark Garlick
The ratio of C to O suggest exoplanet HD 209458b, seen in this artistic rendering, formed far from its star before moving closer.

Scientists have identified six molecules, the most detected simultaneously to date, in the atmosphere of a planet nearly 160 light years away (Nature 2021, DOI: 10.1038/s41586-021-03381-x). The group spotted infrared signatures of H2O, CO, HCN, CH4, NH3, and C2H2 gases circling the exoplanet HD 209458b, a gas giant roughly the size of Jupiter that orbits its star at about one-eighth the distance Mercury orbits the sun. Only H2O, CO, and HCN had been previously observed in HD 209458b’s atmosphere. Study coauthor Matteo Brogi of the University of Warwick and the Italian National Institute for Astrophysics says the new discoveries are thanks to more powerful equipment and the work of other scientists, who used theoretical and experimental methods to deduce the spectroscopy fingerprints his group looked for. The data suggest HD 209458b has about equal amounts of carbon and oxygen. Scientists generally expect a carbon-to-oxygen ratio of 1:2. Brogi and his collaborators propose that the planet may have formed farther from the star, where H2O would be frozen and not available to be swirled into a gas planet. In that scenario, HD 209458b would have moved closer to the star later in its evolution. But the researchers also acknowledge that there’s a lot that’s unknown about chemical and physical processes on exoplanets, and that our assumptions about them may be incorrect. Brogi says the team’s subsequent work on other exoplanets also shows that theories don’t always line up with observations. Experiments, observations, and models will be necessary as scientists learn more about distant planets, he says.



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