Astronomers have detected a hydroxyl radical (OH) — the neutral form of the hydroxide ion — in the dayside atmosphere of the ultrahot Jupiter exoplanet WASP-33b. It is the first time that OH has been detected in the planetary atmosphere outside the Solar System.
An artist’s impression of the ultrahot Jupiter WASP-33b and its parent star. Image credit: Astrobiology Center.
Discovered in 2010, WASP-33b is a gas giant approximately 4.5 times the mass of Jupiter.
The planet orbits WASP-33, a young star located 380 light-years away in the northern constellation of Andromeda.
WASP-33b is much closer to the star than Mercury is to the Sun, and reaches atmospheric temperatures of more than 2,500 degrees Celsius.
“The science of extrasolar planets is relatively new, and a key goal of modern astronomy is to explore these planets’ atmospheres in detail and eventually to search for Earth-like exoplanets,” said Dr. Neale Gibson, an astronomer in the School of Physics at Trinity College Dublin.
“Every new atmospheric species discovered further improves our understanding of exoplanets and the techniques required to study their atmospheres, and takes us closer to this goal.”
In the study, Dr. Gibson and colleagues analyzed high-resolution near-infrared emission spectra of WASP-33b taken using the InfraRed Doppler (IRD) spectrograph on the 8.2-m Subaru Telescope.
“IRD is the best instrument to study the atmosphere of an exoplanet in the infrared,” said Professor Motohide Tamura, director of the Astrobiology Center in Japan.
“These techniques for atmospheric characterization of exoplanets are still only applicable to very hot planets, but we would like to further develop instruments and techniques that enable us to apply these methods to cooler planets, and ultimately, to a second Earth,” said Dr. Hajime Kawahara, an astronomer in the Department of Earth and Planetary Science and the Research Center for the Early Universe at the University of Tokyo.
The astronomers were able to detect the emission signature of OH in the dayside of WASP-33b.
“This is the first direct evidence of OH in the atmosphere of a planet beyond the Solar System,” said Dr. Stevanus Nugroho, an astronomer in the Astrobiology Center, the National Astronomical Observatory of Japan, and the School of Mathematics and Physics at Queen’s University Belfast.
“It shows not only that astronomers can detect this molecule in exoplanet atmospheres, but also that they can begin to understand the detailed chemistry of this planetary population.”
“We see only a tentative and weak signal from water vapor in our data, which would support the idea that water is being destroyed to form hydroxyl in this extreme environment,” added Dr. Ernst de Mooij, an astronomer in the School of Mathematics and Physics at Queen’s University Belfast.
The findings were published in the Astrophysical Journal Letters.
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Stevanus K. Nugroho et al. 2021. First Detection of Hydroxyl Radical Emission from an Exoplanet Atmosphere: High-dispersion Characterization of WASP-33b Using Subaru/IRD. ApJL 910, L9; doi: 10.3847/2041-8213/abec71
