First ever hydroxyl radical detected in ‘ultra-hot Jupiter’ atmosphere

27 Apr 2021

Artist’s impression of WASP-33b. Image: Astrobiology Center

Astronomers used a new instrument to detect the hydroxyl radical in the atmosphere of WASP-33b, the exoplanet known as ‘ultra-hot Jupiter’.

An international team of astronomers has detected a new chemical signature in the atmosphere of an exoplanet.

The team, which includes researchers from Trinity College Dublin and Queen’s University Belfast, found the first direct evidence of a hydroxyl radical (OH) in the atmosphere of a planet outside our solar system.

An OH is a type of free radical and the neutral form of the hydroxide ion. In the Earth’s atmosphere, OH is mainly produced by the reaction of water vapour with atomic oxygen. It is a so-called ‘atmospheric detergent’ and plays a crucial role in the Earth’s atmosphere to purge pollutant gasses such as methane and carbon monoxide.

However, this discovery of OH occurred on the dayside of WASP-33b, a gas-giant planet also known as ‘ultra-hot Jupiter’.

The exoplanet orbits its host star much closer than Mercury orbits the sun, which means its atmospheric temperatures are much higher at 2,500 degrees Celsius – hot enough to melt most metals.

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The team used a new instrument, the Infrared Doppler at the Subaru Telescope in Hawaii, which can detect atoms and molecules through their ‘spectral fingerprints’, or unique features superimposed on the spectrums emitted by stars and planets.

These spectral fingerprints change with the velocity of a planet as it orbits its host star, allowing astronomers to separate its signal from the star.

Dr Neale Gibson, assistant professor at Trinity College Dublin and co-author of the study, said the science of exoplanets is relatively new. “A key goal of modern astronomy is to explore these planets’ atmospheres in detail and eventually to search for ‘Earth-like’ exoplanets,” he said.

“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.”

Prof Chris Watson from Queen’s University Belfast, a co-author on the study, added: “While WASP-33b may be a giant planet, these observations are the testbed for the next-generation facilities like the Thirty Meter Telescope and the European Extremely Large Telescope in searching for biosignatures on smaller and potentially rocky worlds, which might provide hints to one of the oldest questions of humankind: ‘Are we alone?’”

Their study has been published in the Astrophysical Journal Letters.

Jenny Darmody is the deputy editor of Silicon Republic

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