JWST finds sulphur dioxide for the first time on an exoplanet

Although you might not have heard of WASP-39 b, it’s a minor celebrity when it comes to planets.

The James Webb Space Telescope (JWST) has ‘hot Saturn’ in its sights, and in August showed the first clear detection of carbon dioxide in the planet. Now, a series of five papers published on arXiv (currently under review at Nature) has found evidence of another surprising gas – sulphur dioxide.

This is the first-time sulphur dioxide has been discovered in an exoplanet’s atmosphere. The 300-astronomer strong team also discovered the first detection of carbon monoxide in WASP-39b’s atmosphere.

This, and other new information from JWST potentially makes WASP-39b the best explored planet outside the solar system.

“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 JWST,” said one of the team, University of California, Santa Cruz astronomer Natalie Batalha.

“Data like these are a game changer.”

WASP-39 b, (or the International Astronomical Union official name Bocaprins) is around 698 light-years from Earth and is in the area of the Virgo constellation. The exoplanet is what’s known as a ‘hot Saturn’. This means exactly what it sounds like – the planet is very hot and is about the same size of Saturn. WASP-39 b is also ‘puffy’ – it’s less than half the mass of Jupiter but has a radius of 1.27 times the size.  A real marshmallow of a planet.

It’s also incredibly close to its host star – whizzing around the star every four days. This means it would be a bit balmy – temperatures would reach up to 900 °C.

We only know the planet is there because of tiny changes in the light levels of the star it’s orbiting – called WASP-39. You can see how this works below with another exoplanet – WASP-96 b.

Exoplanet wasp 96 b transit light curve
Credit: NASA, ESA, CSA, and STScI

The JWST has a number of instruments perfect for needling the atmosphere of far-flung exoplanets. It carries the Near InfraRed Spectrograph (NIRSpec), the Near InfraRed Camera (NIRCam) and the Near InfraRed Imager and Slitless Spectrograph (NIRISS), which were all used for the new studies. These sensors are able to detect which wavelengths are missing after the light goes through the exoplanet’s atmosphere in incredible detail. 

“I was blown away when I first saw the planet’s spectrum. I knew we had something remarkable, in terms of the level of structure that was in the spectrum and the precision we achieved,” said Imperial College London astrophysicist Dr James Kirk.

“The level of detail provided by JWST is revolutionary. It is incredibly exciting to think that we’re only at the dawn of the JWST era.”

The new papers found that a photochemical reaction is possibly taking place to create the sulphur dioxide. This occurs when the light from the star hits the atmosphere, and the team think that atmospheric water is split into hydrogen and hydroxide, which then reacts with hydrogen sulphide to produce the sulphur dioxide. This is the first time a photochemical by-product has been detected on an exoplanet.

The papers also shed light onto the amount of cloud cover on WASP-39b. It seems that the clouds are not a uniform blanket over the planet and could instead be broken up. JWST isn’t just looking at WASP-39 b. In it’s first year of science it’s going to be observing around 70 exoplanets. But WASP-39 b is a benchmark for those studies, allowing us to uncover secrets about our nearest exoplanets neighbours.

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