Astronomers have detected molecules transformed by sunlight in the atmosphere of an alien planet for the first time.
The conversion of molecules by sunlight is seen on many planets in our Solar System, including Earth, where the process creates ozone.
The discovery of molecules of sulphur dioxide in the atmosphere of a big gas giant known as WASP-39b by the James Webb Space Telescope (JWST) shows this process also happens on planets outside our Solar System.
"This is the first time we see concrete evidence of photochemistry — chemical reactions initiated by energetic stellar light — on exoplanets," Shang-Min Tsai, a researcher at the University of Oxford in the United Kingdom and lead author of a paper explaining the origin of sulphur dioxide in WASP-39b's atmosphere, said in a statement.
WASP-39b is a "hot Saturn", a giant gas planet about the size of Jupiter, but with the mass of Saturn.
Earlier this year, molecules of carbon dioxide were detected around the exoplanet, which lies some 700 light-years away.
The latest analyses, presented in a suite of five papers on the pre-print website Arxiv, provide the first full picture of the alien world's atmosphere.
The detection of sulphur dioxide provides hope that similar signatures might be found around smaller Earth-like exoplanets that lie in their star's habitable zone, commented Nataliea Lowson, an astrophysicist who studies exoplanet atmospheres but is not part of the JWST team.
"This photo chemistry is related to processes on Earth that we know are linked to biosignatures," said Ms Lowson of the University of Southern Queensland.
How does sunlight make sulphur dioxide?
The JWST detected the molecules using a technique known as spectrography, which breaks down light passing through the planet's atmosphere as it crosses in front of its Sun. Each wavelength of light corresponds to a different chemical.
The JWST teams detected much more sulphur dioxide than they expected.
"This means there must be external process at play to maintain the high abundance that they see," Ms Lowson said.
The scientists suspect sunlight is breaking down hydrogen sulphide and water into components — sulphur and oxygen — that make sulphur dioxide.
"This is a process known to occur on giant planets," Ms Lowson said.
However, the JWST team was yet to confirm the presence of hydrogen sulphide in the planet's atmosphere.
"That doesn’t mean it's not there; it might be visible in another wavelength region they didn't observe in," she said.
What else did they discover about this planet?
Along with sulphur dioxide, the JWST teams also detected carbon dioxide, carbon monoxide, sodium, potassium and water vapour.
The amounts of different types of chemicals not only tell us more about the composition of WASP-39b's atmosphere, but how it formed.
Ms Lowson said the ratio of carbon to oxygen indicated the planet formed in a similar way to Jupiter and Saturn.
"The composition from these observations tells us that WASP-39b has roughly the same metal richness as Jupiter and Saturn; three to 10 times that of our Sun," she said.
This composition indicated the planet began its life much further away from its star, said Jonti Horner, also of the University of Southern Queensland.
"This is really exciting," said Professor Horner, who was not involved with the discovery.
"It confirms all our theories, which have said these [large hot planets] form further out and then migrate in."
So where do we go next?
Professor Horner said the new results indicate the JWST will be able to study a wider variety of planets and get more information than astronomers had expected.
WASP-39b is much easier to study than many other alien planets because it is big and orbits very close to its sun.
"It's eight times closer to its star than Mercury is to the Sun," Professor Horner said.
That means it is not only hot — reaching temperatures of up to 900 degrees Celsius — but its atmosphere is also puffed up.
"Because it has a big fluffy atmosphere, it's much easier to study than other [exoplanets] we've found," Professor Horner said.
One of the big challenges for the JWST will be whether or not it is able to detect atmospheres around smaller Earth-like planets like those in the TRAPPIST system.
Professor Horner believes this "could be a stretch too far" for even the JWST.
"[The telescope] can look for planets that are a bit bigger than Earth around stars that are bit smaller than the Sun," he said.
But, he said, this information would still give us a better handle on what planets are out there, what their atmospheres are like and where they form.
"Understanding the places we are looking at is important in the search for life."
