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| An artist’s impression of the exoplanet WASP-39b NASA/JPL-Caltech/Robert Hurt; Center for Astrophysics-Harvard & Smithsonian/Melissa Weiss |
Astronomers analysing data from the James Webb Space Telescope have spotted
signs of chemical reactions driven by star light in the atmosphere of an
exoplanet for the first time.
The James Webb Space Telescope (JWST) has spotted chemical reactions driven
by starlight taking place in the atmosphere of a distant alien world for the
first time, raising hopes that the telescope could help identify exoplanets
that host life.
Many of the compounds found in Earth’s atmosphere, including some that are
essential for life, didn’t exist when the planet first formed — instead,
they were the product of chemical reactions triggered by light from the sun.
These photochemistry reactions also occur in the atmospheres of almost all
the other planets in the solar system, and so were predicted to happen in
exoplanet atmospheres but until now had never been observed.
In August, JWST observations of the exoplanet WASP-39b, a 900°C ball of gas
as massive as Saturn and wider than Jupiter, found the first evidence for
carbon dioxide in an exoplanet atmosphere. But astronomers also spotted a
strange bump in the signature of the planet’s light, which suggested an
unknown element or molecule was absorbing the host star’s light as it passed
through the planet’s atmosphere.
Now, Katy Chubb at the University of St Andrews in the UK and her colleagues
have analysed data on WASP-39b’s light taken from four infrared instruments
on JWST. “The large range of wavelengths covered by the four different
instruments really allows us to build a complete as possible picture of this
atmosphere as we can,” says Chubb.
The team divided into subgroups and used a range of atmospheric models to
mimic the signal from JWST. Only models that included chemical reactions
involving sulphur could reproduce the data, suggesting the bump was caused
by atmospheric sulphur dioxide, says team member Éric Hébrard at the
University of Exeter, UK. “We were very surprised because as soon as we each
independently implemented the sulphur chemistry, it fit right away.”
The levels of sulphur dioxide were far higher than they should be if the
planet is made only from material created when the star system formed. The
only explanation, says Chubb, is that light from the star WASP-39 has caused
a chain of chemical reactions in the planet’s atmosphere to produce the
sulphur dioxide.
“We haven’t been able to probe such process in the deep atmosphere before
the JWST era,” says Nikku Madhusudhan at the University of Cambridge, who
wasn’t involved in the research. “The results are an excellent demonstration
of JWST capability for exoplanets spectroscopy.”
Identifying photochemical reactions on WASP-39b could also help indicate
whether the planet formed further out from its star and moved inwards,
picking up material across the system, or whether it formed at its current
location and simply accumulated material there — early observations of the
oxygen to carbon ratio suggests it formed far away from its star, but more
definitive data will be needed first, says Chubb.
The find also bodes well for observing more compounds produced by
photochemical processes, such as ozone on Earth, says Hébrard. “Even if
[WASP 39B] is very different than what we have on Earth — it’s hot, it’s
hydrogen dominated, you don’t want to live there — having that first
detection of a photochemical product is one way forward.”
And ultimately, such detections could help in one of JWST’s most important
mission goals – searching for signs of an exoplanet that could host life.
“We need to cover a lot more in order to answer those biosignatures problem,
but it’s the first step along the way,” says Hébrard.
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Space & Astrophysics