From: ESO 2213
13 October 2022 -- Using the European Southern Observatory’s Very Large Telescope (ESO’s
VLT), astronomers have discovered the heaviest element ever found in an
exoplanet atmosphere — barium. They were surprised to discover barium at high
altitudes in the atmospheres of the ultra-hot gas giants WASP-76 b and WASP-121
b — two exoplanets, planets which orbit stars outside our Solar System. This
unexpected discovery raises questions about what these exotic atmospheres may
be like.
“The puzzling and counterintuitive part is: why
is there such a heavy element in the upper layers of the atmosphere of these
planets?” says Tomás Azevedo Silva, a PhD student at the University of
Porto and the Instituto de Astrofísica e Ciências do Espaço (IA) in Portugal
who led the study published today in Astronomy & Astrophysics.
WASP-76 b and WASP-121 b are no ordinary exoplanets.
Both are known as ultra-hot Jupiters as they are comparable in size to Jupiter
whilst having extremely high surface temperatures soaring above 1000°C. This is
due to their close proximity to their host stars, which also means an orbit
around each star takes only one to two days. This gives these planets rather
exotic features; in WASP-76 b, for example, astronomers suspect it rains
iron.
But even so, the scientists were surprised to find
barium, which is 2.5 times heavier than iron, in the upper atmospheres of
WASP-76 b and WASP-121 b. “Given the high gravity of the planets, we would
expect heavy elements like barium to quickly fall into the lower layers of the
atmosphere,” explains co-author Olivier Demangeon, a researcher also from
the University of Porto and IA.
“This was in a way an ‘accidental’ discovery,”
says Azevedo Silva. “We were not expecting or looking for barium in
particular and had to cross-check that this was actually coming from the planet
since it had never been seen in any exoplanet before.”
The fact that barium was detected in the atmospheres
of both of these ultra-hot Jupiters suggests that this category of planets
might be even stranger than previously thought. Although we do occasionally see
barium in our own skies, as the brilliant green colour in fireworks, the
question for scientists is what natural process could cause this heavy element
to be at such high altitudes in these exoplanets. “At the moment, we are
not sure what the mechanisms are,” explains Demangeon.
In the study of exoplanet atmospheres ultra-hot
Jupiters are extremely useful. As Demangeon explains: “Being gaseous and
hot, their atmospheres are very extended and are thus easier to observe and
study than those of smaller or cooler planets”.
Determining the composition of an exoplanet’s
atmosphere requires very specialised equipment. The team used the ESPRESSO instrument
on ESO’s VLT in Chile to analyse starlight that had been filtered through the
atmospheres of WASP-76 b and WASP-121 b. This made it possible to clearly
detect several elements in them, including barium.
These new results show that we have only scratched
the surface of the mysteries of exoplanets. With future instruments such as the
high-resolution ArmazoNes high Dispersion Echelle Spectrograph (ANDES),
which will operate on ESO’s upcoming Extremely Large Telescope (ELT),
astronomers will be able to study the atmospheres of exoplanets large and
small, including those of rocky planets similar to Earth, in much greater depth
and to gather more clues as to the nature of these strange worlds.
This research was presented in the paper “Detection
of Barium in the atmospheres of ultra-hot gas giants WASP-76b & WASP-121b”
to appear in Astronomy & Astrophysics (doi:
10.1051/0004-6361/202244489).
https://www.eso.org/public/news/eso2213/
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