Using the European Southern Observatory’s Very Large Telescope (ESO’s VLT), a team has discovered the heaviest element ever detected in the atmosphere by one exoplanet: the barium. The surprise arose when discovering the presence of barium at high altitudes in the atmospheres of ultra-hot gas giants WASP-76 b y WASP-121 b, two exoplanets orbiting stars outside our Solar System. This unexpected discovery raises questions about what these exotic atmospheres can be like.
In the words of Thomas Azevedo Silvaa doctoral student at the University of Porto and the Institute of Astrophysics and Space Sciences (IA) of Portugal, who led the study published in the journal ‘Astronomy & Astrophysics’, “the puzzling and counterintuitive part is: why is there such a heavy element in the upper layers of the atmosphere of these planets?
WASP-76 b y WASP-121 b they are not ordinary exoplanets. Both are in the class of ultra-hot Jupiters, as they are comparable in size to our gas giant, while having extremely high surface temperatures rising above 1000°C. This is due to their close proximity to their host stars, which also means that an orbit around each star is completed in as little as a day or two. This gives these planets quite exotic features; on WASP-76 b, for example, astronomers suspect it’s raining iron.
But even so, the scientific community was surprised to detect 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 fall rapidly to the lower layers of the atmosphere,” explains co-author Olivier Demangeonalso a researcher at the University of Porto and IA.
“This was, in a way, an ‘accidental’ discovery,” says Azevedo Silva. “We weren’t expecting or looking for barium in particular and had to verify that it actually came from the planet, as it had never been seen on any exoplanet before.”
Stranger planets even than thought
The fact that barium was detected in the atmospheres of these two planets suggests that this category of planets might be even stranger than previously thought. Although we occasionally see barium in our own skies, as in the case of the bright green color of fireworks, the question for the scientific community is what natural process could cause this heavy element to be at such high altitudes on these exoplanets. “At the moment, we’re not sure what the mechanisms are,” explains Demangeon.
In the study of exoplanet atmospheres, ultrahot Jupiters are extremely useful. As Demangeon explains: “Being gaseous and hot, their atmospheres are very extensive and therefore easier to observe and study than those of smaller or cooler planets.”
Determining the composition of an exoplanet’s atmosphere requires highly specialized equipment. The team used the ESPRESSO instrument, installed on ESO’s VLT in Chile, to analyze 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, including barium.
These new results show that we have only scratched the surface of the mysteries surrounding exoplanets. With future instruments such as ANDES (ArmazoNes high Dispersion Echelle Spectrograph), which will operate on ESO’s forthcoming Extremely Large Telescope (ELT), the astronomical community will be able to study the atmospheres of large and small exoplanets, including those of planets Earth-like rocky outcrops at much greater depths and gather more clues about the nature of these strange worlds.