Barium, a chemical element with the symbol Ba and atomic number 56, is the heaviest element detected to date in any exoplanetary atmosphere.
This artist’s impression shows an ultrahot exoplanet as it is about to transit in front of its host star. Image credit: M. Kornmesser / ESO.
Exoplanets hotter than about 2,000 K called ultrahot Jupiters are currently the most readily accessible laboratories for the study of exoplanet atmospheres.
Their size and large atmospheric scale heights, combined with the proximity to the host stars, make them appealing targets for the study of light that is transmitted through planetary atmospheres.
Two of the best examples are WASP-76b and WASP-121b. Both planets are inflated ultrahot Jupiters on orbits with periods shorter than two days and equilibrium temperatures close to 2,500 K.
This gives these planets rather exotic features; in WASP-76b, for example, astronomers suspect it rains iron.
Tomás Azevedo Silva, a Ph.D. student at the University of Porto and the Instituto de Astrofísica e Ciências do Espaço, and his colleagues were surprised to find barium, which is 2.5 times heavier than iron, in the upper atmospheres of WASP-76b and WASP-121b.
“This was in a way an ‘accidental’ discovery. 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,” Azevedo Silva said.
“The puzzling and counterintuitive part is: why is there such a heavy element in the upper layers of the atmosphere of these planets?”
WASP-76b and WASP-121b were observed with the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) instrument on ESO’s Very Large Telescope.
“Given the high gravity of the planets, we would expect heavy elements like barium to quickly fall into the lower layers of the atmosphere,” said Dr. Olivier Demangeon, an astronomer at the University of Porto and the Instituto de Astrofísica e Ciências do Espaço.
The fact that barium was detected in the atmosphere of WASP-76b and WASP-121b suggests that ultrahot Jupiters might be even stranger than previously thought.
Although we do occasionally see barium in our own skies, as the brilliant green color 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,” Dr. Demangeon said.
The astronomers also detected cobalt and strontium and found tentative evidence of the presence of titanium in the atmosphere of WASP-121b.
Their paper appears today in the journal Astronomy & Astrophysics.
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T. Azevedo Silva et al. 2022. Detection of barium in the atmospheres of the ultra-hot gas giants WASP-76b and WASP-121b. Together with new detections of Co and Sr+ on WASP-121b. A&A, in press; doi: 10.1051/0004-6361/202244489
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