Heaviest Element Yet Detected in an Exoplanet Atmosphere

Using the European Southern Observatory’s Very Large Telescope (ESO’s VLT), astronomers have spotted the heaviest element ever found in an exoplanet atmosphere– barium. They were surprised to find barium at high altitudes in the atmospheres of the ultra-hot gas giants WASP-76 b and WASP-121 b– two exoplanets, planets that orbit stars outside our Solar System. This not expected discovery raises doubts about what these exotic atmospheres may resemble.

” The confusing and counterintuitive part is: why is there such one heavy element in the upper layers of the atmosphere of these planets?” states Tomás Azevedo Silva, one Ph.D. student at the College of Porto and the Instituto de Astrofísica e Ciências do Espaço (IA) in Portugal who led the research released today in Astronomy & Astrophysics.

WASP-76 b and also WASP-121 b are no ordinary exoplanets. Both are called ultra-hot Jupiters as they are comparable in dimension to Jupiter whilst having extremely high surface temperatures soaring above 1000°C. This results from their close proximity to their host stars, that also means an orbit around each star takes only one to 2 days. This gives these planets instead exotic features; in WASP-76 b, for example, astronomers suspect it rains iron.

The discovery

However, even so, the researchers were surprised to discover barium, that is 2.5 times heavier than iron, in the upper atmospheres of WASP-76 b and WASP-121 b. “Provided the high gravity of the planets, we could expect heavy elements like barium to fall into the lower layers of the atmosphere quickly,” explains co-author Olivier Demangeon, a researcher additionally from the University of Porto and IA.

” This remained, in a way, an ‘accidental’ discovery,” says Azevedo Silva. “We were not expecting or searching for barium in particular and also had to cross-check that this was really coming from the planet since it had never been observed in any exoplanet before.”

The fact that barium was spotted in the atmospheres of both of these ultra-hot Jupiters recommends that this category of planets may be even stranger than before thought. Although we do occasionally observe barium in our own skies, as the brilliant green color in fireworks, the question for researchers is what natural process would cause this heavy element to be at such great altitudes in these exoplanets. “At the moment, we aren’t sure what the mechanisms are,” explains Demangeon.

In the research of exoplanet atmospheres, ultra-hot Jupiters are extremely helpful. As Demangeon explains: “Being gaseous and warm, their environments are very extended and are thus easier to observe as well as study than those of smaller or cooler planets.”

Determining the composition of an exoplanet

Determining the composition of an exoplanet’s atmosphere needs very specialized equipment. The group utilized the ESPRESSO tool on ESO’s VLT in Chile to analyze starlight that had been filtered through the atmospheres of WASP-76 b and also WASP-121 b. This made it possible to detect several elements in them, including barium, clearly.

These new results show that we have just scratched the surface of exoplanets’ secrets. With future instruments like the high-resolution ArmazoNes high Dispersion Echelle Spectrograph (ANDES), that will run on ESO’s upcoming Extremely Large Telescope (ELT), the astronomers will be able to study the atmospheres of exoplanets big and small, including those of rocky planets similar to Earth, in much higher depth and to collect more clues as to the nature of these strange worlds.

Read the original article on PHYS.

More information: T. Azevedo Silva et al, Detection of barium in the atmospheres of the ultra-hot gas giants WASP-76b and WASP-121b, Astronomy & Astrophysics (2022). DOI: 10.1051/0004-6361/202244489