Portuguese astronomers say they have found the element barium in the atmospheres of two ‘ultra-hot Jupiter’ exoplanets

Using the European Southern Observatory’s Very Large Telescope (ESO’s VLT), the astronomers were surprised to discover barium at high altitudes in the atmospheres of the ultra-hot gas giant exoplanets WASP-76 b and WASP-121 b; planets which orbit stars outside our solar system.

This unexpected discovery raises questions about what these exotic atmospheres may be like.

Study lead Tomás Azevedo Silva is a PhD student at the University of Porto and the Instituto de Astrofísica e Ciências do Espaço (IA) in Portugal.

He 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-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 1,000°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; on WASP-76 b, for example, astronomers suspect it rains iron.

See also: Two new ‘super-Earth’ planets found 100 light years away

heavy elements

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.

Co-author Olivier Demangeon, a researcher also from the University of Porto and IA, added: “Given the high gravity of the planets, we would expect heavy elements like barium to quickly fall into the lower layers of the atmosphere.”

Azevedo Silva said: “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.”

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,” Demangeon explained.

exoplanet atmospheres

In the study of exoplanet atmospheres ultra-hot Jupiters are extremely useful.

Demangeon said: “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.

The study is published in Astronomy & Astrophysics.

This artist’s impression shows an ultra-hot exoplanet, a planet beyond our Solar System, as it is about to transit in front of its host star. When the light from the star passes through the planet’s atmosphere, it is filtered by the chemical elements and molecules in the gaseous layer. With sensitive instruments, the signatures of those elements and molecules can be observed from Earth. Using the ESPRESSO instrument of ESO’s Very Large Telescope, astronomers have found the heaviest element yet in an exoplanet’s atmosphere, barium, in the two ultra-hot Jupiters WASP-76 b and WASP-121 b. © ESO/M. Kornmesser.