The Heaviest Element Ever Seen in an Exoplanet’s Atmosphere: Barium

Astronomers have spotted barium in the atmosphere of a distant exoplanet. With its 56 protons, you have to run your finger further down the periodic table than astronomers usually do to find barium. What does finding such a heavy element in an exoplanet atmosphere mean?

It means we’re still learning how strange exoplanets can be.

We’ve made a lot of progress in exoplanet science since astronomers discovered the first ones in the mid-1990s; we now know of thousands of them. In 2001, researchers using Hubble data were the first to observe an exoplanet atmosphere, identifying sodium. Some said it was lucky and wouldn’t happen again. Boy, were they wrong.

Now we have the power to characterize exoplanet atmospheres in more detail than ever. One of our most powerful telescopes is the European Southern Observatory’s (ESO) Very Large Telescope (VLT.) A new study based on observations with the VLT and its ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) instrument presents evidence of barium in the atmosphere of the two ultra-hot gas giants.

The study is “Detection of Barium in the atmospheres of ultra-hot gas giants WASP-76b & WASP-121b.” The lead author is Tomás Azevedo Silva, a Ph.D. student at the University of Porto and the Instituto de Astrofísica e Ciências do Espaço (IA) in Portugal. The journal Astronomy and Astrophysics published the research.

WASP-76b and WASP-121b are both ultra-hot Jupiters. WASP-76b has been in the news because of the reported detection of iron in its atmosphere and the possibility that this tidally-locked planet experiences iron rain. Follow-up studies presented other explanations for the data, but the planet is still of great interest to exoplanet scientists.

This illustration of WASP-76b is based on data available in May 2020. As an object of high interest, data keeps coming in. Image Credit: By Trurle - Own work, CC BY-SA 4.0,
This illustration of WASP-76b is based on data available in May 2020. As an object of high interest, data keeps coming in. Image Credit: By Trurle – Own work, CC BY-SA 4.0,

WASP-121b is also noteworthy. It’s the first exoplanet where researchers found water vapour in its stratosphere. Both planets are extremely close to their stars and have orbital periods of less than two days. Both planets orbit stars similar to the Sun, with surface temperatures above 1000 C (1830 F.).

Artist's concept of the hot Jupiter WASP-121b. A new study finds barium in its atmosphere. Image generated using Engine House VFX. Credit: Bristol Science Centre/University of Exeter
Artist’s concept of the hot Jupiter WASP-121b. A new study finds barium in its atmosphere. Image generated using Engine House VFX. Credit: Bristol Science Centre/University of Exeter

Both of these planets are known for having exotic atmospheres, but the discovery of barium has intensified interest in them. There’s no ready explanation for barium in the upper atmosphere of the planets. “The puzzling and counterintuitive part is: why is there such a heavy element in the upper layers of the atmosphere of these planets?” said lead author Azevedo Silva.

Both Jupiter-sized planets have powerful gravity that should draw heavy elements like barium to their surfaces. But instead, the barium, which is even heavier than iron, is aloft in the upper atmospheres of both planets. “Given the high gravity of the planets, we would expect heavy elements like barium to quickly fall into the lower layers of the atmosphere,” explained co-author Olivier Demangeon, another researcher from the University of Porto and IA.

Barium isn’t up there by itself. These planets’ atmospheres are exotic mixtures of chemicals alien to our atmosphere. The researchers weren’t looking for barium when they found it; they were studying the strange atmospheres as part of their effort to constrain the atmospheric composition and dynamics. They also wanted to lay the “… groundwork for new research regarding their formation and evolution environments,” as they state in their paper.

“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 team also found vanadium, lithium, manganese, and other elements in the atmospheres. But barium is the heaviest element ever found in an exoplanet atmosphere.

The two planets in the study lend themselves well to atmospheric research. Is finding barium in these atmospheres a signal that we can expect to find them in other ultra-hot Jupiters? Maybe. “Both WASP-76b and WASP-121b represent some of the highest S/N datasets currently available, the presence in both
of the studied planets may indicate that this heavy species can be common in the atmospheres of ultra-hot Jupiters,” the paper states.

There’s no barium in Earth’s atmosphere unless we’re watching a fireworks display. How did it get into the upper atmosphere of these planets? “At the moment, we are not sure what the mechanisms are,” explained Demangeon.

The team doesn’t overextend themselves when it comes to those mechanisms. In their paper they write, “The presence of these heavy ionized species at high altitudes in the atmospheres of ultra hot Jupiters may be evidence of unexpected atmospheric dynamics.” Further research will uncover what those dynamics are. Who knows what discoveries are waiting for us in these atmospheres?

Planetary escape may play a role in the detection. Is an unknown mechanism driving heavy elements away? The observations show that the calcium detected on WASP-121b appears to extend beyond the planet’s Roche Lobe. “Finally, we also confirm the clear asymmetric absorption feature of Ca+ on WASP-121b, with an excess absorption at the bluer wavelengths and an effective planet radius beyond the Roche-Lobe, hinting that the signal may arise from planetary atmospheric escape.”

This won’t be the last team of researchers to study these planets. Giant gaseous planets close to their stars present an excellent opportunity to study their atmospheres as their starlight passes through them. And since the orbital period is less than two days, both planets are readily observed with powerful telescopes and instruments. “Being gaseous and hot, their atmospheres are very extended and are thus easier to observe and study than those of smaller or cooler planets,” said Demangeon.

There’s reason for some caution around these results, though. When researchers discovered iron at WASP-76b, and that the iron could possibly fall as rain, people noticed. It isn’t easy to envision a world so different from ours that it can rain iron. But that iron rain discovery was called into question by another study. At this time, scientists aren’t sure if there is iron rain.

Will further research question this barium detection?