Two exoplanets have a strangely heavy element floating in their skies

Astronomers have found barium floating high in the atmospheres of two sweltering exoplanets, and they don’t know how such a heavy element ended up where it is

By Leah Crane

An artist’s impression of an ultra-hot exoplanet, similar to WASP-76b and WASP-121b

ESO/M. Kornmesser

Two massive exoplanets – worlds beyond our solar system – have barium floating high in their atmospheres. This is the heaviest element ever spotted in an exoplanet atmosphere, and astronomers don’t know how it got there.

The ultra-hot gas giants WASP-76b and WASP-121b are two of the most-studied exoplanets, both similar in size to Jupiter but with temperatures higher than 1000°C. Tomás Azevedo Silva at the University of Porto in Portugal and his colleagues observed these sizzling worlds with the Very Large Telescope in Chile and were surprised to find the signature of the metal barium in both of their atmospheres.

“This was an accidental discovery – we just wanted to confirm we already knew everything that was there,” says Azevedo Silva. “We were not looking for barium, and I don’t think anyone was looking for barium, because there is no reason for barium to be there.”

Barium is much heavier than most elements we expect to find high up in planetary atmospheres – it is nearly 2.5 times as heavy as iron. “Our first thought is that something so heavy would sink down in the atmosphere,” says Azevedo Silva. “This raises a lot of questions about how it got there and what’s going on.”

It may be that there is unexpected convection within each of these worlds, churning barium and other heavy elements towards the top of their atmospheres. Azevedo Silva says it could also be related to their proximity to their stars – each of these planets is so close to its star that it completes a full orbit in only a few Earth days.

And because the researchers found it in both the worlds they checked, whatever process lofts barium to the top of these atmospheres could be common in other ultra-hot worlds as well. Understanding how this element ended up so high in the atmosphere could help us figure out how these strange planets formed.

Journal reference: Astronomy & Astrophysics, DOI: 10.1051/0004-6361/202244489

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