Astronomers find an element heavier than expected on a vast infernal world

In 2021, WASP-76b made headlines as the exoplanet with the most heavy metals. The gas giant, larger than Jupiter, orbits its star at a distance closer than Mercury – and astronomers suspected it was so hot it might rain iron. A similarly strange exoplanet, WASP-121b, is so hot and so close to its star that it takes on the shape of a rugby ball.

But iron is light stuff compared to the unexpected results published in astronomy and astrophysics by a team led by Tomás Azevedo Silva and Olivier Demangeon from the University of Porto and the Portuguese Instituto de Astrofísica e Ciências do Espaço. They found that the two best-studied ultra-hot gas giants – WASP-121b and WASP-76b – contain swirling clouds of barium, a metal two and a half times heavier than iron.

WHAT’S NEW – This isn’t the first time astronomers have discovered mind-melting phenomena in the atmosphere of hot Jupiters. In 2021, ESO researchers expressed their suspicion that the incredible temperatures on the hot sides of WASP-76b and WASP-121b are causing clouds of iron to rain down as they pass over the terminator into the planets’ dark sides.

It was these observations that Tomás Azevedo Silva, Olivier Demangeon and this team wanted to confirm. Trying out a new method for analyzing the “waves” of spectroscopic data — the telltale wavelengths of various elements and molecules in a planet’s atmosphere as they pass in front of their star — they tested their method most thoroughly with the two hot Jupiters studied by astronomers: WASP -76b and WASP-121b. They used data from ESO’s Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) at the Very Large Telescope in Chile.

Speak with Vice versa, Azevedo Silva explains what happened next. “We saw what they saw, but we saw some additional things,” he says. “So in a way it was a coincidence … we weren’t looking for something new.”

In addition to the expected iron clouds, Azevedo Silva’s model found barium gas. Iron is heavy enough, but barium — with an atomic weight of 137 — is more than twice as heavy. This was a big surprise for Azevedo Silva. “I remember looking for what might cause barium to be present in those atmospheres and I couldn’t find anything because no one had written anything about barium in the atmospheres because they didn’t expect barium to exist is there!”

WHY IT MATTERS – The existence of something as heavy as barium high up in a gas giant’s atmosphere is odd enough. But its presence there is helping astronomers show that these planets might be even stranger than they thought.

Ultra-hot Jupiters like WASP-76b and WASP-121b are characterized by two factors: First, they are extremely close to their stars. WASP-121b, slightly more massive than Jupiter but almost twice as wide, orbits its star every 30 hours. WASP-76b is slightly less massive but about the same size and orbits every 43 hours.

Second, both are tidal, meaning they have a permanent light side and a permanent night side. This results in extreme temperatures on the day side: over 1000°C. Normally, something as heavy as barium would be expected to be at the bottom and lighter gases in the upper layers of the atmosphere. But the extreme environment may have something to do with why such heavy elements are visible in the upper layers of these gas giants.

Olivier Demangeon suggests that this could be due to the very high temperatures on the daysides creating a permanent layer of heavier material. Because of the tremendous stellar flux, or the amount of energy received from their stars, the thermal inversions that separate layers of the atmosphere — like Earth’s troposphere, stratosphere, mesosphere, thermosphere, and exosphere — “could create those layers and a portion.” separate from the atmosphere, which means that they do not communicate and [elements] will not go from one to the other,” says Demangeon Vice versa.

But that poses an even bigger mystery to Demangeon. Astronomers don’t think these ultra-hot Jupiters have always been this hot. The accepted understanding of planets like these is that they most likely formed in the outer reaches of their planetary systems before “they migrate and migrate and get where they are.” That should Allow time for heavier materials such as barium or iron to settle against the inner shell.

In order for barium or iron to stay in the upper atmosphere, it means “the planet has to migrate very quickly for this to work” — from far-flung regions around its star to a closer orbit than Mercury’s in just a few million years.

WHAT’S NEXT – To better understand how difficult this discovery is, Azevedo Silva says the next step is to work on a more specific model of these giants’ atmospheres. By closely matching absorption lines — the telltale wobbles — to computer models of the atmosphere, astronomers can compare their spectra and “see which ones fit best.”

In the coming months, the James Webb Space Telescope is scheduled to aim its near-infrared spectrograph (NIRSpec) at WASP-76b as part of the public Cycle-1 GO program in hopes of generating a dataset with “a wealth of detail on the 3D atmospheric Physics and chemistry” is the proposed consideration. “That will provide a lot of information about this thermal structure and the stratification of the planet’s atmosphere,” says Demangenon.

Azevedo Silva also suggests finding something as strange and heavy as barium high in the atmospheres of these two gas giants could be the start of a new understanding of ultrahot Jupiters as a class. Barium is “two for two,” he says. “Maybe we just got lucky,” but “maybe it’s a new class of planets, maybe it’s something these ultra-hot Jupiters have in common, something we’ll see as we observe others.”