In 2021, WASP-76b made headlines as the most heavy metal exoplanet. The gas giant, bigger than Jupiter, orbits its star at a distance closer than Mercury — and astronomers suggested 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’s warped into the shape of a rugby ball.
But iron is lightweight 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 in the two best-studied ultra-hot gas giants — WASP-121b and WASP-76b — swirl clouds of barium, a metal two-and-a-half times heavier than iron.
WHAT’S NEW — This isn’t the first time astronomers have found brain-melting phenomena in the atmosphere of hot Jupiters. In 2021, ESO researchers announced their suspicion that the incredible temperatures found on the hot sides of WASP-76b and WASP-121b result in clouds of iron that rain down as they pass over the terminator into the planets’ dark sides.
It was those observations that Tomás Azevedo Silva, Olivier Demangeon, and this team set out to confirm. While trying out a new method of analyzing the “wiggles” of spectroscopic data — the telltale wavelengths of different elements and molecules in the atmosphere of a planet as they pass in front of their star — they checked their method against the two hot Jupiters most thoroughly studied by astronomers: WASP-76b and WASP-121b. They used data from the ESO’s Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) at the Very Large Telescope in Chile.
Speaking with Inverse, Azevedo Silva explains what happened next. “We saw what they saw, but we saw some extra things,” he says. “So in a way it was accident … we were not looking for anything new.”
In addition to the expected clouds of iron, 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 came as a massive surprise to Azevedo Silva. “I remember searching for what could cause barium to be in these atmospheres and I couldn’t find anything because nobody had written anything about barium in the atmospheres because they didn’t expect barium to be there!”
WHY IT MATTERS — The existence of something as heavy as barium high in the atmosphere of a gas giant is strange enough. But its presence there helps indicate to astronomers that these planets might be even stranger than they thought.
Ultra-hot Jupiters like WASP 76b and WASP-121b are distinguished by two factors: first, they’re extremely close to their stars. WASP-121b, which is a little more massive than Jupiter but nearly twice as wide, orbits its star every 30 hours. WASP-76b is a little less massive, but about the same size, and orbits every 43 hours.
Second, both are tidally locked, which means they have a permanent bright side and a permanent night side. This leads to extreme temperatures on the dayside: upwards of 1000C. Usually, with something as heavy as barium, you’d expect it to be on the bottom and lighter gasses to be in the upper layers of the atmosphere. But the extreme environment might have something to do with why such heavy elements are visible in the upper layers of these gas giants.
Olivier Demangeon suggests that this might be due to the very high temperatures on the daysides creating a permanent layer of heavier material. Because of the tremendous stellar flux, or amount of energy received from their stars, the thermal inversions that separate off layers of the atmosphere — like the Earth’s troposphere, stratosphere, mesosphere, thermosphere, and exosphere — might “create these layers and separate a portion of the atmosphere which means they will not communicate and [elements] will not go from one to the other,” Demangeon tells Inverse.
But that suggests an even bigger mystery to Demangeon. Astronomers don’t think these ultra-hot Jupiters were always so 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 arrive where they are.” That should give heavier materials like barium or iron enough time to settle down to the inner envelope.
For barium or iron to remain in the upper atmosphere, he suggests, means that “the planet needs to migrate very fast for this to work” — zipping from far-flung regions around their star to a tighter orbit than Mercury’s in just a few million years.
WHAT’S NEXT — To get a better grasp on how heavy this discovery is, Azevedo Silva says the next step is to work on a more specific model of these giants’ atmospheres. By matching absorption lines — the telltale wiggles — precisely to computer models of the atmosphere, astronomers can compare their spectra and “see which one fits best.”
In the coming months, the James Webb Space Telescope is scheduled to aim its Near Infrared Spectrograph, or NIRSpec, at WASP-76b as part of the public Cycle 1 GO program in hopes of building a dataset with “a wealth of detail on the 3D atmospheric physics and chemistry,” according to the proposed viewing. “That’s going to provide a lot of information about this thermal structure and how the atmosphere of the planet is layered,” says Demangenon.
Azevedo Silva also suggests finding something as strange and heavy as barium lofted high in the atmosphere of these two gas giants might be the start of a new understanding of ultra-hot 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 common to these ultra-hot Jupiters, something we’ll start to see when we observe others.”
