Surprising find: Astronomers have for the first time detected the element barium in the gas envelope of two extrasolar gas giants – the heaviest element ever discovered in the atmosphere of an exoplanet. How this relatively heavy, dense metal gets into the upper atmosphere of these two hot Jupiters and why it doesn’t fall out again immediately is still unclear. Previously unknown chemical and atmospheric processes may be behind it.
Hot Jupiters are the extremists among the exoplanets: These gas giants orbit their star so closely that their day side is open to several thousand degrees is heated. The chemistry in its gas shells is correspondingly exotic: Under these conditions, metals become gaseous and even stable molecules decompose. The night side of some hot Jupiters is still so hot that it rains liquid iron, snows titanium dioxide, or forms clouds of liquid minerals.
At the same time, however, hot Jupiters offer almost perfect conditions for studying their gas envelopes. Because they orbit close to the star and their gas envelope is usually very inflated. This enables astronomers to detect and spectrometrically analyze the light radiating through their atmosphere as the planets pass in front of their star.
Ultra-hot gas giants in sight
Astronomers led by Tomás Azevedo Silva from the University of Porto have now taken a closer look at two of these ultra-hot gas giants. Hot Jupiter, some 640 light-years away WASP-76b takes only 1.8 days to orbit its star. Earlier observations had already shown that its day side is up to 2,400 degrees hot, while the night side is still around 1,500 degrees. These temperature differences create strong winds that blow vaporized iron and other metals such as magnesium, cobalt, chromium or strontium to the night side, where they condense into droplets.
The gas giant, around 850 light-years away WASP-121b is similarly hot and has an orbital period of 30.6 hours. Spectral data show that droplets of metals, but also of minerals such as perosvkite, forsterite and corundum, also form on its night side. These “liquid gems” also form clouds on the hot gas giant and rain down from the sky at night. In order to find out more about these two planetary exotics, Silva and his team examined them with the high-resolution ESPRESSO spectrograph on the Very Large Telescope of the European Southern Observatory in Chile.
element barium on both planets
The results revealed something surprising: In the upper atmospheres of both exoplanets, the astronomers discovered the spectral lines of barium, an alkaline earth metal which, with an atomic mass of around 137, is around 2.5 times heavier than iron. “This is the heaviest element that has so far been detected in the atmosphere of an exoplanet,” the researchers explain. The spectral lines showed that the barium in the planetary gas envelopes occurred as a positively charged ion.
“In a way, this was an accidental discovery because we didn’t expect or look for barium,” says Silva. The alkaline earth metal melts under normal conditions at around 727 degrees and evaporates at 1,637 degrees, so it could be gaseous at least temporarily on the two gas giants. “The presence of barium ions on both exoplanets could indicate that this heavy element is common in the atmospheres of ultra-hot Jupiters,” speculate Silva and his team.
mystery about the location
Even more surprising, however, is the place where the astronomers detected barium: “What is actually puzzling and paradoxical is: Why is there such a heavy element in the upper layers of the atmosphere of these planets?” asks Silva. Given the high gravity of the planets, one would expect heavy elements like barium to sink rapidly and only occur in the lower layers of the planetary gas envelope.
This raises the question of what natural process could cause this heavy element to be present at such high altitudes on these two exoplanets. “We are not yet clear about the mechanisms,” explains Silva’s colleague Olivier Demangeon. According to the astronomers, however, it could well be that there are processes in the atmosphere of hot Jupiter that are unknown from other planets. (Astronomy and Astrophysics, 2022; doi: 10.1051/0004-6361/202244489)
Quelle: European Southern Observatory (ESO)