How does barium get into the exoplanet atmosphere?

The two exoplanets WASP-76b and WASP-121b contain the heavy element barium in their upper atmosphere. This is shown by high-resolution spectra obtained at the Very Large Telescope of the European Southern Observatory ESO in Chile. It is the heaviest element to date that has been detected in the atmosphere of planets around other stars. And it poses a problem for the astronomers: With the strong gravitational pull of the two planets, there should be no such heavy substances high in the atmosphere, the discoverers write in the journal Astronomy

"The barium should fall down from the upper atmosphere very quickly," explains Olivier Demangeon of the University of Porto in Portugal. "At the moment we have no idea what mechanism transports this element up." The two planets, which are 640 and 860 light-years away from Earth, are "hot Jupiters" - i.e. gas planets that are similar in size and mass to Jupiter in resemble our solar system.

However, they orbit their central star in a very narrow orbit with an orbital period of less than two days. Due to the proximity of the star, the temperature of the atmosphere is about 2000 degrees Celsius.

Such planets are particularly worthwhile observation objects for astronomers. "Because they are mostly gas and very hot, they have extensive atmospheres," says Demangeon, "so we can observe and study their atmospheres much more easily than those of smaller and cooler planets." The position of the planetary orbits also helps the sky researchers in these observations: They are oriented in such a way that the planets regularly pass in front of their central star when viewed from Earth.

During these "transits," the planets obscure part of the star, dimming its light - and these regular eclipses help in discovering the planets. Crucial to studies like Demangeon and his colleagues' is that a small portion of starlight shines through the atmosphere of each planet. In this part of the starlight, the atmosphere leaves a kind of fingerprint: the materials that make up the atmosphere absorb starlight at characteristic wavelengths.

Using these "spectral lines", the researchers can then identify the substances that make up the planet's atmosphere. However, the whole process is expensive and complicated. First, the researchers need a high-resolution spectrograph, a special additional device on the telescope that breaks down the radiation into its wavelengths. And then the astronomers have to eliminate all influences of the star and also of the earth's atmosphere from the data.

Demangeon and his colleagues were initially able to confirm a large number of substances that had already been indicated by earlier observations. The researchers have also discovered new cobalt and strontium. And then they came across spectral lines from barium - and at first doubted that these actually came from the exoplanets. "We weren't looking for barium," says Azevedo Silva from the University of Porto, "because we didn't expect any barium there." Only after further checks were the scientists convinced of their surprising discovery.

According to the researchers, the detection of barium in the atmospheres of two hot Jupiters indicates that such heavy elements could often be found in the atmospheres of extremely hot Jupiters. There must be previously unknown atmospheric currents that transport such substances into the upper atmosphere.