Barium, the Heaviest Element Recorded in Exoplanet Atmospheres

This animation illustrates the transit spectroscopy method used in this study to detect barium on WASP-76b and WASP-121b. As light from a star passes through the atmosphere of an orbiting exoplanet, it gets filtered by the elements and molecules in the atmosphere. Once this filtered light reaches our telescopes on Earth, we can determine the chemical composition of the exoplanet atmosphere. Credit: SciTech Daily.

Two exoplanets, WASP-76b and WASP-121b, have clouds of barium in their upper atmospheres. Barium is the heaviest element ever recorded in an exoplanet atmosphere thus far. These two exoplanets, which orbit stars outside our solar system, are both “hot Jupiters,” meaning they are large gas giants that orbit close to their stars. This discovery is rather puzzling since the high gravity of these exoplanets should cause heavy elements like barium to quickly sink into the lower layers of their atmospheres.

WASP-76b is a metal-rich exoplanet less massive than Jupiter but nearly the same size, which orbits its star at a distance less than that at which Mercury orbits our Sun. Due to the high temperatures at this location — up to 2000˚C — astronomers predicted it would be hot enough to rain iron metal. A study led by Tomás Azevedo Silva from the University of Porto sought to determine whether iron rained on WASP-76b. The study instead detected the presence of barium, a metal 2.5 times heavier than iron, in the upper atmosphere. The research team used data from the European Southern Observatory’s Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) at the Very Large Telescope (VLT) in Chile. Barium was detected using transit spectroscopy. In this method, as a planet passes in front of its star, the light passes through the planet’s atmosphere, where some wavelengths are absorbed by molecules, and others pass through unimpeded. By measuring the fraction of light able to penetrate at different wavelengths, the atmosphere’s chemical composition can be inferred. The presence of barium may suggest that it is common in the atmospheres of ultra-hot Jupiter-like exoplanets and presents evidence of unanticipated atmospheric dynamics. Fortunately, the JWST will soon be aimed at WASP-76b to better understand the thermal structure of its atmosphere and how it can retain heavy metals in its outer envelope. READ MORE