Detecting barium in an exoplanet atmosphere

This animation illustrates the transit spectroscopy method that astronomers use to study the atmospheres of exoplanets. As the planet passes in front of its star, the light from the star gets filtered by elements and molecules in the gaseous layer, which alters the stellar light that we observe here on Earth, and leaves a unique chemical fingerprint in the observed spectrum.

Astronomers used this transit technique to discover barium, the heaviest element yet detected in the atmosphere of an exoplanet, in two exoplanets, WASP-76 b and WASP-121 b. The star consists of many elements, including hydrogen and small traces of barium. As the stellar light is filtered through the planet’s atmosphere, the fingerprint of barium becomes stronger, indicating its elusive presence. Note that in the actual spectra of WASP-76 b and WASP-121 b there are many elements other than hydrogen and barium, the two we chose to illustrate in this animation. Hydrogen is the simplest element in the Universe, with only one proton in its nucleus; barium is much heavier, having 56 protons in its nucleus.

Barium was discovered in the two ultra-hot Jupiters WASP-76 b and WASP-121 b using the extremely sensitive ESPRESSO instrument on ESO’s Very Large Telescope. How barium can exist at such high altitudes in these excotic atmospheres remains a mystery to astronomers.