Scientists using the European Southern Observatory's (ESO) Very Large Telescope have found the heaviest element ever discovered within the atmosphere of an exoplanet — twice.
Barium, which is two and half times heavier than iron, was observed in the atmospheres of the ultra-hot gas giants WASP-76 b and WASP-121 b — two exoplanets, aka planets that orbit stars outside our Solar System.
Both planets are known as ultra-hot Jupiters due to being similar in size to Jupiter and with surface temperatures that soar above 1,000 degrees Celsius.
This is due to them orbiting very close to their host stars, which means a year on WASP-76 b or WASP-121 b takes only one to two days.
This close proximity gives these planets unique features in WASP-76 b, for example, astronomers theorise it rains liquid iron.
The barium discovery has perplexed astronomers, who found the element by accident.
Tomás Azevedo Silva, a PhD student at the University of Porto and the Instituto de Astrofísica e Ciências do Espaço in Portugal, led the study published on Thursday in Astronomy & Astrophysics.
"We were not expecting or looking for barium in particular and had to cross-check that this was actually coming from the planet since it had never been seen in any exoplanet before," Mr Azevedo Silva said.
"The puzzling and counterintuitive part is: why is there such a heavy element in the upper layers of the atmosphere of these planets?"
Study co-author Olivier Demangeon expanded on why it was so unusual to find barium on these exoplanets.
"Given the high gravity of the planets, we would expect heavy elements like barium to quickly fall into the lower layers of the atmosphere," he explained.
The question for scientists is what natural process could cause this heavy element to be at such high altitudes in these exoplanets.
"At the moment, we are not sure what the mechanisms are," said Mr Demangeon.
Barium is so reactive it does not occur in its pure form on Earth but it can be purified and is used in paint, glassmaking and to achieve a brilliant green colour in fireworks.
Scientists say that the discovery of barium on these exoplanets is just scratching the surface of what can be learnt from the mysterious planets.
