‘Hot Saturn’ WASP-96b: Bizarre Exoplanet Has No Clouds In Its Atmosphere But Plenty Of Sodium

For the first time ever, astronomers detected sodium atoms in the atmosphere of an exoplanet, which suggests it is cloud-free.

Blue exoplanet.
Alex Farias / Shutterstock

For the first time ever, astronomers detected sodium atoms in the atmosphere of an exoplanet, which suggests it is cloud-free.

“Hot Jupiters” aren’t the only eccentric exoplanets out there. It turns out that “hot Saturns” can be quite whimsical too. For instance, “hot Saturn” WASP-96b seems to be “entirely cloud-free,” says astronomer Nikolay Nikolov, lead author of a recent study on the atmosphere of this peculiar exoplanet.

This strange gas giant (the planet, that is) lies 980 light-years away in the Phoenix constellation. Like all “hot Saturns,” the exoplanet is similar in mass to Saturn and 20 percent bigger than Jupiter. It also orbits its parent star, WASP-96, very closely.

According to Sci-News, it only takes 3.4 days for the “hot Saturn” to complete a full orbit around WASP-96. Such close quarters with the parent star means that the exoplanet is very hot, reaching 1,881 degrees Fahrenheit (or 1,027 degrees Celsius).

Hot exoplanets, especially those as incandescent as WASP-96b, are typically known to have thick atmospheres, with plenty of clouds and hazes. The same goes for some of the coldest planets in the cosmos. However, this strange exoplanet doesn’t fit the mold, Nikolov’s team uncovered.

The researchers managed to take a close look at WASP-96b with the help of the European Southern Observatory’s Very Large Telescope (VLT) in Chile. Known as the world’s most powerful exoplanet-hunting telescope, the VLT enabled the team to gauge the composition of WASP-96b’s atmosphere as they observed the “hot Saturn” passing in front of its parent star.

Their observations pointed to a unique discovery, reveals a news release from the University of Exeter in the U.K., which led the study. For the first time ever, the astronomers found the complete fingerprint of sodium in the spectrum of an exoplanet. This discovery comes just a few days after the first-ever detection of helium in the atmosphere of exoplanet WASP-107b.

“Until now, sodium was revealed either as a very narrow peak or found to be completely missing,” said Nikolov, who is a research fellow at the British university.

Nevertheless, WASP-96b’s atmosphere was found to contain plenty of sodium present in “levels similar to those found in our own solar system,” notes the news release.

According to the university, these high levels of sodium would make the “hot Saturn” appear bluish in color to a distant observer because the sodium would absorb the yellow-orange light from the exoplanet’s full spectrum.

Finding sodium in WASP-96’s atmosphere led to another breakthrough. Since the spectral characteristics of sodium atoms can’t be observed in planets with thick atmospheres because their clouds would obscure the view — the element’s “tent-shaped” profile can only be spotted deep in a planet’s atmosphere, notes Nikolov — the team deduced that WASP-96b’s atmosphere has no clouds.

“WASP-96b is the only exoplanet that appears to be entirely cloud-free and shows such a clear sodium signature, making the planet a benchmark for characterization,” Nikolov pointed out.

This exoplanet’s “spectrum is in excellent agreement with cloud-free, solar-abundance models,” his team wrote in their paper, published yesterday in the journal Nature.

This important discovery has now given astronomers the opportunity to observe the full range of possible atmospheres on other planets, which can help shed more light into cloud formation and composition, noted Nikolov.

“WASP-96b will also provide us with a unique opportunity to determine the abundances of other molecules, such as water, carbon monoxide and carbon dioxide with future observations,” said study co-author Dr. Ernst de Mooij, from Dublin City University in Ireland.