Surprisingly Water-Rich Atmosphere Discovered On "Hot Saturn" Exoplanet

Artist's Concept of WASP-39b NASA, ESA, G. Bacon and A. Feild (STScI)

A British-American team of researchers has used the Hubble and Spitzer space telescopes to better understand a peculiar and currently unique planet, WASP-39b. The object is a hot Saturn, meaning it's roughly the same size as the ringed planet but much closer to its star.

The observations, reported in the Astronomical Journal, revealed that although the researchers expected to see water, they were surprised by how much water they found. In fact, this exoplanet’s atmosphere is three times richer in water than Saturn’s own. This peculiar fact suggests that this particular planet must have formed away from its parent star, where icy comets and asteroids collided with it, and then migrated inward.

“WASP-39b shows exoplanets can have much different compositions than those of our solar system,” co-author David Sing of the University of Exeter, said in a statement.

“Hopefully this diversity we see in exoplanets will give us clues in figuring out all the different ways a planet can form and evolve.”

The exoplanet is located 700 light-years away in the constellation Virgo, and 7 million kilometers (4,350,000 miles) from its star, which is about twenty times closer than the Earth is from the Sun. It is tidally locked with one face always facing the star. That side reaches a temperature of 776.7°C (1,430°F). This proximity gives WASP-39b some unusual looks, too. It has the mass of Saturn but it is so puffy that it is bigger than Jupiter.

Crucially for these observations, the hot atmosphere is free of high altitude clouds. Researchers were able to peer right down into the planet and gather the information about the water content. Without it, the team wouldn’t have been able to reconstruct its likely formation.

“We need to look outward so we can understand our own solar system,” explained lead investigator Hannah Wakeford of the Space Telescope Science Institute in Baltimore, Maryland, and the University of Exeter. “But exoplanets are showing us that planet formation is more complicated and more confusing than we thought it was. And that’s fantastic!”

The team hopes that when the James Webb Space Telescope goes online next year, they will be able to perform an even better analysis. Studying carbon and oxygen, for example, will allow a better understanding of where and how the planet formed. 

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