Giant, waterlogged "hot Saturn" hints at breadth of exoplanet diversity

Water is not only a key ingredient in supporting life, it's also a major clue as to how planets form, and NASA has found a lot of the stuff in the atmosphere of a giant exoplanet called Wasp-39b. The planet is as massive as Saturn but has three times as much water as the famous ringed planet. Although this "hot Saturn" is far from habitable, it does provide insights into the wide variety of planets in the universe.

Located 700 light years from Earth in the constellation of Virgo, Wasp-39b is not what one would call a garden spot. Its mass is only 0.28 that of Jupiter, but it's radius is 1.27 greater than the largest planet in our solar system. It's also 20 times closer to its star, Wasp-39, than the Earth is to the Sun, which it orbits once every four days.

The planet is tidally locked with one side always facing its parent star, with a dayside temperature of a scorching 1,430° F (776.7° C). Powerful winds circulate this heat from the day to the night side, which means both hemispheres are just as hot. In addition, it probably lacks Saturn's characteristic rings.

But it's the amount of water on Wasp-39b that interests a team of scientists led by Hannah Wakeford of the Space Telescope Science Institute in Baltimore, and the University of Exeter. They were confident that traces of water would be found on the exoplanet as such signs have been found in the gas giants of the Solar System, but finding three times as much was a surprise – and a very informative one at that.

The presence of water was detected by the Hubble and Spitzer space telescopes using spectrographs to break up the light from its star as it passed through the unusually high, clear, cloudless atmosphere of Wasp-39b.

The discovery of its atmosphere having three times the amount of water of Saturn indicates that the exoplanet had its origins much farther out than its present orbit, where it would have been bombarded by comets and other icy objects before spiraling in the inner part of its system. NASA says that it may even have destroyed other planetary bodies in its wanderings.

"WASP-39b shows exoplanets can have much different compositions than those of our solar system," team member David Sing of the University of Exeter. "Hopefully, this diversity we see in exoplanets will give us clues in figuring out all the different ways a planet can form and evolve."

Wakeford plans to book time on NASA's James Webb Space Telescope, which is due to launch later this year, to record a more complete spectrogram of the exoplanet's atmosphere with an emphasis on how much carbon and oxygen is there so as to gain a better understanding of its formation.

Source: NASA