Water on Hot Exoplanet Surprises Scientists, Tells Solar System's Origin Story

An artist’s rendering shows the scorching hot exoplanet WASP-39b. NASA/ESA

An alien planet the size of Saturn has a surprising amount of water in its atmosphere, scientists have announced.

The Hubble Space Telescope found the water vapor as it was making detailed observations of the scorching hot exoplanet called WASP-39b, which is 700 light-years away from Earth. According to a new study, the foreign planet has three times as much water as Saturn.

Scientists also found a lot of heavier elements in the inflated atmosphere. Combined with the water vapor, that suggests the planet was bombarded with metal-rich ice while it was forming.

The scientists call the observations the most detailed profile of an exoplanet's atmosphere ever created. It was made in part by combining new observations from Hubble, the Spitzer Space Telescope and the Very Large Telescope—possible in part because the exoplanet does not have clouds high up in its atmosphere—with older data.

WASP-39b was first discovered in 2011.

One thing that makes WASP-39b special is its star, which is similar to the sun. The planet itself is comparable in size to our own Saturn, although it doesn't have any rings. But it orbits so near to the star—closer than Mercury is to the sun—that it completes a revolution in four days and is sweltering. The study puts the atmosphere's temperature in the neighborhood of 1,400 degrees Fahrenheit.

The makeup of water and metals in this "hot Saturn" exoplanet's atmosphere suggests it is an immigrant to the inner region of its alien solar system, the scientists say: It's possible that "WASP-39b formed beyond the snow line in the planet-forming disk of the host star."

An exoplanet 700 light-years from Earth has a lot more water in its atmosphere than astronomers had expected to find. NASA, ESA, and G. Bacon (STScI)

A "snow line" refers to the distance from a host star where a planet's materials become solid. If this giant planet was coming together beyond that line, the pieces building it up could have been rich in metals, which would explain the elements the scientists are detecting now.

The new information about WASP-39b could help astronomers better understand our planetary neighborhood.

"We need to look outward to help us understand our own solar system," lead investigator Hannah Wakeford said in a statement on Hubble's website. "Exoplanets are showing us that planet formation is more complicated and more confusing than we thought it was. And that's fantastic."

According to Hubble, next year's expected launch of the James Webb Space Telescope—a joint effort of NASA, the European Space Agency and the Canadian Space Agency to study the origin of the universe and alien planets—could offer more information about WASP-39b, such as its carbon and oxygen content. Knowing how much of those lighter elements are in the atmosphere may tell scientists more about how the exoplanet formed.