Scientists have found compelling evidence for a stratosphere on an enormous planet outside our solar system. The planet's stratosphere -- a layer of atmosphere where temperature increases with higher altitudes -- is hot enough to boil iron. WASP-121b, located approximately 900 light years from Earth, is a gas giant exoplanet commonly referred to as a "hot Jupiter."

An international team of researchers, led by the University of Exeter with contributions from the University of Maryland, made the new discovery by observing glowing water molecules in WASP-121b's atmosphere using NASA's Hubble Space Telescope. The research is published in the August 3, 2017 issue of the journal Nature.

Previous research spanning the past decade has indicated possible evidence for stratospheres on other exoplanets, but this is the first time that glowing water molecules have been detected -- the clearest signal yet to indicate an exoplanet stratosphere.

"When it comes to distant exoplanets, which we can't see in the same detail as other planets here in our own solar system, we have to rely on proxy techniques to reveal their structure," said Drake Deming, a professor of astronomy at UMD and a co-author of the study. "The stratosphere of WASP-121b so hot it can make water vapor glow, which is the basis for our analysis."

To study the gas giant's stratosphere, scientists used spectroscopy to analyze how the planet's brightness changed at different wavelengths of light. Water vapor in the planet's atmosphere, for example, behaves in predictable ways in response to certain wavelengths of light, depending on the temperature of the water. At cooler temperatures, water vapor blocks light from beneath it. But at higher temperatures, the water molecules glow.

The phenomenon is similar to what happens with fireworks, which get their colors when metallic substances are heated and vaporized, moving their electrons into higher energy states. Depending on the material, these electrons will emit light at specific wavelengths as they lose energy. For example, sodium produces orange-yellow light and strontium produces red light.

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The water molecules in the atmosphere of WASP-121b similarly give off radiation as they lose energy, but it is in the form of infrared light, which the human eye is unable to detect.

"Theoretical models have suggested that stratospheres may define a special class of ultra-hot exoplanets, with important implications for the atmospheric physics and chemistry," said Tom Evans, research fellow at the University of Exeter and lead author of the study. "When we pointed Hubble at WASP-121b, we saw glowing water molecules, implying that the planet has a strong stratosphere."

WASP-121b has a greater mass and radius than Jupiter, making it much puffier.

The exoplanet orbits its host star every 1.3 days, and the two bodies are about as close as they can be to each other without the star's gravity ripping the planet apart. This close proximity also means that the top of the atmosphere is heated to a blazing hot 2,500 degrees Celsius -- the temperature at which iron exists in gas rather than solid form.

"This new research is the smoking gun evidence scientists have been searching for when studying hot exoplanets," said Professor David Sing, an associate professor of astrophysics at the University of Exeter and a co-author of the research paper. "We have discovered this hot Jupiter has a stratosphere, a common feature seen in most of our solar system planets."

In Earth's stratosphere, ozone traps ultraviolet radiation from the sun, which raises the temperature of this layer of atmosphere. Other solar system bodies have stratospheres, too -- methane is responsible for heating in the stratospheres of Jupiter and Saturn's moon Titan, for example. In solar system planets, the change in temperature within a stratosphere is typically less than 100 degrees Celsius. However, on WASP-121b, the temperature in the stratosphere rises by 1,000 degrees Celsius.

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"We've measured a strong rise in the temperature of WASP-121b's atmosphere at higher altitudes, but we don't yet know what's causing this dramatic heating," said Nikolay Nikolov, co-author and research fellow at the University of Exeter. "We hope to address this mystery with upcoming observations at other wavelengths."

Vanadium oxide and titanium oxide gases are candidate heat sources, as they strongly absorb starlight at visible wavelengths, much like ozone absorbs UV radiation. These compounds are expected to be present in only the hottest of hot Jupiters, such as WASP-121b, as high temperatures are required to keep them in the gaseous state. Indeed, vanadium oxide and titanium oxide are commonly seen in brown dwarfs, 'failed stars' that have some commonalities with exoplanets.

NASA's forthcoming James Webb Space Telescope will be able to follow up on the atmospheres of planets like WASP-121b with higher sensitivity than any telescope currently in space.

"This super-hot exoplanet is going to be a benchmark for our atmospheric models, and will be a great observational target moving into the Webb era," said Hannah Wakeford, a research fellow at the University of Exeter and a co-author of the research paper.