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Exoplanet shines with glowing water atmosphere

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Staff Writer | August 3, 2017
Scientists have found compelling evidence for a stratosphere on an enormous planet outside our solar system.
Hot Jupiter
Space   Approximately 900 light years from Earth
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.

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.


 

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