UMD Scientists Find First Clear Evidence of an Upper Level Atmosphere on an Exoplanet

An international team of researchers from UMD, University of Exeter, NASA and other institutions has found compelling evidence for a stratosphere on an enormous planet outside of the solar system. Previous research spanning the past decade has indicated possible evidence for stratospheres on planets in other solar systems, but this finding on planet, WASP-121b, 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.”

The planet, located approximately 900 light years from Earth, is a gas giant exoplanet commonly referred to as a “hot Jupiter.” The scientists made the new discovery using NASA’s Hubble Space Telescope. The research is published in the August 3, 2017 issue of the journal Nature.

In our solar system all the planets have some kind of atmosphere, but the composition, density and number of layers vary greatly. Earth’s atmosphere has five layers, of which the troposphere is closest to the planet surface and in which temperature falls as the altitude increases. The stratosphere is the next layer and here temperatures increase with higher altitudes, a phenomena also found in the stratosphere of WASP-121b. The last three layers of Earth’s atmosphere are the mesosphere, the thermosphere, and the last layer, the exosphere, which merges with the emptiness of outer space .

To study the stratosphere of WASP-121b, 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.

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.

Though researchers have not been able to positively identify the cause of the heating they hope upcoming observations at other wavelengths to address this mystery.

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.