NASA’s Hubble Space Telescope has discovered the strongest evidence to date for a stratosphere on a planet outside our solar system, WASP-121b.
The stratosphere is a layer of atmosphere in which temperature increases with higher altitudes. The discovery was made by an International team of researchers led by the University of Exeter by observing glowing water molecules in WASP-121b's atmosphere.
Mark Marley, co-author of the study described the result as exciting, as the discovery shows that a common trait of most of the atmospheres in our solar system, which is a warm stratosphere, can also be found in the atmospheres of exoplanets.
"We can now compare processes in exoplanet atmospheres with the same processes that happen under different sets of conditions in our own solar system,” Marley added.
The scientists used spectroscopy, in order to study the exoplanet’s stratosphere, to analyse how the planet's brightness changed at different wavelengths of light.
Key Analysis Points
• The Water vapour in the planet's atmosphere behaves in predictable ways in response to different wavelengths of light, depending on the temperature of the water.
• At cooler temperatures, water vapour in the planet's upper atmosphere blocks light of specific wavelengths radiating from deeper layers towards space.
• However at higher temperatures, the water molecules in the upper atmosphere glow at the wavelengths, instead of blocking them.
• The phenomenon is similar to what occurs with fireworks, which get their colours from chemicals emitting light. When metallic substances are heated and vaporized, their electrons move into higher energy states. Depending on the material, these electrons will emit light at specific wavelengths as they lose energy.
• 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 is not visible to the naked human eye.
Elaborating on the finding, Dr. Tom Evans, lead author and research fellow at the University of Exeter said, “Theoretical models have suggested that stratospheres may define a special class of ultra-hot exoplanets, with important implications for the atmospheric physics and chemistry. When we pointed Hubble at WASP-121b, we saw glowing water molecules, implying that the planet has a strong stratosphere."
• Located approximately 900 light years from Earth, the exoplanet is a gas giant commonly referred to as a 'hot Jupiter'.
• However, it has 1.2 times greater mass and 1.9 times greater radius than Jupiter.
• It orbits around its host star every 1.3 days in very close proximity.
• It is so close to its star that if it got any closer, the host star’s gravity would start ripping it apart.
• The extreme proximity also means that the top of the exoplanet’s atmosphere is heated to a blazing hot 2,500 degrees Celsius, the temperature that is hot enough to boil some metals like iron, which in such conditions exists in gas rather than solid form.
In the case of stratospheres in our solar system, the Earth’s stratosphere is warm as ozone gas traps ultraviolet radiation from the sun in this layer raising its temperature and methane is responsible for heating in the stratospheres of Jupiter and Saturn's moon Titan.
However, while the change in temperature within the stratosphere in the planets within our solar system is typically around 100 degrees Fahrenheit (about 56 degrees Celsius), on WASP-121b, the temperature in the stratosphere rises by 1,000 degrees (560 degrees Celsius).
Scientists are not yet clear what chemicals are causing the rise in temperature in WASP-121b's atmosphere.
The research was published in the August 2017 issue of a leading scientific journal called Nature.
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