An international team of astronomers believe they have collected the strongest evidence yet of a stratosphere in an exoplanet’s atmosphere by detecting glowing water molecules around the planet.
The scientists report that their detection has a five sigma confidence level, which means the feature is a true stratosphere with 99.99994 percent certainty. The object of the study, published in Nature, is WASP-121b – a hot Jupiter located 880 light-years from Earth. Curiously, it orbits its star around the poles, not the equator.
Using the Hubble Space Telescope and NASA’s infrared imager Spitzer, the team managed to distinguish the elements that likely made up the exoplanet atmosphere, as well as the temperature of said elements. The planet is estimated to have a temperature of about 2,700/2,800°C (4,900/5,080°F), hot enough to boil lead, and its stratosphere is about 1,000°C (1,832°F) hotter. In the solar system stratospheres, the increase is about 100°C (212°F).
"We used Hubble to detect glowing water molecules in the atmosphere of WASP-121b, which implies that the upper layers of the atmosphere must be hotter than the lower layers, i.e. a stratosphere," lead author Dr Tom Evans told IFLScience. "If the upper layers were cooler than the lower layers – as you might expect the atmosphere to cool off towards space – you would see the opposite, and this water gas would block out infrared light at specific wavelengths rising from the hotter deeper layers."
A stratosphere is an unusual layer and its temperature profile is a bit counterintuitive. Our experience tells us that the higher we go in altitude, the cooler it gets, but this notion is flipped in the stratosphere of Earth. The temperature increases the further you are from the planet, due to the absorption of ultraviolet radiation by the ozone layer.
"The presence of a stratosphere on WASP-121b implies that a large amount of the starlight beating down on the planet is getting absorbed high up in the atmosphere by 'something'. The glowing water gas is a symptom of this heating of the upper atmosphere, but almost certainly not the cause of the heating," Dr Evans continued.
WASP-121b doesn’t have an ozone layer, but it could be rich in more complex molecules that are expected at high temperature. In this and previous studies, researchers detected hints of molecules like titanium oxide and vanadium oxide, which could be responsible for the stratosphere, although more observations are needed to confirm that.
"Our understanding of these atmospheres is still very basic, so we've got lots of work to do in understanding the physics and chemistry behind why some hot planets form stratospheres and some don't appear to," Dr Evans added. "The first step in this direction will be to identify what is causing the stratospheres in the first place – what is the 'mystery absorber'? And is it the same absorber for all stratospheres?"
At least a few models have suggested that if an exoplanet is exposed to strong radiation, it could develop a stratosphere. However, researchers haven’t provided solid estimates for the likelihood of this feature in the universe and, before this study, there have only been two other claims for stratospheres on exoplanets. One has since been revised, while the second one has not been characterized with the same level of detail as WASP-121b.
Precise observations of exoplanetary atmospheres are going to be the next big thing. Thanks to more targets, better analytical tools, and several new instruments, the next decade will see astronomers reach a better understanding of what the atmospheres of exoplanets are truly like.
