Astronomers have discovered an exoplanet which provides the strongest evidence about the existence of a stratosphere on a planet along with glowing water molecules around it.
ALSO READ: NASA continues research on pulsars 50 years after the accidental discovery: Top 5 things to know
A stratosphere refers to a layer in which the temperature rises with higher altitudes.
"This result is exciting because it shows that a common trait of most of the atmospheres in our solar system -- a warm stratosphere -- also can be found in exoplanet atmospheres," said Mark Marley, study co-author based at NASA's Ames Research Center in California's Silicon Valley.
"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.
Data from NASA's space telescope Hubble was used by the researchers to analyse the WASP-121b – an exoplanet which is classified as hot Jupiter.
The WASP-121 system is located at an estimated distance of 900 light years from Earth, which is considered to be close by galactic standards. The mass of WASP-121b is around 1.2 times that of gas giant Jupiter. The orbit period of Jupiter around the Sun is 12 years, while WASP-121b completes its orbital period in just 1.3 days.
The exoplanet is very close to its parent star and if it gets any closer, it could get ripped apart because of the star's gravity, a NASA statement revealed. The atmosphere of this exoplanet is heated to a high temperature of 2,500 Celsius (4,600 degrees Fahrenheit), which is scorching enough to boil some metals.
According to the previously conducted research, the possible signs of a stratosphere were detected on WASP-33b as well as some other hot Jupiters. The latest study by astronomers is believed to be the best evidence so far, as it is the pioneering proof about the signature of hot water molecules which the researchers observed.
"Theoretical models have suggested stratospheres may define a distinct class of ultra-hot planets, with important implications for their atmospheric physics and chemistry," said Tom Evans, lead author and research fellow at the University of Exeter, United Kingdom. "Our observations support this picture," he added.
The scientists studied how the different molecules present in the atmosphere of the WASP-121b reacted to specific wavelengths of light. This analysis was carried out with the help of Hubble's capabilities for spectroscopy.
"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," says astronomer Drake Deming from the University of Maryland.
"The stratosphere of WASP–121b is so hot it can make water vapour glow, which is the basis for our analysis."
Water vapour 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.
Starlight is able to penetrate deep into a planet's atmosphere, where it raises the temperature of the gas there. This gas then radiates its heat into space as infrared light. However, if there is cooler water vapour at the top of the atmosphere, the water molecules will prevent certain wavelengths of this light from escaping to space. But if the water molecules at the top of the atmosphere have a higher temperature, they will glow at the same wavelengths.
"The emission of light from water means the temperature is increasing with height," said Tiffany Kataria, study co-author based at NASA's Jet Propulsion Laboratory, Pasadena, California. "We're excited to explore at what longitudes this behaviour persists with upcoming Hubble observations."
The phenomenon is similar to what happens 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: sodium produces orange-yellow and strontium produces red in this process, for example. The water molecules in the atmosphere of WASP-121b similarly emit radiation as they lose energy, but in the form of infrared light, which the human eye is unable to detect, a NASA statement revealed.
In Earth's stratosphere, ozone gas 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 around 56 degrees Celsius (100 degrees Fahrenheit). On WASP-121b, the temperature in the stratosphere rises by 560 degrees Celsius (1,000 degrees). Scientists do not yet know what chemicals are causing the temperature increase in WASP-121b's atmosphere. Vanadium oxide and titanium oxide are candidates, as they are commonly seen in brown dwarfs, "failed stars" that have some commonalities with exoplanets. Such compounds are expected to be present only on the hottest of hot Jupiters, as high temperatures are needed to keep them in a gaseous state.
"This super-hot exoplanet is going to be a benchmark for our atmospheric models, and it will be a great observational target moving into the Webb era," said Hannah Wakeford, study co-author who worked on this research while at NASA's Goddard Space Flight Center, Greenbelt, Maryland.
