The oversized planet WASP-18b, located 325 light-years from Earth, is wrapped in a smothering stratosphere loaded with carbon monoxide and devoid of water, a NASA-led team has found.
The formation of a stratosphere layer in a planet's atmosphere is attributed to sunscreen-like molecules, which absorb ultraviolet (UV) and visible radiation coming from the star and then release that energy as heat.
The study suggests that the "hot Jupiter" WASP-18b, a massive planet that orbits very close to its host star, has an unusual composition, and the formation of this world might have been quite different from that of Jupiter as well as gas giants in other planetary systems.
"The composition of WASP-18b defies all expectations," said Kyle Sheppard of NASA's Goddard Space Flight Center in the US.
"We don't know of any other extrasolar planet where carbon monoxide so completely dominates the upper atmosphere," said Sheppard, lead author of the research published in the Astrophysical Journal Letters.
On Earth, ozone absorbs UV in the stratosphere, protecting our world from a lot of the Sun's harmful radiation.
For the handful of exoplanets with stratospheres, the absorber is typically thought to be a molecule such as titanium oxide, a close relative of titanium dioxide, used on Earth as a paint pigment and sunscreen ingredient.
The researchers looked at data collected for WASP-18b as part of a survey to find exoplanets with stratospheres.
The heavyweight planet, which has the mass of 10 Jupiters, has been observed repeatedly, allowing astronomers to accumulate a relatively large trove of data.
This study analysed five eclipses from archived Hubble data and two from Spitzer.
From the light emitted by the planet's atmosphere at infrared wavelengths, beyond the visible region, it is possible to identify the spectral fingerprints of water and some other important molecules.
The analysis revealed WASP-18b's peculiar fingerprint, which does not resemble any exoplanet examined so far. To determine which molecules were most likely to match it, the team carried out extensive computer modelling.
"The only consistent explanation for the data is an overabundance of carbon monoxide and very little water vapor in the atmosphere of WASP-18b, in addition to the presence of a stratosphere," said Nikku Madhusudhan a co-author of the study from the University of Cambridge.
"This rare combination of factors opens a new window into our understanding of physicochemical processes in exoplanetary atmospheres," said Madhusudhan.
The findings indicate that WASP-18b has hot carbon monoxide in the stratosphere and cooler carbon monoxide in the layer of the atmosphere below, called the troposphere.
The team determined this by detecting two types of carbon monoxide signatures, an absorption signature at a wavelength of about 1.6 micrometres and an emission signature at about 4.5 micrometres.
This is the first time researchers have detected both types of fingerprints for a single type of molecule in an exoplanet's atmosphere.