A NASA-led team of scientists has determined that WASP-18b, a “hot Jupiter” located 325 light-years from Earth, has a smothering stratosphere that is loaded with carbon monoxide, or CO, but has no signs of water. “The composition of WASP-18b defies all expectations,” said Kyle Sheppard of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“We don’t know of any other extrasolar planet where carbon monoxide so completely dominates the upper atmosphere,” Sheppard said. The findings, published in the Astrophysical Journal Letters, come from a new analysis of observations made by the Hubble and Spitzer space telescopes.
The formation of a stratosphere layer in a planet’s atmosphere is attributed to “sunscreen”-like molecules, which absorb ultra violet (UV) and visible radiation coming from the star and then release that energy as heat. The new 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.
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.
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 modeling.
“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,” Madhusudhan said.”Hot Jupiters” are gas giants like Jupiter but much hotter, with orbits that take them feverishly close to their stars.