Further studies of a planet discovered in 2009 has led to findings that the ‘hot Jupiter’ has a stratosphere loaded with carbon monoxide and devoid of water suggesting that its formation might have been quite different from that of Jupiter as well as gas giants in other planetary systems.
Scientists explain that stratosphere is formed in a planet’s atmosphere thanks to “sunscreen”-like molecules that absorb UV and visible radiation coming from the star and then release that energy as heat. Instead the composition of WASP-18b defies all expectations as the carbon monoxide completely dominates the upper atmosphere of WASP-18b.
Ozone on Earth 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 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 micrometers and an emission signature at about 4.5 micrometers. This is the first time researchers have detected both types of fingerprints for a single type of molecule in an exoplanet’s atmosphere.
In theory, another possible fit for the observations is carbon dioxide, which has a similar fingerprint. The researchers ruled this out because if there were enough oxygen available to form carbon dioxide, the atmosphere also should have some water vapor.
To produce the spectral fingerprints seen by the team, the upper atmosphere of WASP-18b would have to be loaded with carbon monoxide. Compared to other hot Jupiters, this planet’s atmosphere likely would contain 300 times more “metals,” or elements heavier than hydrogen and helium. This extremely high metallicity would indicate WASP-18b might have accumulated greater amounts of solid ices during its formation than Jupiter, suggesting it may not have formed the way other hot Jupiters did.