WASP-18b, a hot-Jupiter exoplanet located approximately 325 light-years from Earth, has a stratosphere that’s loaded with carbon monoxide, but has no signs of water, according to a team of astronomers led by Dr. Kyle Sheppard at NASA’s Goddard Space Flight Center.
The hot-Jupiter WASP-18b. Image credit: NASA’s Goddard Space Flight Center.
“The composition of WASP-18b defies all expectations. We don’t know of any other exoplanet where carbon monoxide so completely dominates the upper atmosphere,” said Dr. Sheppard, lead author of the paper published in the Astrophysical Journal Letters (arXiv.org preprint).
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 formation of a stratosphere layer in a planet’s atmosphere is attributed to ‘sunscreen’-like molecules, which absorb UV and visible radiation coming from the star and then release that energy as heat,” the astronomers explained.
“WASP-18b has an unusual composition, and the formation of this alien world might have been quite different from that of Jupiter as well as gas giants in other planetary systems.”
They looked at data collected for WASP-18b, which has the mass of 10 Jupiters, as part of a survey to find exoplanets with stratospheres.
The planet has been observed repeatedly, allowing the team to accumulate a relatively large trove of data.
The study analyzed 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’s possible to identify the spectral fingerprints of water and some other important molecules.
The analysis revealed WASP-18b’s peculiar fingerprint, which doesn’t resemble any exoplanet examined so far.
To determine which molecules were most likely to match it, the researchers 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 co-author Dr. Nikku Madhusudhan, from the University of Cambridge.
“This rare combination of factors opens a new window into our understanding of physicochemical processes in exoplanetary atmospheres.”
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.
In theory, another possible fit for the observations is carbon dioxide, which has a similar fingerprint. The authors 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.
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Kyle B. Sheppard et al. 2017. Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b. ApJL 850, L32; doi: 10.3847/2041-8213/aa9ae9
