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Exoplanet WASP-18b: Carbon Monoxide Atmosphere, No Water

Exoplanet WASP-18b: Carbon Monoxide Atmosphere, No Water

NASA JPL
November 29, 2017

A NASA-led team has found evidence that the oversized exoplanet WASP-18b is wrapped in a smothering stratosphere loaded with carbon monoxide and devoid of water. The findings 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 ultraviolet (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 and 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 Greenbelt, Maryland, lead author of the paper published in the Astrophysical Journal Letters. “We don’t know of any other extrasolar planet where carbon monoxide so completely dominates the upper atmosphere.”

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, located 325 light-years from Earth, 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 analyzed five eclipses from archived Hubble data and two from Spitzer.

 

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image credit: NASA GSFC

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