Dry exoplanet is smothered in carbon monoxide

NASA scientists have found surprising evidence that the exoplanet WASP-18b, a "hot Jupiter" lacks water and has a stratosphere consisting almost entirely of carbon monoxide.
By Delila James | Dec 06, 2017
NASA scientists have found surprising evidence that the exoplanet WASP-18b a "hot Jupiter" lacks water and has a stratosphere consisting almost entirely of carbon monoxide.

The study is published in Astrophysical Journal Letters.

"The composition of WASP-18b defies all expectations," said lead author Kyle Sheppard of NASA's Goddard Space Flight Center in Greenbelt, Maryland, in a statement. "We don't know of any other extrasolar planet where carbon monoxide so completely dominates the upper atmosphere."

WASP-18b, which orbits very close to its star, is huge, with a mass a 10 Jupiters. It is located some 325 light-years from Earth.

To arrive at their conclusions, the researchers analyzed the light emitted by the planet's atmosphere at infrared wavelengths. They also studied five archived eclipses from the Hubble Space Telescope and two from Spitzer.

"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 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."

While carbon dioxide has a similar fingerprint to carbon monoxide and also could explain the phenomena, the researchers ruled this out because the atmosphere should contain some water vapor if there were enough oxygen around to form carbon dioxide.

The researchers anticipate better opportunities to study WASP-18b and other exoplanets with the launch of the James Webb Space Telescope.

"The expected launch of the James Webb Space Telescope and other future space-based observatories will give us the opportunity to follow up with even more powerful instruments and to continue exploring the amazing array of exoplanets out there," said co-author Avi Mandell, an exoplanet scientist at Goddard.

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