"Death Planet" Atmosphere Almost Entirely Carbon Monoxide With Absolutely No Water

WASP-18b, a so-called hot Jupiter planet. NASA's Goddard Space Flight Center

Bad news first: There's definitely no water on distant planet WASP-18b. But the good news: The planet's atmosphere is keeping scientists on their toes—in fact, they say it's like nothing they have ever seen before. Although the planet was first spotted in 2009, a team of astronomers is doing new research on its atmosphere and have published a new paper on the topic in The Astrophysical Journal Letters.

"The composition of WASP-18b defies all expectations," lead author Kyle Sheppard, a doctoral student in astronomy at the University of Maryland, said in a press release. "We don't know of any other extrasolar planet where carbon monoxide so completely dominates the upper atmosphere."

That news has led some to nickname WASP-18b the "Death Planet" although it's not really any more brutal than any other planet of its type, which is known as a "hot Jupiter." In fact, it's not even really any more brutal than literally Jupiter and its neighbor Saturn, which vaporized an entire spacecraft in a matter of minutes earlier this year.

Most of the other hot Jupiters scientists have spotted in other solar systems are brutal in a different way, with atmospheres that are typically full of chemicals like titanium oxide and vanadium oxide. And some of the team's results make sense for that model: They found signs that chemicals are absorbing sunlight in WASP-18b's upper atmosphere, just as these chemicals would do.

But when the scientists analyzed the light they saw from the planet, which is a messy mix of information about all the chemicals that make it up, they realized it didn't look like any other planets that have been studied so far.

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Specifically, the models they ran suggest the atmosphere had to contain basically just carbon dioxide and carbon monoxide and nothing else. And the scientists knew that if Venus-like carbon dioxide was the culprit, there should have been enough oxygen in the atmosphere to also produce water, which was nowhere to be seen.

A sun-absorbing upper atmosphere built out of carbon monoxide, however, has never been spotted before. "This rare combination of factors opens a new window into our understanding of physicochemical processes in exoplanetary atmospheres," paper co-author Nikku Madhusudhan, an astronomer at the University of Cambridge in the United Kingdom, said in the press release.

Fortunately for astronomers, the James Webb Space Telescope, due to launch in 2019, should offer more information about WASP-18b and other exoplanets.