JWST Finds Ammonia and Puzzling Chill in Giant Exoplanet's Atmosphere

Astronomers using the James Webb Space Telescope have detected water, methane, ammonia and carbon dioxide in the atmosphere of a giant exoplanet orbiting a small, cool star, a discovery that complicates existing theories of how such large planets form.

The planet, HATS-6 b, sits roughly 500 light-years from Earth and circles an M dwarf star once every three days. It is close in size to Jupiter, a fact that puzzles researchers because M dwarfs are thought to lack the material and time needed to build planets that large. A University of Maryland-led team reported the findings in the Astronomical Journal.

Ammonia proved to be the most significant find. This is only the second time the compound has been detected in a distant planet's atmosphere using transmission spectroscopy, the technique of analyzing starlight filtered through a planet's air as it passes in front of its star. Lead author Giannina Guzman Caloca, of the University of Maryland, said the result strengthens the case that giant planets orbiting M dwarf stars form through different chemical pathways than the hot Jupiters found around sun-like stars.

Water, hydrogen and carbon compounds have turned up before in the atmospheres of giant exoplanets, Guzman Caloca said, but ammonia has almost never been detected. Its presence, and its concentration, offers a new clue to the conditions under which HATS-6 b took shape.

The planet's temperature presented a second puzzle. Standard models predict HATS-6 b should run close to 800 degrees Fahrenheit given how tightly it orbits its star. Early observations instead point to a much cooler planet, closer to 250 degrees Fahrenheit, an outcome the researchers call physically improbable at that orbital distance without some additional explanation.

Guzman Caloca said the likeliest cause is a thick layer of cloud and haze reflecting starlight back into space before it can heat the planet, in much the way clouds shroud Venus. If confirmed, the finding would mark one of the clearest examples yet of clouds significantly altering a giant exoplanet's temperature.

HATS-6 b is one of seven targets in NASA's Giant Exoplanets around M dwarf Stars program, known as GEMS, which is designed to compare these planets systematically against the better-studied giants that orbit sun-like stars. Researchers say the program should help determine whether HATS-6 b's atmosphere is typical of planets in its class or an outlier.

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