Astronomers have potentially detected a stratosphere in the atmosphere of a hot Jupiter.

The study of exoplanet atmospheres is in its early years. Astronomers have been able to detect some compounds, including water, and infer compositions based on a planet’s glow or starlight filtered through its clouds.

hot Jupiter WASP-121b
Artist's concept of the hot Jupiter WASP-121b and its host star.
Engine House VFX / At-Bristol Science Centre / University of Exeter

One thing that’s eluded observers, though: a stratosphere. Earth’s atmosphere has both a troposphere — the lower layer, where the temperature drops the higher you go — and a stratosphere, where things turn on their heads and the temperature increases with altitude. This temperature inversion exists because of ozone, which absorbs the Sun’s ultraviolet rays and heats up the stratosphere.

Previous claims of exoplanet stratospheres have often not withstood follow-up scrutiny. But reporting in the August 3rd Nature, Thomas Evans (University of Exeter, UK) and colleagues think they’ve found this long-sought layer, in the atmosphere of the hot Jupiter WASP-121b.

WASP-121b clearly hasn’t read the latest dermatologist recommendations, because the hot Jupiter is sunning itself in the blaze of its yellow F-type star, whipping around the star in a little more than a day at a distance less than one-tenth that of Mercury’s average distance from the Sun. The star roasts the gas giant, heating it to roughly 2500 kelvin (4000°F).

By watching the hot Jupiter pass behind its star with the Hubble and Spitzer Space Telescopes, Evans’s team teased out light that came from the exoplanet. Although they couldn’t resolve individual spectral lines in this light, the researchers were able to study the shape that multiple, mushed-together lines made. Among the shapes they uncovered, they caught a bump in the exoplanet’s spectra that they think is emission from water.

We should only detect emission from water molecules if the part of the upper atmosphere they’re in is hotter than the gas that lies below — if the gas below were hotter, the water would absorb some of its glow, an effect not seen here. Thus, the team concludes that WASP-121b has a stratosphere. That implies it has some sort of ozone-mimicking compound in its upper atmosphere, perhaps vanadium oxide (VO), although the team couldn’t determine whether VO is present.

The result is not incontrovertible, as Kevin Heng (University of Bern, Switzerland) lays out in a thoughtful Nature perspective piece. The stratosphere detection depends on several assumptions, any of which might not hold water (pun intended). It’s also possible that we’ve had “a failure of imagination,” overlooking some physics that could explain the water emission without an inverted-temperature layer, says Jonathan Fortney (University of California, Santa Cruz), who was also not involved in the study.

Caveats aside, Fortney adds, “This is the most compelling paper on this topic in a decade.” It’s especially nice because NASA’s upcoming James Webb Space Telescope could confirm or refute the result after its launch next year.


Reference: T. M. Evans et al. “An ultrahot gas-giant exoplanet with a stratosphere.” Nature. August 3, 2017.

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August 4, 2017 at 9:01 pm

"Hot Jupiter" indeed. You only have to put your tongue a short distance into your cheek to start wondering at what point one should transition from speaking of a "stratosphere" to speaking of a "corona".

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