Stratosphere detected on distant exoplanet


Exoplanet WASP-121b is so hot that water vapour in its upper atmosphere glows brightly enough to be seen by the Hubble Space Telescope.


An artist's impression of WASP-121b and its host star.
An artist's impression of WASP-121b and its host star.
Engine House VFX, At-Bristol Science Centre, University of Exeter

Glowing water molecules detected through infrared radiation provided astronomers with a key clue that allowed them to identify the presence of a stratosphere on a distant planet.

A stratosphere is a layer of upper atmosphere in which temperatures rise as the altitude increases and gases absorb ultraviolet radiation from a host star.

The Earth’s stratosphere contains about 20% of our planet’s atmospheric gases. The layer beneath it, the troposphere, is characterised by a temperature range that decreases as altitude rises.

A report published in the journal Nature establishes the existence of a stratosphere around an exoplanet dubbed WASP-121b, about 900 light years away.

Although astronomers have long considered the presence of stratospheres on some exoplanets likely, this is the first time one has been definitively identified. An earlier claim, in 2015, concerning a planet dubbed WASP-33b, is considered debatable, because of the comparatively low quality of the data obtained.

Both WASP-121b and WASP-33b are known as “hot Jupiters”. Also known as “roasters”, these are planets that, like Jupiter, are gas giants, but also have very short orbital periods, generally less than 10 days.

WASP-121b has a greater diameter and mass than Jupiter, and orbits its host star every 1.3 days.

To make their findings, a team of scientists led by David Sing from the University of Exeter, UK, analysed data showing glowing water vapour obtained by NASA's Hubble Space Telescope.

The vapour provided a valuable diagnostic tool for the astronomers, because it behaves in ways that are regular and predictable in relation to known temperatures and certain wavelengths of light. Broadly, if the water vapour is cool it will block light reflecting from beneath it. If it is warm, it will glow.

“When it comes to distant exoplanets, which we can’t see in the same detail as other planets here in our own solar system, we have to rely on proxy techniques to reveal their structure," say co-author Drake Deming, from the University of Maryland in the US.

“The stratosphere of WASP-121b is so hot it can make water vapor glow, which is the basis for our analysis.”

The glow, however, is not visible to the human eye, because it is in the infrared part of the spectrum. Using the Hubble Telescope, however, made it easy to detect.

Unlike many other planets that are currently the focus of astrophysical interest, WASP-121b is definitely not considered a possible candidate for life. Apart from its dizzying rate of orbit, if it were any nearer its host star gravity would rip it apart.

The temperature at the top of its stratosphere is calculated to be 2500 degrees Kelvin (2227 Celsius) – and therein lies a question that provides Sing and his colleagues with their next research target.

Earth’s stratosphere increases in temperature from bottom to top by around 100 degrees Celsius. WASP-121b’s racks up 10 times that gradient.

It is suspected that the extreme temperatures are caused by the presence of vanadium oxide and titanium oxide gases, both thought to be present on super-hot gas giants, but as yet the evidence is lacking.

“We’ve measured a strong rise in the temperature of WASP-121b’s atmosphere at higher altitudes, but we don't yet know what’s causing this dramatic heating,” says co-author Nikolay Nikolov.

“We hope to address this mystery with upcoming observations at other wavelengths.”

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Andrew Masterson is an author and journalist based in Melbourne, Australia.
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