Exoplanets are a constant source of surprise and wonder. The skies of HAT-P-7b are decorated with clouds made of ruby and sapphire, KELT-9b is hotter than most stars, and now astronomers have noticed something strange about a gas giant called WASP-19b. In the upper atmosphere of this "hot Jupiter" sits a layer of titanium oxide, which has flipped the usual atmospheric temperature structure on its head.

Although it's close to the same mass as Jupiter, WASP-19b sits far, far closer to its host star, orbiting it every 19 hours or so. Cozying up that close means the planet is boiling away at about 2,000º C (3,632º F).

Astronomers from the European Southern Observatory watched WASP-19b for over a year, analyzing the composition of its atmosphere by observing how the light from the star passes through it. Different molecules and gases will affect the wavelengths of the light seen on the other side, and the layers of different materials can be identified by observing changes in the exoplanet's radius, when it's viewed at different wavelengths.

Then, it's a matter of comparing those readings to atmospheric models. In doing so, the team determined that WASP-19b's hazy atmosphere contained small amounts of titanium oxide (TiO), as well as water and sodium.

"Detecting these molecules is no simple feat," says Elyar Sedaghati, lead author of the study. "We used an algorithm that explores many millions of spectra spanning a wide range of chemical compositions, temperatures, and cloud or haze properties in order to draw our conclusions."

The team also studied the effects of TiO on the exoplanet's climate, and found that it likely performs a similar function to Earth's ozone layer, albeit with the opposite effect on the temperature at altitude. The ozone on Earth and the titanium oxide on WASP-19b both absorb light from their parent stars, but the TiO releases the energy locally, into the stratosphere. That means that temperatures are higher in the upper atmosphere and cooler at lower altitudes – the opposite of how things work here on Earth.

"Titanium oxide can substantially affect the behavior of WASP-19b's atmosphere," says Ryan MacDonald, co-author of the study. "From altering its temperature structure, to driving strong winds, we are now one step closer to uncovering the nature of this extreme world."

This is the first time titanium oxide has been detected in the atmosphere of a gas giant exoplanet, and the finding should help astronomers develop a better understanding of the composition and processes going on in the atmospheres of these distant worlds.

"TiO has been predicted to exist in hot Jupiters for over a decade but its conclusive detection has proved elusive in the past," says Nikku Madhusudhan, co-author of the study. "The clear detection of the molecule is a major observational advancement – it is an exciting time in exoplanetary science."

The research was published in the journal Nature.

Source: European Southern Observatory

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