A hot Jupiter with titanium skies

Even if it were possible to reach the planet known as WASP-19b – which it isn’t, being 800 light years away – it would be most unpleasant place to visit.

Some of the reasons for this are obvious. WASP-19b is a “hot Jupiter”, large and so close to its parent star that a single orbit takes just 19 hours. Measurements taken by the European Southern Observatory (ESO) in Germany have established its atmospheric temperature at 2000 degrees Celsius. 

Three years of analysis by a team of astronomers led by the ESO’s Elyar Sedaghati now shed fresh light on the reasons for the super-high temperature.

In a paper published in the journal Nature, the team reveals the presence of titanium oxide in WASP-19b’s atmosphere – along with water and sodium.

Titanium oxide is rare on Earth, but is known to exist in some abundance in the atmospheres of cool stars.

It has long been suspected to be present above the surface of hot Jupiter planets. Recent research confirming the existence of a stratosphere around another hot Jupiter – WASP-121b – suggested that the molecule might play a role in inducing that planet’s atmospheric temperature of 2500 degrees Celsius.

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The work by Sedaghati and colleagues, however, is the first time the presence of titanium oxide has been confirmed.

“Detecting such molecules is, however, no simple feat,” explains Sedaghati.

“Not only do we need data of exceptional quality, but we also need to perform a sophisticated analysis. 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.”

To get that data, the team used an instrument connected to ESO’s Very Large Telescope, known as FORS2, which stands for “focal reducer/low dispersion spectrograph 2”. The instrument provides imaging, polarimetry and spectroscopy data, using two simultaneous magnifications across a wide wavelength range.

“The presence of titanium oxide in the atmosphere of WASP-19b can have substantial effects on the atmospheric temperature structure and circulation,” says team member Ryan MacDonald.

The molecule, in sufficient density, acts as a heat absorber: effectively a shield that prevents external heat entering and internal heat escaping. As a result, WASP-19b is in the grip of a permanent thermal inversion: much hotter above the ground than on it.

The team collected FORS2 data for more than a year, focusing on different light wavelengths as they passed through the atmosphere. These produced apparent changes to the planet’s radius, which were then compared to existing atmospheric models. Variations from prediction allowed deductions regarding the chemical content of the atmosphere.

The findings will be useful for refining observations for other hot Jupiters, but might also provide new methods of investigating the atmospheres of potentially habitable – or inhabited – planets.

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