An Atmosphere of Heavy Metals

Researchers have found strong evidence of titanium oxide in the atmosphere of a hot giant planet, adding new insights to the complex motions of these planets’ extreme atmospheres.

hot exoplanet with titanium oxide

An artist's impression of the exoplanet WASP-19b where astronomers have found titanium oxide.
ESO/M. Kornmesser

Astronomers might have observed one of the molecules that govern the atmospheric structure of a hot Jupiter.

An international team of researchers used the European Southern Observatory’s Very Large Telescope to detect titanium oxide in the atmosphere of WASP-19b, a Jupiter-mass planet that orbits its host star in just 19 hours and has a surface temperature of 2000°C.

WASP-19b belongs to a class of exoplanet known as hot Jupiters, giant balls of smoldering gas closely circling their host stars. In such planets, astronomers think titanium oxide could play a role similar to that of ozone on Earth’s atmosphere, absorbing radiation from incoming starlight and redistributing it thought the atmosphere.

However, observing a planet like WASP-19b is challenging because of its distance and its faint glow relative to the light of its star.

In this case, Elyar Sedaghati (European Southern Observatory and Technical University of Berlin) and colleagues captured the star’s light as it passed through a sliver of WASP-19b’s atmosphere during several of the planet’s transits, when the planet passes in front of the star from our point of view.

The planet’s atmosphere can absorb certain wavelengths of light as it passes through, introducing subtle changes that telescopes like the Very Large Telescope in Chile can pick up. This technique is called transmission spectroscopy. Analyzing those changes, scientists reconstruct the atmosphere’s chemical makeup.

In WASP-19b’s atmosphere, Sedaghati’s team found titanium oxide, water, and a thick, light-scattering haze. They also detected the presence of sodium.

But it’s worth noting that this is not a direct observation of each substance’s spectral line — they are sort of blended together in the transmission spectrum they obtained from the star’s light. To disentangle the different chemical species, Sedaghati’s team had to match their observations to existing atmospheric models. To do so, they used a computer algorithm that shuffles several parameters, such as the planet’s temperature — as well as possible environmental scenarios such as presence or absence of clouds or haze — to come up with the chemical mix that would most likely produce the observed effects.

Previous research has used similar methods to ferret out lighter elements such as hydrogen, carbon, sodium, and potassium in hot giant atmospheres.

The discovery of titanium oxide, if it holds up, is important because of the effect it has on an exoplanet’s atmosphere. It’s thought to act as a heat absorber: where it exists, it captures the heat of the host star’s light, creating a layer of the atmosphere where temperature increases instead of decreases with altitude. Earth has a similar inversion layer, due to ozone in the lower stratosphere.

Astronomers have already captured hints of such an inversion in other hot Jupiters, such as WASP-33b and WASP-121b, but this study represents the first solid evidence of the molecule that could be the culprit of such inversions in these incredibly hot atmospheres.

 

References: Sedaghati et al. “Detection of titanium oxide in the atmosphere of a hot Jupiter.” Nature, September 2017.

Heng. “Astronomy: Ozone-like layer in an exoplanet atmosphere.Nature, August 03, 2017

Haynes et al. “Spectroscopic Evidence for a Temperature Inversion in the Dayside Atmosphere of the Hot Jupiter WASP-33b.” Astrophysical Journal, May 06, 2015.

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