The temperature of a planet’s atmosphere decreases with increasing altitude, unless a shortwave absorber that causes a temperature inversion exists. Ozone plays this role in the Earth’s atmosphere. In the atmospheres of highly irradiated exoplanets, the shortwave absorbers are predicted to be titanium oxide and vanadium oxide. Using high-resolution transmission spectroscopy, astronomers have now detected titanium oxide in the atmosphere of the ultrahot Jupiter WASP-189b. Moreover, they have detected metals, including neutral and singly ionized iron and titanium, as well as chromium, magnesium, vanadium and manganese.
WASP-189 is a 730-million-year-old A-type star located 322 light-years away in the constellation of Libra.
Also known as HD 133112, the star is larger and more than 2,000 degrees Celsius hotter than the Sun, and so appears to glow blue.
Discovered in 2018, WASP-189b is a transiting gas giant about 1.6 times the radius of Jupiter
It sits around 20 times closer to the star than Earth does to the Sun, and completes a full orbit in just 2.7 days.
“WASP-189b has a high equilibrium temperature of 2,368 degrees Celsius due to its close proximity to its hot A-type host star,” said Lund University astronomer Bibiana Prinoth and colleagues.
“It is one of the brightest transiting planet systems currently known, making it very amenable for spectroscopic studies of its atmosphere.”
“The system is well characterized, thanks to extensive photometric observations with CHEOPS, including a precise measurement of the orbital parameters.”
The astronomers observed the spectrum of WASP-189 during three transit events with the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at ESO’s 3.6-m telescope in La Silla Observatory, Chile.
“We measured the light coming from the planet’s host star and passing through the planet’s atmosphere,”
“The gases in its atmosphere absorb some of the starlight, similar to ozone absorbing some of the sunlight in Earth’s atmosphere, and thereby leave their characteristic ‘fingerprint’.”
“With the help of HARPS, we were able to identify the corresponding substances.”
“The gases that left their ‘fingerprints’ in the atmosphere of WASP-189b included iron, chromium, vanadium, magnesium and manganese.”
The researchers were also able to detect titanium oxide in the atmosphere of WASP-189b.
“Titanium oxide absorbs short wave radiation, such as ultraviolet radiation,” said University of Bern’s Professor Kevin Heng.
“Its detection could therefore indicate a layer in the atmosphere of WASP-189b that interacts with the stellar irradiation similarly to how the ozone layer does on Earth.”
“In our analysis, we saw that the ‘fingerprints’ of the different gases were slightly altered compared to our expectation,” Prinoth said.
“We believe that strong winds and other processes could generate these alterations.”
“And because the fingerprints of different gases were altered in different ways, we think that this indicates that they exist in different layers — similarly to how the fingerprints of water vapor and ozone on Earth would appear differently altered from a distance, because they mostly occur in different atmospheric layers.”
The results were published in the journal Nature Astronomy.
B. Prinoth et al. Titanium oxide and chemical inhomogeneity in the atmosphere of the exoplanet WASP-189b. Nat Astron, published online January 27, 2022; doi: 10.1038/s41550-021-01581-z