Artist's impression of spectra of exoplanet
Credit: INAF/M Galliani

Simulation accurately predicts oxygen in atmosphere of hottest known exoplanet

Exoplanets and planetary physics

A team including astrophysicist Dr Mitchell Young from Oxford University has detected the fingerprints of atomic oxygen in the atmosphere of the hottest known exoplanet, KELT-9b. Dr Young developed one of the most advanced computer models capable of simulating the atmospheres of hot exoplanets to enable the group to make their prediction; the agreement between the model and the observations is a milestone in the exploration of planets outside of the solar system.

Exoplanets are planets that orbit stars other than the sun. Ever since the first exoplanets were detected just over two decades ago, scientists have tried to characterise their atmospheres and explain why they appear so different from the planets in the solar system. In order to observe the atmospheres of these distant planets, scientists look for their fingerprints in the spectra of exoplanets when the planets transit their host stars. During a transit, the planet passes in front of the host star as seen from the Earth. If the planet has an atmosphere, part of the light from the host star passes through it, allowing scientists to study the physical characteristics and composition of the atmosphere.

KELT-9b: the hottest exoplanet

With a dayside temperature of more than 4000 °C, KELT-9b, discovered in 2017, is by far the hottest exoplanet known to date. This planet is a gas giant similar to Jupiter, with the distinction that the temperature in its atmosphere is high enough to melt iron. This is because it orbits close to a hot and bright host star within just about 36 hours. Scientists have been struggling to understand the nature of such a hot and peculiar object, and why it does not simply boil away, being so close to its host star.

The team detected the fingerprints of atomic oxygen in the planet’s spectrum. Given that KELT-9b is a very hot gas giant planet, this detection is not an indication of the presence of life, but it is the first definite detection of oxygen atoms in the atmosphere of an exoplanet. The detection was inspired by new simulations of the planet’s atmosphere. The team’s advanced computer model allows researchers to reproduce exoplanet atmospheres on a desktop computer and predict their temperature, structure and composition with unprecedented accuracy. The model did not only match previous observations of other species in the atmosphere but it also predicted that oxygen atoms should be able to be detected.

Model in agreement with observations

The team went back to analyse previous observations of the planet obtained by using the 3.6-m telescope at the Calar Alto observatory in Spain. The results confirmed the model’s prediction; the oxygen signatures were there all along but had been missed by earlier analysis. The model also agrees with other observations remarkably well, giving confidence that the physics in the model captures the reality of hot exoplanet atmospheres to an unprecedented degree. The results show that while KELT-9b loses some gas over time from its hot atmosphere, it is not in danger of boiling away any time soon. Its proximity to the star does lead to strong turbulence and fast wind storms in its atmosphere though; the observations indicate that wind speeds can reach up to 40,000 km/h, which is remarkable considering that the strongest wind speeds recorded on Earth are of about 400 km/h, while those of Jupiter are of about 1,500 km/h.

The agreement between the model and the observations is a milestone in the exploration of the planets outside of the solar system. It shows that realistic models of exoplanets can now be created which will significantly improve our ability to understand the atmospheres of the hottest exoplanets. While similar observations of the atmospheres of smaller, cooler planets are not yet possible, one day they will be.

‘We view this work as a dress rehearsal for future work in looking for oxygen in the atmospheres of different planets in the galaxy, including smaller, possibly habitable Earth-like worlds,’ concludes Dr Young.

High-resolution detection of neutral oxygen and non-LTE effects in the atmosphere of KELT-9B, Francesco Borsa et al, Nature Astronomy, 22 December 2021