Astronomers Measure the Layers of an Exoplanet's Atmosphere

The number of planets discovered beyond our Solar System has grown exponentially in the past twenty years, with 4,919 confirmed exoplanets (and another 8,493 awaiting confirmation)! Combined with improved instruments and data analysis, the field of study is entering into an exciting new phase. In short, the focus is shifting from discovery to characterization, where astronomers can place greater constraints on potential habitability.

In particular, the characterization of exoplanet atmospheres will allow astronomers to determine their chemical makeup and whether they have the right characteristics to support life. In a new study led by the University of Lund, an international team of researchers characterized the atmosphere of one of the most extreme exoplanets yet discovered. This included discerning what could be several distinct layers that have particular characteristics.

The study, which recently appeared in the journal Nature Astronomy, was conducted by researchers from the Lund Observatory (University of Lund), the National Centre of Competence in Research (NCCR) PlanetS, the Max Planck Institute for Extraterrestrial Physics, the National Institute for Astrophysics (INAF), the European Southern Observatory (ESO), and multiple universities and research institutes from the UK, Canada, and Chile.

As Earth’s atmosphere demonstrates, planetary atmospheres do not consist of a single uniform envelope but many layers, each with characteristic properties. The lowest layer of our atmosphere, which extends from sea level to the highest mountain peaks (the troposphere), is where most meteorological phenomena occur since it contains the most water vapor of any layer. Above that is the stratosphere, which contains the ozone layer that shields the surface from potentially harmful ultraviolet radiation.

Next is the mesosphere, which is very thin and cold but still dense enough that meteors will burn up as they pass through it. The thermosphere is next, where temperatures increase again with altitude (due to Solar heating). The uppermost layer is the exosphere, which is too thin for any meteorological phenomena to occur. However, the Aurora Borealis and Aurora Australis sometimes occur in the lower part of the exosphere, overlapping into the thermosphere.

For the sake of their study, the international team examined the exoplanet known as WASP-189b, a “Hot Jupiter” located 322 light-years from Earth. This planet was discovered in 2018 using the Wide-Angle Search for Planets (WASP) consortium, while extensive follow-up observations were conducted in 2020 using the ESA’s CHaracterising ExOPlanets Satellite (CHEOPS). These revealed a planet about twice the radius of Jupiter that orbits its host star 20 times closer than Earth orbits the Sun – leading to daytime temperatures of 3,200 °C (5,790 °F).

Using more recent observations with the High Accuracy Radial velocity Planet Searcher (HARPS), a spectrograph integrated with the 3.6-meter telescope at the ESO’s La Silla Observatory, the team was able to examine the atmosphere of this Hot Jupiter for the first time. These spectral observations revealed an atmosphere with the chemical “fingerprints” of iron, chromium, vanadium, magnesium, and manganese.

As Lund doctoral student Bibiana Prinoth (who was the lead author on the study) explained in a University of Bern press release:

“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.”

In addition to the previously-mentioned minerals, the team was interested to find traces of titanium oxide gas. This substance has a melting point of 1,843 °C (3350 °F) and is very scarce on Earth, where it is typically used as a pigment known as “titanium white.” Because of its particular properties, this gas may play an important role in the atmosphere of WASP-180b – similar to how ozone played an important role in the evolution of Earth’s atmosphere.

Like ozone, Titanium oxide absorbs short-wave electromagnetic radiation, including ultraviolet light. Therefore, the detection of this compound could indicate that there is a layer in WASP-189 b’s atmosphere that interacts with stellar radiation the same way the Earth’s Ozone Layer does. Already, researchers have found hints of this and other layers on the ultra-hot Jupiter-like planet. As Prinoth explains:

“In our analysis, we saw that the ‘fingerprints’ of the different gases were slightly altered compared to our expectation. 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 vapour and ozone on Earth would appear differently altered from a distance, because they mostly occur in different atmospheric layers.”

These results may change how astronomers investigate exoplanets. In the past, astronomers tended to assume that the atmosphere of exoplanets existed as uniform layers and tried to characterize them as such. But these latest results demonstrate that even the atmospheres of extreme planets – like ultra-Hot Jupiters – have complex three-dimensional structures. AS co-author Kevin Heng, a professor of astrophysics at the University of Bern and a member of the NCCR PlanetS, concludes:

“We are convinced that to be able to fully understand these and other types of planets – including ones more similar to Earth, we need to appreciate the three-dimensional nature of their atmospheres. This requires innovations in data analysis techniques, computer modelling and fundamental atmospheric theory.”

Further Reading: Bern University, Nature Astronomy