Rare-Earth metals in the atmosphere of a glowing-hot exoplanet

KELT-9 b is the hottest exoplanet known to date. In the summer of 2018, a joint team of astronomers from the universities of Bern and Geneva found signatures of gaseous iron and titanium in its atmosphere. Now these researchers have also been able to detect traces of vaporized sodium, magnesium, chromium, and the rare-Earth metals scandium and yttrium. Exoplanets are planets outside our solar system that orbit around stars other than the Sun. Since the discovery of the first exoplanets in the mid-90's, well over 3'000 exoplanets have been discovered. Many of these planets are extreme compared to the planets in our solar system: Hot gas giants that orbit incredibly close to their host stars, sometimes within periods of less than a few days. Such planets do not exist in our solar system, and their existence has defied predictions of how and why planets form. For the past 20 years, astronomers from all over the world have been working to understand where these planets come from, what they are made of, and what their climates are like. An extremely hot gas giant KELT-9 is a star located 650 light years from the Earth in the constellation Cygnus. Its exoplanet KELT-9 b exemplifies the most extreme of these so-called hot-Jupiters because it orbits very closely around its star that is almost twice as hot as the Sun. Therefore, its atmosphere reaches temperatures of around 4'000 C. In such heat, all elements are almost completely vaporized and molecules are broken apart into their constituent atoms - much like is the case in the outer layers of stars. This means that the atmosphere contains no clouds or aerosols and the sky is clear, mostly transparent to light from its star.

The atoms that make up the gas of the atmosphere absorb light at very specific colors in the spectrum, and each atom has a unique "fingerprint" of colors that it absorbs. These fingerprints can be measured with a sensitive spectrograph mounted on a large telescope, allowing astronomers to discern the chemical composition of the atmospheres of planets that are many light-years away.

The exoplanet as a treasure trove

A team of researchers from the Universities of Bern and Geneva collaborated to use this technique, and made an interesting discovery: "Using the HARPS-North spectrograph on the Italian National Telescope on the island of La Palma, we found iron and titanium atoms in the hot atmosphere of KELT-9 b", explains Kevin Heng, Director and Professor at the Center for Space and Habitabilty (CSH) at the University of Bern and a member of the National Centre of Competence in Research PlanetS.

The team observed the KELT-9 system for a second time last summer, with the goal of confirming their previous detections, but also to proceed to search for additional elements that could be present in the data as well. Their survey included 73 atoms, among which some so-called rare-Earth metals. These substances are less common on Earth, but are applied in advanced materials and devices.

Jens Hoeijmakers, who is the first author of the study which is now published in the Journal Astronomy and Astrophysics and who is a Postdoc at the CSH in Bern and at Geneva Observatory, says: "Our team predicted that the spectrum of this planet could well be a treasure trove where a multitude of species can be detected that have not been observed in the atmosphere of any other planet before."

After careful analysis, the researchers indeed found strong signals of vaporized sodium, magnesium, chromium and the rare-Earth metals scandium and yttrium in the spectrum of the planet. The latter three of these have never been detected robustly in the atmosphere of an exoplanet before.

"The team also advanced their interpretation of this data, and were able to use these signals to estimate at what altitude in the planet's atmosphere these atoms are absorbing", says Jens Hoeijmakers. What is more, the researchers also know more about strong global wind patterns high up in the atmosphere that blow the material from one hemisphere to the other.

"With further observations, many more elements may well be discovered by using the same technique in the atmosphere of this planet in the future, and perhaps also on other planets that are heated to similarly high temperatures", explains Jens Hoeijmakers.

Kevin Heng adds: "The chances are good that one day we will find so-called biosignatures, i.e. signs of life, on an exoplanet, using the same techniques that we are applying today. Ultimately, we want to use our research to fathom the origin and development of the solar system as well as the origin of life."

Iron and Titanium in the Atmosphere of an Exoplanet

Exoplanets, planets in other solar systems, can orbit very close to their host star. When, in addition to this, the host star is much hotter than our Sun, then the exoplanet becomes as hot as a star. The hottest "ultra-hot" planet was discovered last year by American astronomers. Today, an international team, led by researchers from the University of Geneva (UNIGE), who joined forces with theoreticians from the University of Bern (UNIBE), Switzerland, discovered the presence of iron and titanium vapours in the atmosphere of this planet. The detection of these heavy metals was made possible by the surface temperature of this planet, which reaches more than 4,000 degrees [Celsius]. This discovery is published in the journal Nature [www.nature.com]. KELT-9 is a star located 650 light-years from Earth in the constellation Cygnus (the Swan). With a temperature of over 10,000 degrees [Celsius], it is almost twice as hot as the Sun. This star is orbited by a giant gas planet, KELT-9b, which is 30 times closer than the Earth's distance from the Sun. Because of this proximity, the planet circles its star in 36 hours and is heated to a temperature of over 4,000 degrees. It's not as hot as the Sun, but hotter than many stars. At present, we do not yet know what an atmosphere looks like and how it can evolve under such conditions.

That is why NCCR PlanetS researchers affiliated with the University of Bern recently performed a theoretical study on the atmosphere of the planet KELT-9b.

"The results of these simulations show that most of the molecules found there should be in atomic form, because the bonds that hold them together are broken by collisions between particles that occur at these extremely high temperatures," explains Kevin Heng, professor at the UNIBE. This is a direct consequence of the extreme temperature. Their study also predicts that it should be possible to observe gaseous atomic iron, in the planet's atmosphere using current telescopes.

Light Reveals the Chemical Components of the Atmosphere
The UNIGE FOUR ACES team, which is also part of the NCCR PlanetS at the Department of Astronomy of the Faculty of Science of the UNIGE, had observed this planet precisely as it was moving in front of its host star (i.e., during a transit). During transit, a tiny fraction of the light from the star filters through the planet's atmosphere, and analysis of this filtered light can reveal the chemical composition of the atmosphere.

This is achieved with a spectrograph, an instrument that spreads white light into its component colours, called a spectrum. If present among the components of the atmosphere, iron vapour would leave a well-recognisable fingerprint in the spectrum of the planet.

Using the HARPS-North spectrograph, built in Geneva and installed on the Telescopio Nazionale Galileo in La Palma, astronomers discovered a strong signal corresponding to iron vapour in the planet's spectrum. "With the theoretical predictions in hand, it was like following a treasure map," says Jens Hoeijmakers, a researcher at the Universities of Geneva and Bern and lead author of the study, "and when we dug deeper into the data, we found even more," he adds with a smile. Indeed, the team also detected the signature of another metal in vapour form: titanium.

This discovery reveals the atmospheric properties of a new class of so-called "ultra-hot Jupiter." However, scientists believe that many exoplanets have completely evaporated in environments similar to KELT-9b. Although this planet is probably massive enough to withstand total evaporation, this new study demonstrates the strong impact of stellar radiation on the composition of the atmosphere. Indeed, these observations confirm that the high temperatures reigning on this planet break apart most molecules, including those containing iron or titanium.

In cooler giant exoplanets, these atomic species are thought to be hidden within gaseous oxides or in the form of dust particles, making them hard to detect. This is not the case on KELT-9b. "This planet is a unique laboratory to analyze how atmospheres can evolve under intense stellar radiation," concludes David Ehrenreich, principal investigator with the UNIGE's FOUR ACES team.