Iron and titanium in the atmosphere of Kelt-9b

An international team of astronomers discovered singly-ionized Titanium and Iron in the atmosphere of the hot-Jupiter exoplanet named KELT-9b (HD 195689b), orbiting a star located 650 light years from Earth in the constellation Cygnus (the Swan) with a temperature of over 10,000 degrees (almost twice as hot as the Sun).

Artist’s view of a sunset over KELT-9b

Artist’s view of a sunset over KELT-9b. By artist/author: Denis Bajram LICENSE CC BY-NC-SA 4.0 / Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International

The chemical composition of an exoplanet is a key ingredient in constraining its formation history. Iron is the most abundant transition metal, but has never been directly detected in an exoplanet due to its highly refractory nature. KELT-9b is the archetype of the class of ultra-hot Jupiters that straddle the transition between stars and gas-giant exoplanets and serve as distinctive laboratories for studying atmospheric chemistry, because of its high equilibrium temperature of almost 4050 K. Due the proximity to its star, 30 times closer than the Earth’s distance from the Sun, the orbital period of the planet is only 36 hours. Thank’s to this work, scientists can finally undestand about the planet’s atmospheres under such hard conditions.

“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 University of Bern.

These results have been obtained using the method of the cross-correlation applied to 27 high-resolution spectra obtained with the HARPS-N spectrograph installed at the Telescopio Nazionale Galileo, thanks to a Director's Discretionary Time program.

Schematic of the orbital geometry of the exoplanet

a, Schematic of the orbital geometry of the exoplanet (represented by the black filled circle). The arrow indicates the orbital trajectory of the exoplanet as it transits across the stellar disk. The blue and red shading represent the blue- and redshifted regions on the stellar disk caused by stellar rotation. b, The obscuration of part of the stellar disk results in an enhancement in flux of the stellar absorption line (as in blue shaded area). d, As the exoplanet progresses in its orbit, its projected orbital velocity shifts from being blueshifted (at point A) to being redshifted (at point C). c, These two distinct signatures show up in the cross-correlation function (CCF; grey scale; top) as a Doppler shadow (deficit in the CCF at the instantaneous velocity of the obscured part of the stellar disk) and a bright streak (enhancement in the CCF at the instantaneous radial component of the orbital velocity of the planet), respectively. To isolate the signature of the planet, we model and subtract the Doppler shadow from the CCF (bottom). The systemic radial velocity of the KELT-9 system (−20 km s−1) is indicated. (https://www.nature.com/articles/s41586-018-0401-y)

The observations were performed when the planet 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 and titanium would leave a well-recognisable fingerprint in the spectrum of the planet. Using the HARPS-North spectrograph, astronomers discovered a strong signal corresponding to singly-ionized iron and singly-ionized titanium in the planet’s spectrum.

Once more the high performances ensured by HARPS-N combined with the development of new techniques were fundamental tools in performing this new discovery. Kelt-9b will probably attract new investigations in the near future. Stay tuned!

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