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Exoplanets and their Atmospheres

Latest news (9 Apr 2021): we detected 6 molecular species simultaneously in the atmosphere of a hot, giant exoplanet. Read the full research on Nature.

The discovery of planets orbiting other stars radically changed our idea of how planetary systems form and evolve. The astonishing diversity of the thousands of exoplanets currently known inspires two fundamental questions: how common is our Solar System? Is there another Earth out there?
I am addressing these questions by studying the atmospheres of exoplanets. I am developing novel and robust observational tools to infer their composition and structure. So far my research has focussed on giant planets orbiting close to their parent stars, called hot Jupiters. In the years to come, thanks to new discoveries and better instruments, smaller and cooler objects will be targeted. Eventually, these observational techniques will be applicable to terrestrial planets to hunt for the signatures of life.

How do we study exoplanet atmospheres? Up to a few years ago, only the atmospheres of transiting planets could be studied. When a planet crosses the disk of its parent star or it is occulted by it, we observe a small decrease in the total light coming from the system. By measuring this dimming at various wavelengths, it is possible to infer the structure and composition of a planet atmosphere, and how the incoming energy from the star gets redistributed.

At very high spectral resolution (R > 20,000), molecular bands are resolved into the individual lines. Each species shows a unique pattern, and this "fingerprint" can be recognised by matching observations with models through cross correlation. Artist image of exoplanet tau Bootis b, with link to the ESO press release In addition, since planets move along the orbit, their spectrum is subject to a rapidly-changing Doppler shift. Therefore, we can effectively disentangle the signature of an exoplanet from the stationary, dominant absorption of the Earth's atmosphere. With infrared high-resolution spectroscopy, the radial velocity of a transiting planet was measured directly for the very first time, and carbon monoxide was detected in the atmosphere of τ Boötis b. Thanks to the direct detection of the planet radial velocity, we can now study the atmospheres of non-transiting planets and estimate their true masses and the orbital inclinations.

Multiple molecules, when detected, allow us to put constraints on the elemental abundances of carbon and oxygen (C/O ratio), and in general on the planet metallicity. With enough planets observed, we hope to unveil the preferential formation mechanism(s) of planets and their primordial location in protoplanetary disks.

Planet rotation, as well as winds, leave a very specific imprint on the shape of the spectral lines. At very high spectral resolution, the broadening and distortion caused by global atmospheric circulation has been detected for both young, giant planets on large orbits and transiting hot Jupiters.

Concept rendering of the Extremely Large Telescope, from the ESO websiteThe future of high-resolution spectroscopy for exoplanet atmospheres is bright. Modern spectrographs collect more photons and cover a larger fraction of the planet spectrum at once, increasing our sensitivity to smaller and cooler planets. Upcoming missions are also designed to find smaller planets around bright stars, which are ideal candidates for characterisation. Combining other techniques, such as low-resolution spectroscopy and direct imaging, with high-resolution spectroscopy will drastically improve the measurements of atmospheric parameters in the coming years.
Eventually, in the era of the Extremely Large Telescope, we will turn our instruments towards terrestrial planets in the habitable zones of M-dwarf stars, looking for potential biomarkers in their atmospheres.