The past two decades have seen a revolution in our understanding of our Solar System and how it fits in the broader extrasolar planet context. Currently, over 3000 exoplanetary systems have been detected, populated by a large variety of exoplanets in excess of 4000. If hot Jupiters and terrestrial-like planets in the habitable zone of their host-star have been highlighted, one of the biggest surprise was that the most abundant planets exhibit radius between those of the Earth and Neptune, and are of a type not present in our Solar System. Nicknamed, “transitional planets”, they may either have a rocky core enveloped in a H2-He gaseous envelope or contain a significant amount of H2O-dominated ices/fluids. Are they super-Earths, ocean-planets, mini-Neptunes, magma covered or frozen planets? Speculations on the nature of exoplanets, on their chemical composition, dynamical regimes and formation history are numerous but the answers are currently vague and few. In front of this situation, several national and international agencies have or will deploy space missions, facilities and instruments dedicated to the characterization of exoplanets and to the study of their atmospheres. This will teach us the big picture of planetary formation and evolution, and why our Solar System is so special in many aspects. This is arguably one of the hottest topic for the whole astrophysical community and a prime driver in the design of future observatories.

However, the challenges in this endeavor are daunting. From the extraction of a feeble signal in the observational dataset, to the numerical modelling of their chemical and dynamical structures, these exotic worlds will confront the community with many hurdles in data analysis, experimental and theoretical chemistry and spectroscopy, numerical modeling of fluid dynamics and complex chemical networks.

The goal of this school is to provide a new generation of scientists the theoretical background, the methods, and the tools that will allow them to tackle the above-listed challenges. This will develop in the French and European planetary science community a high level of expertise required to exploit the data from future ELT or JWST and Ariel missions. If rightly and dedicately trained, the European astrophysical community has the best assets to become the world-leading community on exoplanet characterization.

Our program will cover the broad range of expertise needed to obtain, analyze and model spectroscopic observations of exoplanets. We will present the facilities and instruments, running or upcoming, which will be used for exoplanet characterization through various techniques (direct detection, transit, light curves), the reduction techniques and tools, and the atmospheric parameters retrieval methods. In this data analysis process, spectroscopy takes a very important place, and both experimental and theoretical advances will be presented. Developments, both theoretical and experimental, in thermochemistry, photochemistry, heterogenous chemistry and cloud microphysics are also extremely important to understand the chemical composition and evolution of these objects. Finally, modelling the dynamics of their atmosphere requires new numerical codes able to explore a broader extent of dynamical regimes and describe physical interactions. Our school will also be based on the latest discoveries on Solar System planets from the most recent space missions (JUNO, Cassini, …), expose what lessons can be learnt for exoplanetary studies and present how planetary science in the Solar System has coped in the past with similar hurdles in terms of spectroscopy, chemistry and modelling.

Our school, which will be held when the JWST 6.5m space telescope will start its nominal operation, very shortly after the release of the early-science data, and on the eve of the ELT advent, will timely foster the capacities of the community to explore the diversity of these new worlds.

The stakes


The question of the characterization of the atmospheres of exoplanets emerged as soon as the first parameters resulting from the detection (mass, radius, equilibrium temperature) were obtained thanks to the detections of exoplanets by various methods (radial velocities, transits, direct detection, microlenses, etc.). This characterization which requires the use of
spectroscopy is today accessible only on a limited number of objects (a few tens for more than 4000 detected exoplanets) and is still very limited. However, the era of large telescopes with the ELT (2026) and the space missions James Webb Space Telescope (2021) and Ariel (2029) will bring new and more precise observations open to interpretation. On the French side, the training of a new generation of scientists able to study these new data is of great importance to allow research to play a leading role in these discoveries. The interdisciplinarity of the measurements (astrophysics, planetology, spectroscopy, molecular chemistry, instrumentation) makes it particularly attractive to bring together in a single school several key disciplines in this field. The structuring of the community within INSU is ensured by the involvement of national programs (PNP, PNPS and PCMI), with the inter-program exoplanet group. A coordination between CNES and INSU is already present, through the
inter-agency committees that follow the JWST and Ariel space missions. On the thematic level, the INSU foresight had already insisted on the need for structuring emerging research on exoplanets at its Lalonde-les-Maures seminar in 2015. This necessity has been confirmed at the 2020 seminar.