Most of the exoplanets are hot and therefore their spectra are very complex and contain numerous unassigned features. The large number of exoplanetary observations acquired via a combination of new, larger telescopes and space missions will  therefore only be exploitable if and only if the adequate laboratory and theoretical data are made available for their analysis, without the current limitations.

Our consortium of five French laboratories and multiple associated partners proposes to improve the existing high-temperature spectroscopy data for several molecular species detected in exoplanets. Our strategy consists in producing experimental and theoretical data that are then applied to observations.

The provision of infrared (IR) laboratory data of methane, acetylene, ethylene and ethane, between 500 and 2500 K will help to refine thermal profiles and provide information on the gaseous composition, the hazes and their temporal variability. Currently available data are insufficient in several ways:

• All datasets are affected by large gaps below 1.65 μm, an important region in observations;
• High-temperature spectroscopic data cannot be extrapolated from low-T atmospheric databases such as HITRAN and GEISA. Such extrapolations fail to reproduce high J rovibrational transitions and hot band transitions involving highly excited  vibrational levels and thus opacity calculations at high temperatures present large uncertainties;
• Accurate molecular models are still missing to generate complete high-T line lists for hydrocarbon absorbers which dominate the spectrum of brown dwarfs, exoplanets and Asymptotic Giant Branch stars and play a primary role in the physical chemistry of their outer atmospheres.

Based on state-of-the-art theoretical calculations and new models, extensive line lists (including positions, intensities, profiles, etc.) will be generated and validated by laboratory experiments: (i) emission spectroscopy at thermal equilibrium above 500 K in the 1.4–17 μm region; (ii) high sensitivity laser absorption spectroscopy by Cavity Ring Down Spectroscopy (CRDS) in non-thermal equilibrium and hypersonic relaxation in the 1.5–1.7 μm region; (iii) direct absorption and CRDS at high sensibility from 500 to 1000 K in the 1.26–1.71 μm region for weak line measurements; (iv) room temperature absorption down to 0.8 μm to reach highly excited vibrational states. These complementary interdisciplinary researches will permit a breakthrough towards interpretation of high-resolution exoplanetary observations.

The feasibility of this challenging project is attested by our previous successful experience and established expertise in experiments, spectra analyses and theory: our low-T experimental data and ab initio predictions for methane, ethylene and their isotopologues are currently the most accurate available, while first CH 4 line lists produced already at 2000 K above 2 μm show good agreement with observations.

At the end of our project, the scientific community will benefit from experimental and synthetic spectra in the 0.8–17μm region for hydrocarbons and their isotopologues (¹²CH₄ , ¹³CH₄ , CH₃D, C₂H₂ , C₂H₄ and C₂H₆) in large temperature ranges (up to 2500 K). For each of these species, we will provide rovibrational assignments, broadening coefficients (by H₂) and cross-sections, directly usable in radiative transfer codes.

Our consortium will establish the framework for a large international collaboration, which will benefit from a mutualisation of unique access to observations, special facilities, manpower, software and experimental means.