ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large Survey) is one the three candidates for the next ESA medium class (M4) mission expected to be launched in 2026. This mission will be devoted to observing spectroscopically in the infrared (IR) a large population (500-1000) of known transiting planets in our Galaxy, opening a new discovery space in the field of extrasolar planets and enabling the understanding of the physics and chemistry of these far away worlds.
ARIEL is based on a 1 meter-class telescope and a spectrometer covering the band from 1.2 to 7.8 micron, in addition to photometric bands in the visible and in the near-IR to monitor the stellar activity, measure the albedo and detect clouds. During its four years operations from an L2 orbit ARIEL will continuously observe exoplanets transiting their host star. The ARIEL consortium includes academic institutes and industry from twelve countries in Europe (UK, FR, IT, DE, AU, NL, BE, SP, PL, PT, IE, DK)
More than 3,000 exoplanets have been discovered. Ongoing and planned ESA and NASA missions from space such as GAIA, Cheops, PLATO, Kepler II and TESS will increase the number of known systems to tens of thousands. Ground based surveys using a variety of direct and indirect techniques will contribute further. Of all these exoplanets we know very little, i.e. their orbital data and, for some of these, their physical parameters such as their size and mass. In the past decade, pioneering results have been obtained using transit spectroscopy with Hubble, Spitzer and ground-based facilities, enabling the detection of a few of the most abundant ionic, atomic and molecular species and to constrain the planet’s thermal structure. Despite these early successes, current data are very sparse, i.e. there is not enough wavelength coverage. Future general purpose facilities with large collecting areas will allow the acquisition of better exoplanet spectra, compared to the currently available, especially from fainter targets but a breakthrough in our understanding of planet formation and evolution mechanisms will only happen through the observation of the planetary bulk and atmospheric composition of a statistically large sample of planets. ARIEL will provide a complete, statistically significant sample of gaseous and rocky planets with temperatures hotter than ~500K, as these types of planets will allow direct observation of their bulk properties, enabling us to constrain models of planet formation and evolution.