The Exoplanet Characterisation Observatory (EChO) was a proposed space telescope as part of the Cosmic Vision roadmap of the European Space Agency, and competed with four other missions for the M3 slot in the programme. On February 19, 2014, the PLATO mission was selected in favour of the other candidates in the programme, including EChO.
EChO would be the first dedicated mission to investigate exoplanetary atmospheres, addressing the suitability of those planets for life and placing our Solar System in context. EChO is intended to provide high resolution, multi-wavelength spectroscopic observations. It would measure the atmospheric composition, temperature and albedo of a representative sample of known exoplanets, constrain models of their internal structure and improve our understanding of how planets form and evolve. It will orbit around the L2 Lagrange point, 1.5 million km from Earth in the anti-sunward direction.
^Tinetti, G.; Cho, J. Y. K.; Griffith, C. A.; Grasset, O.; Grenfell, L.; Guillot, T.; Koskinen, T. T.; Moses, J. I.; Pinfield, D.; Tennyson, J.; Tessenyi, M.; Wordsworth, R.; Aylward, A.; Van Boekel, R.; Coradini, A.; Encrenaz, T.; Snellen, I.; Zapatero-Osorio, M. R.; Bouwman, J.; Du Foresto, V. C.; Lopez-Morales, M.; Mueller-Wodarg, I.; Pallé, E.; Selsis, F.; Sozzetti, A.; Beaulieu, J. P.; Henning, T.; Meyer, M.; Micela, G.; Ribas, I. (2011). "The science of EChO". Proceedings of the International Astronomical Union6: 359. Bibcode:2011IAUS..276..359T. doi:10.1017/S1743921311020448.edit