The Exoplanet Characterization Observatory (EChO) was a candidate mission for the M3 launch opportunity in ESA’s Cosmic Vision programme. The mission was designed to measure the chemical composition and structure of hundred(s) of exoplanet atmospheres, with uninterrupted spectroscopic coverage from visible to infrared wavelengths. EChO targets would extend from gas giants (Jupiter- or Neptune-like) to super-Earths, in the very hot to temperate zones of F to M-type host stars, opening up the way to large-scale, comparitive planetology that would firmly place our own Solar System in the context of other planetary systems in the Milky Way. In February 2014 the PLATO mission was selected for the M3 launch opportunity.
EChO would be the first dedicated mission to investigate exoplanetary atmospheres, addressing the suitability of those planets for life and placing the 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.