  The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse
planet population. Observations to date have shown that our Solar System is
certainly not representative of the general population of planets in our Milky
Way. The key science questions that urgently need addressing are therefore:
What are exoplanets made of? Why are planets as they are? What causes the
exceptional diversity observed as compared to the Solar System?
  EChO (Exoplanet Characterisation Observatory) has been designed as a
dedicated survey mission for transit and eclipse spectroscopy capable of
observing a large and diverse planet sample within its four-year mission
lifetime. EChO can target the atmospheres of super-Earths, Neptune-like, and
Jupiter-like planets, in the very hot to temperate zones (planet temperatures
of 300K-3000K) of F to M-type host stars. Over the next ten years, several new
ground- and space-based transit surveys will come on-line (e.g. NGTS, CHEOPS,
TESS, PLATO), which will specifically focus on finding bright, nearby systems.
The current rapid rate of discovery would allow the target list to be further
optimised in the years prior to EChO's launch and enable the atmospheric
characterisation of hundreds of planets. Placing the satellite at L2 provides a
cold and stable thermal environment, as well as a large field of regard to
allow efficient time-critical observation of targets randomly distributed over
the sky. A 1m class telescope is sufficiently large to achieve the necessary
spectro-photometric precision. The spectral coverage (0.5-11 micron, goal 16
micron) and SNR to be achieved by EChO, thanks to its high stability and
dedicated design, would enable a very accurate measurement of the atmospheric
composition and structure of hundreds of exoplanets.
