The Exoplanet Characterization Observatory, EChO

Fig.1: EChO

EChO will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. The observatory will provide medium resolution, simultaneous multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. The use of passive cooling, few moving parts and well established technology gives a low-risk and potentially long-lived mission.

EChO will build on observations by Hubble, Spitzer and ground-based telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. However EChO's configuration and specifications are designed to study a number of systems in a consistent manner that will eliminate the ambiguities affecting prior observations. EChO will simultaneously observe a broad enough spectral region to constrain from one spectrum the temperature structure of the atmosphere, the abundances of the major carbon and oxygen molecules, the expected photochemically-produced species and magnetospheric signatures. The spectral range and resolution of the 4 channels are tailored to separate bands belonging to up to 30 molecules and retrieve the composition and temperature structure of planetary atmospheres.

The EChO mission will focuss on transiting expoplanets:
During a primary transit, when a planet passes in front of its star, the star's light passes through the limb of the planet's atmosphere, effectively providing an atmospheric transmission spectrum. During a secondary eclipse the planet passes behind its star; the dip in the flux reveals the emission, and at optical wavelengths, the reflection, spectrum of the planet. An orbital lightcurve can be used to obtain the horizontal gradients of the temperature and composition of exoplanets. These combined approaches provide complementary information, including the temperature and composition profiles over ~3 decades of pressure, the planet's cloud opacity and composition, and disk-wide temperature and composition variations. In all cases, instead of spatially separating the light of the planet from that of the star, EChO exploits temporal variations to extract the planet's signal. The mission will study a large range of processes that shape the structure of planets.

The investigation of exoplanetary atmospheres requires a dedicated space mission that is fine-tuned to this purpose. Such a mission must be capable of capturing a snapshot of the planet's atmosphere and separating time variable characteristics from steady state conditions. It must be able to observe many systems, including the dimmer planets that approach the size of Earth and be optimised to eliminate systematic errors.
Lastly, it must have a spectrometer with sufficient resolution to capture the spectral characteristics of the constituents that reveal the chemical and dynamical processes of the atmosphere. EChO fulfils all of these requirements.