The Exoplanet Characterisation Observatory (EChO) will 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 will provide high resolution, multi-wavelength spectroscopic observations. It will 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.

EChO: What are exoplanets made of?

The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. It will place our Solar System in context and by addressing the suitability of planets for life will allow us to address some of the fundamental questions of the Cosmic Visions programme:

• What are the conditions for planet formation and the emergence of life?
• Are systems like our Solar System rare or very common? How does the Solar System work?

EChO will provide high 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.

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 light-curve 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, a few of which we highlight here.

EChO will provide constraints on the formation of the exoplanets. Primary and secondary observations will readily indicate (through the CH₄, CO, CO₂ and H₂ features) the carbon and oxygen elemental abundance of the atmospheres which point to the formation mechanism of the planet: whether by the accretion of solids (as did our planets) or by gas collapse (as do stars). The planet's C and O abundance can be compared to measurements of C and O in the primary star to determine whether the atmosphere's abundance matches that of the star. A significant overabundance of heavy elements in the planet indicates formation by core accretion. A mild enrichment of heavy elements indicates formation by gas collapse.

EChO will characterise an exoplanet's climate. The composition and thermal structure of the planet's atmosphere will be compared to the primary star's incident light and measured reflection to determine the partitioning of stellar radiation. Such measurements probe the greenhouse effect, which shapes planetary atmospheres and renders them habitable (as on Earth) or uninhabitable (as on Venus). Planetary climates are also affected by atmospheric dynamics: Earth's circulation redistributes heat from equator to pole. EChO will study the dynamics of exoplanetary atmospheres through the derived vertical and horizontal temperature and compositional gradients.

EChO will study exoplanetary chemistry. Thermal equilibrium models will indicate if an exoplanet's atmosphere is in equilibrium. With the exception of the deep atmospheres of the Giant Planets, atmospheric equilibrium is rare in the Solar System, so we expect to find atmospheres that are out of equilibrium. We plan to study photochemistry, the most important non-equilibrium chemical process which on Earth is responsible for O₃ production, through the identification and measured abundances of principal photochemical products.

Monitoring stellar variability simultaneously with exoplanet atmospheric data is a key aspect of the mission. The best available indicator of chromospheric flux in the wavelength ranges accessible to EChO is the H Balmer alpha line at 0.66\,µm. Emission in the core of the line can be used to determine how variations in the stellar chromosphere affect planetary atmospheres, and to distinguish stellar variability from planetary variability. In addition, EChO will search for H₃+, which indicates if a Jovian-type planet has a magnetosphere and is an indicator of the effects that stars of different types have on planetary atmospheres.

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.

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 target list for EChO includes planets ranging from Jupiter-sized, with an orbital semi-major axis one tenth that of Mercury and equilibrium temperatures, T_eq up to 2000 K, to those of a few Earth masses, with T_eq ~300 K. The list will include planets with no Solar System analog, such a as the recently discovered planet GJ1214b, whose density lies between that of terrestrial and gaseous planets.

As the number of detected exoplanets grows exponentially each year, and the mass of those detected steadily decreases, the target list will be constantly adjusted to include the most interesting systems.

Payload Instrument

We have baselined a dispersive spectrograph design covering continuously the 0.4-16 µm spectral range in 6 channels (1 VIS, 5 IR) which allows the spectral resolution to be adapted to the target brightness from several tens (Lambda ≥ 11 µm) to several hundreds (Lambda ≤ 11 µ m). Thus optimising for the scientific objectives over the observation spectral range. The instrument is mounted behind a 1.2/1.5 m class telescope passively cooled and detectors with performance optimised precisely to each wavelength range, actively cooled in order to reduce the dark current to the required level. Stability and accuracy of the photometry is critical to the success of EChO and the design of the whole detection chain and satellite will be dedicated to achieving a high degree of photometric stability and repeatability. Calibration is also critical and requires detailed monitoring of the detector performance using both internal and external calibration sources.

EChO will be placed in a grand halo orbit around L2. This orbit, in combination with a nested thermal shield design, provides a highly stable thermal environment for the passive cooling of the instrument and telescope. The orbit and thermal shield design will also provide a high degree of visibility of the sky over the year and an ability to repeatedly observe several tens of targets whatever the epoch in the year.

The internal assessment study at ESTEC will start soon.

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