The discovery of almost two thousand exoplanets has revealed an unexpectedly diverse planet population.We see gas giants in few-day orbits, whole multi-planet systems within the orbit of Mercury, and newpopulations of planets with masses between that of the Earth and Neptune – all unknown in the SolarSystem. Observations to date have shown that our Solar System is certainly not representative of the generalpopulation of planets in our Milky Way. The key science questions that urgently need addressing aretherefore: What are exoplanets made of? Why are planets as they are? How do planetary systems work andwhat causes the exceptional diversity observed as compared to the Solar System? The EChO (ExoplanetCharacterisation Observatory) space mission was conceived to take up the challenge to explain this diversityin terms of formation, evolution, internal structure and planet and atmospheric composition. This requires indepthspectroscopic knowledge of the atmospheres of a large and well-defined planet sample for whichprecise physical, chemical and dynamical information can be obtained.In order to fulfil this ambitious scientific program, EChO was designed as a dedicated survey mission fortransit and eclipse spectroscopy capable of observing a large, diverse and well-defined planet sample withinits four-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signal from the starand planet are differentiated using knowledge of the planetary ephemerides, allows us to measureatmospheric signals from the planet at levels of at least 10-4 relative to the star. This can only be achieved inconjunction with a carefully designed stable payload and satellite platform. It is also necessary to providebroad instantaneous wavelength coverage to detect as many molecular species as possible, to probe thethermal structure of the planetary atmospheres and to correct for the contaminating effects of the stellarphotosphere. This requires wavelength coverage of at least 0.55 to 11 μm with a goal of covering from 0.4to 16 μm. Only modest spectral resolving power is needed, with R~300 for wavelengths less than 5 μm andR~30 for wavelengths greater than this.The transit spectroscopy technique means that no spatial resolution is required. A telescope collecting area ofabout 1 m2 is sufficiently large to achieve the necessary spectro-photometric precision: for the Phase A studya 1.13 m2 telescope, diffraction limited at 3 μm has been adopted. Placing the satellite at L2 provides a coldand stable thermal environment as well as a large field of regard to allow efficient time-critical observationof targets randomly distributed over the sky. EChO has been conceived to achieve a single goal: exoplanetspectroscopy. The spectral coverage and signal-to-noise to be achieved by EChO, thanks to its high stabilityand dedicated design, would be a game changer by allowing atmospheric composition to be measured withunparalleled exactness: at least a factor 10 more precise and a factor 10 to 1000 more accurate than currentobservations. This would enable the detection of molecular abundances three orders of magnitude lower thancurrently possible and a fourfold increase from the handful of molecules detected to date. Combining thesedata with estimates of planetary bulk compositions from accurate measurements of their radii and masseswould allow degeneracies associated with planetary interior modelling to be broken, giving unique insightinto the interior structure and elemental abundances of these alien worlds.EChO would allow scientists to study exoplanets both as a population and as individuals. The mission cantarget super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planettemperatures of 300 K - 3000 K) of F to M-type host stars. The EChO core science would be delivered by athree-tier survey. The EChO Chemical Census: This is a broad survey of a few-hundred exoplanets, whichallows us to explore the spectroscopic and chemical diversity of the exoplanet population as a whole. TheEChO Origin: This is a deep survey of a subsample of tens of exoplanets for which significantly highersignal to noise and spectral resolution spectra can be obtained to explain the origin of the exoplanet diversity(such as formation mechanisms, chemical processes, atmospheric escape). The EChO Rosetta Stones: Thisis an ultra-high accuracy survey targeting a subsample of select exoplanets. These will be the bright"benchmark" cases for which a large number of measurements would be taken to explore temporalvariations, and to obtain two and three dimensional spatial information on the atmospheric conditionsthrough eclipse-mapping techniques.If EChO were launched today, the exoplanets currently observed are sufficient to provide a large and diversesample. The Chemical Census survey would consist of > 160 exoplanets with a range of planetary sizes,temperatures, orbital parameters and stellar host properties. Additionally, over the next ten years, severalnew ground- and space-based transit photometric surveys and missions will come on-line (e.g. NGTS,CHEOPS, TESS, PLATO), which will specifically focus on finding bright, nearby systems. The current5rapid 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.
Springer Netherlands: Springer Netherlands, 2017.