Abstract
We demonstrate the effectiveness of the Exoplanet Characterisation Observatory mission concept for constraining the atmospheric properties of hot and warm gas giants and super Earths. Synthetic primary and secondary transit spectra for a range of planets are passed through EChOSim (Waldmann & Pascale 2014) to obtain the expected level of noise for different observational scenarios; these are then used as inputs for the NEMESIS atmospheric retrieval code and the retrieved atmospheric properties (temperature structure, composition and cloud properties) compared with the known input values, following the method of Barstow et al. (2013a). To correctly retrieve the temperature structure and composition of the atmosphere to within 2 {\sigma}, we find that we require: a single transit or eclipse of a hot Jupiter orbiting a sun-like (G2) star at 35 pc to constrain the terminator and dayside atmospheres; 20 transits or eclipses of a warm Jupiter orbiting a similar star; 10 transits/eclipses of a hot Neptune orbiting an M dwarf at 6 pc; and 30 transits or eclipses of a GJ1214b-like planet.
Highlights
The Exoplanet Characterisation Observatory (EChO; Tinetti et al 2012) was a proposed European Space Agency mission for the Cosmic Visions M3 launch
As in Barstow et al (2013a), we find that CO and NH3 are generally poorly retrieved, probably due to their being fewer lines in the case of CO and a lack of high temperature lines in the list used for NH3, so we do not show results for these gases. 3.1 Hot Jupiter For all scenarios, assuming a single primary transit is observed, we recover all parameters correctly to within 2-σ for the lowest reduced χ2 case
We have demonstrated the efficacy of EChO for characterising a range of planets with H2-He dominated atmospheres
Summary
The Exoplanet Characterisation Observatory (EChO; Tinetti et al 2012) was a proposed European Space Agency mission for the Cosmic Visions M3 launch. Its goal was to survey the atmospheres of a range of extrasolar planets from the L2 Lagrange Point, over a period of 5 years This would enable us to better understand the thermochemistry, dynamics and evolution of planetary atmospheres. In order to determine the number of individual transits/eclipses required for a successful simultaneous retrieval of these properties, it is necessary to calculate the expected level of noise on the spectrum for the different planet cases. This is done using the EChOSim instrument simulation package (Waldmann & Pascale 2014). We require that gas volume mixing ratios are correctly retrieved to within 2 σ, and that the temperature structure is correctly retrieved to within σ at altitudes where there is sensitivity
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