Abstract
Transit-spectroscopy of exoplanets is one of the key observational techniques used to characterize extrasolar planets and their atmospheres. The observational challenges of these measurements require dedicated instrumentation and only the space environment allows undisturbed access to earth-like atmospheric features such as water or carbon dioxide. Therefore, several exoplanet-specific space missions are currently being studied. One of them is EChO, the Exoplanet Characterization Observatory, which is part of ESA's Cosmic Vision 2015–2025 program, and which is one of four candidates for the M3 launch slot in 2024. In this paper we present the results of our assessment study of the EChO spectrometer, the only science instrument onboard this spacecraft. The instrument is a multi-channel all-reflective dispersive spectrometer, covering the wavelength range from 400 nm to 16μm simultaneously with a moderately low spectral resolution. We illustrate how the key technical challenge of the EChO mission — the high photometric stability — influences the choice of spectrometer concept and fundamentally drives the instrument design. First performance evaluations underline the suitability of the elaborated design solution for the needs of the EChO mission.
Highlights
One of the most exciting developments in modern astronomy is the detection of more than 850 extrasolar planets in the last two decades
We identified the following spectrometer concepts as possible candidates for the Exoplanet Characterization Observatory (EChO) instrument: (1) Multi-channel dispersive spectrometer with allreflective optics: Six channels separated by dichroic mirrors, each containing a grating covering one octave of the spectrum, camera optics, and a dedicated detector
A first set of performance analyses was conducted as part of the assessment study
Summary
One of the most exciting developments in modern astronomy is the detection of more than 850 extrasolar planets in the last two decades. During a secondary eclipse event, called “occultation”, the planet passes behind the star and the obscured planet radiation (emitted or reflected) can be measured (e.g. Charbonneau et al, 2005; Deming et al, 2005). In both cases, the prime observable is the eclipse depth, i.e., the fraction by which the total system flux is reduced during an eclipse event, measured in one or more photometric bands or by a spectrograph. A third method is to measure the flux variation of the star–exoplanet system during a half or full orbital period This phase-resolved information can be used to determine 1D or 2D maps of the planet brightness temperature (Harrington et al, 2006; Knutson et al, 2007), providing an even deeper insight into the physics and structure of the exoplanet and its atmosphere
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