The EChO science case

Abstract

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 new populations of planets with masses between that of the Earth and Neptune-all unknown in the Solar System. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? How do planetary systems work and what causes the exceptional diversity observed as compared to the Solar System? The EChO (Exoplanet Characterisation Observatory) space mission was conceived to take up the challenge to explain this diversity in terms of formation, evolution, internal structure and planet and atmospheric composition. This requires in-depth spectroscopic knowledge of the atmospheres of a large and well-defined planet sample for which precise physical, chemical and dynamical information can be obtained. In order to fulfil this ambitious scientific program, EChO was designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large, diverse and well-defined planet sample within its 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allows us to measure atmospheric signals from the planet at levels of at least 10 −4 relative to the star. This can only be achieved in conjunction with a carefully designed stable payload and satellite platform. It is also necessary to provide broad instantaneous wavelength coverage to detect as many molecular species as possible, to probe the thermal structure of the planetary atmospheres and to correct for the contaminating effects of the stellar photosphere. This requires wavelength coverage of at least 0.55 to 11 μm with a goal of covering from 0.4 to 16 μm. Only modest spectral resolving power is needed, with R~300 for wavelengths less than 5 μm and R~30 for wavelengths greater than this. spectroscopy technique means that no spatial resolution is required. A telescope collecting area of about 1 m 2 is sufficiently large to achieve the necessary spectro-photometric precision: for the Phase A study a 1.13 m 2 telescope, diffraction limited at 3 μm has been adopted. Placing the satellite at L2 provides a cold and stable thermal environment as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. EChO has been conceived to achieve a single goal: exoplanet spectroscopy. The spectral coverage and signal-to-noise to be achieved by EChO, thanks to its high stability and dedicated design, would be a game changer by allowing atmospheric composition to be measured with unparalleled exactness: at least a factor 10 more precise and a factor 10 to 1000 more accurate tha...

Alfnoor: A Retrieval Simulation of the Ariel Target List

“…On the opposite, scattered spectra can inform us on the stability and the redundancy in the information content of Ariel spectra. Previous studies have used both types to simulate observations by future telescopes (Barstow et al 2012;Tinetti et al 2015Tinetti et al , 2018Feng et al 2016Feng et al , 2018Rocchetto et al 2016;Batalha et al 2017;Mollière et al 2017;Blumenthal et al 2018;Changeat et al 2019aChangeat et al , 2019bEdwards et al 2019b;Lustig-Yaeger et al 2019). Feng et al (2018) results predicted that the retrieved uncertainties should be similar in both scattered and unscattered runs but that the retrieved mean could be different.…”

The Astrophysics of Visible-light Orbital Phase Curves in the Space Age

“…Future exoplanet characterization missions, like the proposed FINESSE, EChO, and ARIEL missions (Deroo et al 2012;Tinetti et al 2015Tinetti et al , 2016, will be able to measure spectroscopic phase curves with a high spectral resolution across a wide wavelength range from the optical to the IR. Unlike current space-based facilities (HST, Spitzer) such missions will be specifically designed for this task and will have the telescope time to measure the spectroscopic phase curve for a sample of planets.…”

SVEEEETIES: singular vector expansion to estimate Earth-like exoplanet temperatures from infrared emission spectra

“…Line et al (2013) emphasize that the second approach is invalid for most current exoplanet spectra due to their limited information content. Barstow et al (2013b,a) use optimum estimation (Irwin et al 2008) to assess the retrievability of the atmosphere of the super-Earth GJ1214b and hot Jupiters, respectively; they compared three different parameterizations and concluded that the proposed EChO space telescope (Tinetti et al 2015) should have been able to characterize the temperature of gas giants with about 200 K precision. The τ-REX (Waldmann et al 2015b) employs a two-stage hybrid approach combining the parametric and layer-by-layer representation.…”