EChO will provide an unprecedented view of the atmospheres of exoplanets transiting nearby stars. A mission dedicated to the differential technique of transit spectroscopy, EChO will build on the successes of the Hubble Space Telescope, Spitzer Space Telescope and ground-based optical telescopes.
EChO will measure atmospheric transmission, reflection and emission spectra over a continuous wavelength range that spans the optical to thermal infrared. Through the detailed measurement of the spectral energy distribution and spectral emission from molecules such as water (H2O), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and ammonia (NH3) it will be possible to establish many critical atmospheric parameters, including chemical composition and abundances, energy budgets, thermal structure and potentially temporal and spatial variation of atmospheric structure.
With a sample of about 100 exoplanets covering a range of masses (Jupiter-class gas giants down to super Earths) as well as temperatures (hot planets at temperatures above 700 Kelvin, warm planets between 400 Kelvin and 700 Kelvin, and those with temperatures more similar to Earth, from 250 Kelvin to 350 Kelvin) that orbit a variety of host stars (of stellar type F, G, K and M), EChO will not only probe the physics and chemistry of exoplanetary atmospheres, but will also start to determine the mechanisms that drive the formation of exoplanets themselves.
EChO will have the sensitivity to detect key spectral diagnostic features in the atmospheres of hot Jupiters, and also of more temperate super Earths that are expected to be found around M-dwarfs. This offers the tantalizing possibility to study the habitable zones of planetary systems that are close cousins of our own Solar System.
EChO will exploit small differences in the light originating from spatially unresolved observations of an exoplanet and its host star at different points in the planetary orbit:
By differencing observations made either side of, and during, transits or eclipses, it is possible to isolate the (very small) contribution from the exoplanet. In the most favourable cases, such as the brightest hot Jupiters, it will be possible to recover an absorption or transmission spectrum of the exoplanetary atmosphere with sufficient signal-to-noise in a single transit. Where this cannot be done, the source will be revisited over the course of the mission and observations taken over multiple transits will be co-added. Careful monitoring of, and accounting for, variations in stellar output and hence in the overall measured light curve will be key to the success of this co-addition. By measuring very small changes in the contribution from the planetary atmosphere during the exoplanet’s orbit around its host star one can establish the longitudinal brightness distribution of the exoplanet, which is intimately linked to the dynamics and thus the physics and chemistry of the atmosphere.
The differential technique gives rise to the most stringent requirement of the mission. EChO will observe exoplanets for which the contrast between exoplanet and host star is typically 10-3 to 10-4, but can be as low as 10-5 depending on the exoplanet/stellar system and the observing wavelength. This level of measurement stability needs to be reached in order to separate the contribution of the exoplanet from the much larger signal from the host star. The contrast ratio translates to a requirement on photometric stability of 10-4 (3 sigma) or better over an interval of about 10 hours (equivalent to the time taken by a representative exoplanet to transit, plus equal periods on either side of transit)
The broad and continuous/simultaneous spectral coverage that EChO will offer (0.4 – 11 microns, with a goal to reach 16 microns) is unique amongst current facilities used for transit spectroscopy, and also to any currently under development. It gives access to a large number of spectral features that can be used to probe the atmospheres of exoplanets at a range of temperatures. Crucially, it provides the means by which to separate temporal variations in the observed astronomical signal that originate from the exoplanetary atmosphere, and those due to variability in the output from the host star: spectral features in the visible waveband can be used to monitor, and correct for the effects of stellar activity on the observed light curves.
The visible waveband can also be used to determine the albedo of hotter planets (temperatures > 700 Kelvin) as well as to establish the contribution of Rayleigh/Mie scattering by possible cloud cover.
The 1-5 micron waveband is essential to characterise the atmospheres of hot/warm exoplanets, with the peak of the blackbody emission at temperatures between a few 1000 Kelvin and 400 Kelvin falling in this band. The fundamental vibration modes of many of the target molecules of EChO are also found at these wavelengths. With a resolving power of R~300, features can be separated and used to recover molecular abundances.
Wavelengths longer than 5 microns, on the other hand, play an important role in characterising temperate exoplanets, which have temperatures similar to Earth (~250 Kelvin). Virtually no photons shorter than 5 microns are emitted by sources at ~300 Kelvin. In this case the peak of the blackbody emission curve falls within the 5-11 micron band, as do fundamental vibration modes of the biomarker ozone (O3) and many molecules including sulphur dioxide (SO2), ethylene (C2H4), methane(CH4) and nitrogen dioxide(NO2). Spectral features in this waveband are broader than those in the near-infrared, and a modest, lower resolving power (a few tens) is sufficient to detect molecules.
One objective is to separate rotational components that, in combination with spectral features of the same molecule found in the near-infrared, will significantly improve the accuracy of models of the atmospheric structure of hot/warm planets. For this, a higher resolving power would be needed (R>100).
At even longer wavelengths, the 11-16 micron waveband hosts the spectral signature of carbon dioxide (CO2), as well as signatures of hydrogen cyanide (HCN) and hydrocarbons (acetylene(C2H2) and ethane (C2H6)) that are prevalent in hydrogen-rich atmospheres. The carbon dioxide feature is particularly important for the recovery of the thermal profile of temperate planet atmospheres. Resolving powers of a few tens are sufficient for this.