EChO would have provided an unprecedented view of the atmospheres of exoplanets transiting nearby stars. A mission dedicated to the differential technique of transit spectroscopy, EChO would have built on the successes of the Hubble Space Telescope, Spitzer Space Telescope and ground-based optical telescopes.
EChO would have measured atmospheric transmission, reflection and emission spectra over a continuous wavelength range that spanned 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 would have been 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.
EChO would have observed a sample of ~100 exoplanets, spanning a mass range from Jupiter-class gas giants down to super Earths, a temperature range spanning hot planets with equilibrium temperatures as high as a few 1000 Kelvin down to planets with temperatures more similar to Earth at 250-350 Kelvin and planets orbiting host stars with a variety of spectral types (F, G, K and M). By covering this broad parameter space, EChO would not only have probed the physics and chemistry of exoplanetary atmospheres, but would also have started to determine the mechanisms that drive the formation of exoplanets themselves.
EChO would have had the sensitivity to detect key spectral diagnostic features in the atmospheres of hot Jupiters, as well as temperate super-Earths around M-dwarfs. This would offer the tantalizing possibility to study exoplanets in the habitable zones of planetary systems that are close cousins of our own Solar System.
EChO would have exploited small differences in the light from spatially unresolved observations of an exoplanet and its host star at different points in the planetary orbit:
- During primary transit, a planet passes in front of its host star. A fraction of the stellar light passes through the limb of the atmosphere of the planet. The combined signal is made up, therefore, of the (dominant) contribution from the star, the transmission spectrum through the atmosphere of the planet and the emission spectrum from the night side of the planet.
- During a secondary eclipse, a planet moves behind its host star, and so the observed signal is from the star alone.
- On either side of the eclipse/transit, light reflected by the planet, as well as emission from the day- and night-sides of the planet and from the star, contributes to the observed signal.
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 would have been possible to recover an absorption or transmission spectrum of the exoplanetary atmosphere with sufficient signal-to-noise in a single transit. Where this could not have been done, the source would have been revisited over the course of the mission and observations taken over multiple transits would have been co-added. Careful monitoring of, and accounting for, variations in stellar output and hence in the overall measured light curve are key to the success of this co-addition.
The differential technique gives rise to the most stringent requirement of the mission. EChO would have observed exoplanets for which the contrast between exoplanet and host star is typically 10-3 to 10-4, but could be as low as 10-5 depending on the exoplanet/stellar system and the observing wavelength. A high level of photometric (measurement) stability would need to have been reached over a time interval equivalent to the time taken by the exoplanet to transit its host star plus equal periods on either side of transit in order to separate the contribution of the exoplanet from the much larger signal from the host star. In the case of orbital phase measurements, the period over which high photometric stability would have had to been achieved extends to a fraction of the orbital period of the exoplanet, or a few days.
The broad and continuous/simultaneous spectral coverage that EChO would have offered (0.55 – 11 microns, with a goal to reach 0.4 - 16 microns) is unique amongst current facilities used for transit spectroscopy, and also to any currently under development. It would have given 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 would have provided 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 could have been used to monitor, and correct for the effects of stellar activity on the observed light curves.
The visible waveband could also have been 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 could have been 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 would have been to separate rotational components that, in combination with spectral features of the same molecule found in the near-infrared, would have significantly improved the accuracy of models of the atmospheric structure of hot/warm planets. For this, a higher resolving power would have been 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.