A large part of our present knowledge of exoplanet atmospheres come from observations done with the Spitzer space telescope. With this mission drawing to an end, there will be a lull in the amount of observational data, at least from space. This may be partly made up by improvements in measurements from the ground, but it is hard to imagine that ground based measurements will get close to the capacity of Hubble and Spitzer in this field, let alone exceed them.
What next? Three space missions under developments have the potential to measure eclipse and transit spectra for exoplanets. FINESSE (NASA) and EChO (ESA) are proposed small space telescopes dedicated to these studies, whereas the James Webb Space Telescope can devote some of its time and instruments to this topic. The scientific objectives would be similar for all three platforms: to gather high-accuracy emission and transmission spectra for about 200 transiting planets, from hot Jupiters to super-Earths, with a sufficient spectral resolution to study molecular features in the near infrared. The dedicated missions would compensate small mirror size by devoting large amounts of time to each object.
The JWST has two instruments that could be used for transit and eclipse spectroscopy: MIRI from 5 to 28 microns, and NIRspec from 1 to 5 microns, with a maximum resolution of 100. The present launch date of JWST is 2019 as far as I know, but as is often the case for space missions, it tends to recede by almost a year every year, and struggle to get off the “time diagram diagonal”.
The FINESSE mission is selected for further study by NASA for the “Explorer” type of missions, i.e. relatively small mission with a compact timeframe. It is potentially the fastest project, with an earliest launch date of 2016. The FINESSE telescope would have a 76-cm mirror, and a wavelength coverage 0.7 microns to 20 microns, with a resolution of 1000. The accuracy of the spectra would be slightly worse than JWST’s NIRspec at the red end, and much better than JWST at the blue end.
Simulated exoplanet spectrum for the super-Earth GJ 1214 b as measured with FINESSE and the JWST. (Note that present measurements show that in reality GJ 1214 b exhibits none of the molecular features predicted in these models, see these posts).
EChO is an ESA candidate mission, currently considered as a possible “M-type” (medium size) programme. The target launch date is 2022. Echo’s technical specifications are still being discussed. The mirror would be in the 1.2-1.5 m range, and there could be two channels, a 5-15 microns instrument and a 0.4-5 m instrument. In an ideal world, a first mission, like NASA’s “TESS” or the moribund ESA “PLATO” would first detect smaller transiting planets around bright stars, then the spectroscopic mission would reap the benefit by collecting the spectra.
FEATURE IMAGE: model of the JWST from the NASA website (model by Stuart Beardow).