ESA EChO home page

This proposed mission would rely on a 1.5 m telescope and a 0.4-16 µm spectrometer, with average resolution but exquisite photometric stability, to perform transmission spectroscopy of transiting exoplanets. It would also do time-resolved spectroscopy of stellar systems, allowing to determine reflection spectra of exoplanets as their phase varies as seen from earth. The latter would also allow, in some cases, to get coarse cartographic data and constrain atmospheric models. Oh, and potentially detect biosignature O₃ on temperate super-earths around M-dwarf stars.

It is one of four contenders for the M3 slot (2022 launch). I like how they get a significant part of Darwin's science output by trading spatial resolution (hard) for sensitivity and time resolution (easier). I don't know, I just find it amazing

Science summary on the mission website

Edit: Reflection, not emission. The temperature difference between day and night would probably not be significant enough for that.

Sounds like a great mission. My only caveat is that with a lower bound of ~1.5 Earth-radii in the habitable zone, a lot of the planets they can characterize will probably be better classfied as sub-Neptunes (i.e., not primarily rocky).

That would stille be valuable science. Moreover, planets such as Gliese 581d might be within its reach (not transiting, so reflection only. Not sure if it's possible, trying to find out...). And once the technique is perfected, getting better sensitivity only requires a bigger telescope (read: light bucket). No need to formation-fly spacecraft with nanometer precision.