ESA EChO home page
This proposed mission would rely on a 1.5 m telescope and a 0.4-16 Ám 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.