Contour map showing the required S/N of the planetary spectrum per pixel, as function of planetary equilibrium temperature and D/H value, for an HDO detection at an S/N of five. The considered wavelength region is 3.6 to 3.8 μm. The D/H values of the Solar System planets are indicated by the horizontal white dotted lines. The colored symbols indicate the lowest detectable D/H values for various exoplanets. Cyan and red symbols indicate limits for CRIRES+ at VLT and METIS at ELT, respectively, assuming a single night of observation (10 h). Filled circles stand for those targets which can be angularly separated from their host star, assuming a stellar flux reduction at the planet position by a factor 100 and 1000 for the VLT and ELT, respectively. Star-symbols denote planets that cannot be spatially resolved, for example hot Jupiters, hence no flux suppression is possible. ^* GJ 1214 b-like planet is a hypothetical non-transiting twin of GJ 1214 b at half the distance from Earth. The hatched area indicates the region where the required S/N per pixel is larger than 5, implying very weak HDO lines, which may be difficult to recover accurately with planet atmospheric modeling.