Apparent spectrum of Proxima Cen b at a distance of 1.29 pc, assuming it is identical to Earth. Left panel: planet synthetic thermal spectrum, assuming the Earth’s P–T profile and abundances is shown as a black solid line. As a comparison, measurements by the Nimbus 4 satellite are shown as a pink solid line, which largely overlap. The red solid line shows the reflected light assuming a surface albedo of 30%, including the attenuation of the planetary atmosphere. The two vertical black dashed lines denote the wavelength region of interest for probing HDO (3.6–3.8 μm). Right panel: same but focused on the 3.6–3.8 μm region, and showing high-resolution spectra. At these wavelength, the reflected starlight is ~15 times stronger than the intrinsic thermal emission from the planet.