Example of our calculated CC functions for carbon dioxide, which is clearly detected in the transit spectrum of a temperate planet orbiting an M3V star 12 pc from Earth, using 100 transits. Two CCs are illustrated—the CC that models synthetic data including the planet transmission signal, and a CC from a pure noise source. For the latter CC, we generate noise at the same amplitude as in the simulated data. The negative peak on the CC indicates a detection of exoplanetary absorption, see Section 2.2 for an explanation of our sign convention. The S/N is 8 for this simulated detection.