Simulated spectra for the observation of an Earth-twin. Top: comparison of the various input signals in our example of an Earthlike planet at 10 pc. The solid lines indicate the total flux received from the respective sources: planet (red), star (black), exozodiacal dust (gray), and local zodiacal dust (blue). The dotted lines with the same color code indicate the fraction of the flux that contributes to the shot noise. Bottom: detected planet signal (red line) and total shot noise (dashed blue line) averaged over each wavelength bin and for an integration time of 200 000 s (values refer to the y axis on the left). The resulting spectral S/N is shown as the black line and is calculated as described in Sect. 2.3 (values refer to the y axis on the right). Integrated over the full wavelength range, one obtains a S/N ≈ 9.7.