Revealing bio-lines of exoplanets by Fourier spectroscopy E. N. Ribak a* , E. Schwartz a , and S. G. Lipson a a Department of Physics, Technion – Israel Institute of Technology, Haifa 32000, Israel ABSTRACT Earth-like extra-solar planets have luminosities which are many orders of magnitude less than those of their parent star. We propose and test a method for identifying molecular spectral bands in light from such a planet by looking at an off- center part of an infrared Fourier transform interferogram. This results in superior sensitivity to narrow spectral bands, which are expected in the planet’s spectrum, but are absent in the parent star. We support this by stronomical observa- tions to illustrate the method in the visible. The results suggest that this method is applicable to searches for planet bio- lines, and for differentiating between narrow lines and wide lines in other astronomical scenarios. Keywords: Exoplanets, habitable zone, Fourier spectroscopy, astrobiology 1. INTRODUCTION There is a huge intensity contrast between an Earth-like extra-solar planet (exo-earth) and a parent star (typical sun-like). This enormous difference is an obstacle for imaging and spectroscopic analysis of a distant light source observed on the ground. We suggest a method of using selected parts of a Fourier interferogram of the combined light sources (both planet and sun) in order to increase the signal to noise ratio, or planet to star ratio, and identify the specific spectral fea- tures of the planet, by reducing the background due to the parent star 1,2 . An exo-earth is expected to reflect and emit a luminosity which is many orders of magnitude less than that of the parent star. However, there are special spectral features where the contrast is not so high, and luckily these are related to life- related molecules. It so turns out that the narrow molecular spectral bands from such a planet are very different from the stellar spectral lines. This can result in superior sensitivity to narrow spectral bands, which are expected in the planet Figure 1. The principle of the observation, showing the spectrum and below its Fourier transform (the fringes envelope). The black-body spectra of the star and planet exhibit fringes at low frequencies (short path difference, not measured), whereas narrow lines are at higher frequencies. A filter letting through only the infra-red (IR) reduces even further the background. The signal to noise ratio is equal to the height of the envelope compared to its mean (broken lines), and improves when block- ing the shorter wave lengths. *eribak@physics.technion.ac.il wave length λ path difference d spectrum S (λ) fringe envelope V (d) IR filter visible and IR IR only measurement range