Revealing habitable exoplanets through their spectral features

Abstract

The extremely low signal contrast between an Earth-like extra-solar planet (exoplanet) and a parent star is a difficult obstacle in their detection, imaging and spectroscopic analysis. We suggest a method of using selected parts of the Fourier interferogram of the combined light sources (both planet and sun) in order to increase the signal to noise ratio and identify the specific spectral features from the planet in the background of the parent star. A habitable exoplanet is expected to reflect and emit a luminosity which is many orders of magnitude less than that of the parent star. However, its spectral features are much different, being much narrower than its sun. Narrower lines are more coherent, so their Fourier spectrum extends to much larger delays. Thus they can be discriminated for by looking at an off-center part of a Fourier spectrogram. As the center (with the shorter delay) has all the power from the star's wider features, these will not affect the result. Now all the power will be distributed at the longer delays (where the exoplanets lines appear), improving the signal to noise ratio. We support this idea by realistic simulations which include photon and thermal noise, and show it to be feasible at a luminosity ratio of 10-6 in the infra-red for a Sun-like star and an Earth-like planet. We also carried out a laboratory experiment to illustrate the method. The results suggest that this method should be applicable to a very large number of candidate stars.