Array configurations to detect and characterize extrasolar planets with a space infrared interferometer

We list here some of the basic design requirements that have to be fulfilled by an infrared nulling interferometer, in order to properly detect and characterize extrasolar planets. Three main constrains can be identified: the interferometer must provide a very strong suppression of the starlight, a good spectral coverage (from 7 to 18 microns) with a fixed baseline, and be able to clearly distinguish planets from local dust disc emission. We review and compare here the possibilities of existing projects and present a new pupil geometry with five 1.5m telescopes deployed in an elliptical array. The telescope array, whose dimensions are about 50mx25m, has been optimized so that the exozodiacal emission is strongly extinguished, very weakly modulated by rotation about the line of sight, and concentrated at a few even frequencies. The planet's signal on the contrary is strongly modulated at many distinctive frequencies. A simple cross correlation method recovers a single image of a solar system twin at 10 pc distance, in about 30h. The spectroscopy of the planets can then be undertaken with the same baseline and would reveal absorption features of water, ozone and carbon dioxide of an Earth-like planet at 10 pc in less than one month.