Direct imaging of habitable exoplanets with ELTs

The unprecedented angular resolution soon to be offered by extremely large telescopes (ELTs), together with recently developed high contrast imaging techniques (coronagraphy and wavefront control), can enable direct imaging and spectroscopic characterization of potentially habitable planets around nearby M-type stars. While the habitable zone of M stars is challenging to resolve, the planet to star contrast and the apparent brightness of the planet are highly favorable, thus providing the only opportunity for direct imaging and spectroscopic characterization of habitable planets from the ground. The key to imaging and characterizing such planets lies in the ability to perform high contrast imaging (approximately 1e-5 raw contrast) at 1 to 2 λ/D with high photometric efficiency. We demonstrate that technical solutions to this challenge exist: a full throughput coronagraph concept offering sub-λ/D inner working angle at high contrast on segmented aperture is shown, and schemes to achieve the necessary level of pointing and low order wavefront error control are presented. Demonstrations of these key techniques are ongoing in laboratories and on ground-based telescopes, already yielding encouraging results. We conclude that a highly specialized, but relatively simple, high contrast imaging system can be build for ELTs within this decade, and that it would likely provide the first opportunity to acquire high quality spectra of habitable planets, before space-based telescope can provide similar capabilities for brighter F-G-K type stars.