Direct imaging of habitable planets with ELTs
Subaru Telescope University of Arizona
ELTs will offer unprecedented angular resolution and collecting area, two of the essential ingredients for direct imaging and characterization of habitable exoplanets. If equipped with high performance AO system and coronagraph, ELTs will therefore be very powerful instruments for high contrast imaging of the immediate surroundings of nearby stars. An analysis of the expected performance of such a system, both from fundamental principles and extrapolations using current technologies developed for 10-m class telescopes, reveals that it will be able to image a large number of extrasolar planets. One of the most interesting finding of this analysis is that ELTs should be able to directly image potentially habitable planets in reflected light, and that reflected light imaging in the near-IR is easier than thermal emission imaging with an ELT (in part thanks to the ability to chose the most nearby stars as targets, as opposed to more distant young stars). While an Earth analog at 10pc would be at the very limit of detection even with a nearly optimal system (using a high sensitivity and high speed ExAO system), adopting a slightly broader definition of habitable planets including Super-Earths around late-type stars shows that detection and spectroscopic characterization of habitable planets will be within reach. For such targets, the reflected light contrast is more favorable and the planet is intrinsically brighter (since late type stars are more numerous, and can therefore be selected at small distance). The key to enable these detections is angular resolution, since the habitable zone around late type stars is very close to the star. An ExAO system for ELT should therefore use techniques optimal for high contrast imaging at a few diffraction limits from the central star. Such techniques are currently being developed in laboratories, and some of them are being deployed on existing telescopes: small inner working coronagraphs can now deliver full sensitivity images down to 1 λ/D, and new wavefront sensing techniques fully utilizing coherence over the entire pupil allow more than 10000x gain in sensitivity over conventional seeing-limited wavefront sensors. This science goal is complementary to direct imaging and characterization of habitable exoplanets from space, which favors F-G-K type stars at larger angular separation. Since no such space mission is planned for the near future (and will therefore likely not fly before 2025-2030), ground-based ELTs may in fact offer the first opportunity for detailed characterization of nearby habitable worlds.