Fundamental limitations on Earth-like planet detection with ELTs
C. Cavarroc¹, A. Boccaletti¹, P. Baudoz¹, T. Fusco², D. Rouan^1
1 LESIA, observatoire de Paris-Meudon, FRANCE ² ONERA Châtillon, FRANCE
A forthcoming step in the study of extrasolar planetary systems is the direct detection and characterization of Earth-like planets. An asset of the ELTs in that context is their very high angular resolution and their increased number of collected photons. The ratio between a terrestrial planet and its star 10^-10 is such an ambitious goal that a complete study needs to be carried out. We started with a simple analyse of the fundamental limitations for the detection of extraterrestrial planets with ELTs. We considered an extreme adaptive optics device upstream a perfect coronagraph. Even with the high Strehl ratio provided, the direct detection level is only 10^-6 at typical exo-Earth angular distances. A calibration device is therefore mandatory to reach the contrast between a terrestrial planet and its star in the near infra-red.

We considered a realistic model taking into account dynamic aberrations left uncorrected by the adaptive optics system, static aberrations of optical system and differential static aberrations due to the calibration channel. Numerical simulations prove that, after the calibration, the limitations are set by the static aberrations which cannot be neglected anymore: to achieve the contrast of 10^-10, differential aberrations must be controlled at about 10 pm. Beyond this limitation due to the speckle noise and despite the great collecting area, another limitation is set by the photon noise. A comparison of our work with previous studies will be made.

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