Adaptive optics and site requirements for the search of earth-like planets with ELTs O. Lardière a , P. Salinari a , L. Jolissaint b , M. Carbillet a , A. Riccardi a , S. Esposito a a Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy lardiere@arcetri.astro.it, salinari@arcetri.astro.it, riccardi@arcetri.astro.it, marcel@arcetri.astro.it, esposito@arcetri.astro.it b Herzberg Institute of Astrophysics, 5071 W. Saanich Road, Victoria, B.C. V9E 2E7, Canada laurent.jolissaint@nrc-cnrc.gc.ca ABSTRACT Since 1995, exoplanets discoveries have triggered a renewal of the permanent question about the possible presence of life outside the solar system. Direct detection and characterisation of earth-like extrasolar planets orbiting main-sequence stars are now among the most exciting and challenging astronomical topics where new major scientific results from space missions and also from ground-based ELT are expected. To scale the performances of an ELT for exoplanets searching, we examine the relative impact of three fundamental parameters (the actuator pitch, the telescope diameter and the site) on the image contrast. Then, we calculate the planet/star flux ratio needed to reach SNR=3 in 10h (only the photon noise is considered) from long-exposure AO-PSF computed with PAOLA (a fast analytical code developed by one of us: L.J.) for different ELT sizes and AO parameters under different observational conditions (atmospheric turbulence, star magnitude) with or without coronagraphy. We find that an actuator pitch of 0.1m is optimal for exoplanet searching in the visible and near-IR from 10 to 40pc. Lastly, we emphasize that the site choice is as important as the telescope size choice is: a 15m telescope is just enough for coronagraphic search for exo-earths at 10pc (SNR=3 in 10h) from the promising Dome C (Antarctica), while a 30m ELT is needed to succeed the same observation in the same time from the Mauna Kea. Keywords: Extremely Large Telescope, adaptive optics, high dynamic-range imaging, planetary systems. 1. INTRODUCTION Since 1995, more than 117 extrasolar planets in 102 planetary systems have been discovered by indirect observations. These discoveries have triggered a renewal of the permanent question about the possible presence of life outside the solar system. Direct detection and characterisation of earth-like extrasolar planets orbiting main-sequence stars are now among the most exciting and challenging astronomical topics where new major scientific results from space missions and also from ground-based ELT are expected. At 10pc, the Earth is seen at 0.1 arcsec from the Sun. This angular separation is largely greater than the resolution of an ELT thanks to the large aperture considered (D>15m), even with a very partial adaptive optics (AO) correction. However, the brightness contrast between the planet and its parent star (10 -9 for Jupiter at 5AU, 2.10 -10 for Earth at 1AU) is more problematic because it requires very good optics figuring and above all an extremely good correction of the atmospheric turbulence. An AO corrected point spread function (AO-PSF) is characterized by a central core, resembling the diffraction-limited telescope PSF, surrounded by a wide halo containing the residual energy (≈1-Strehl) of the non-phased light which can hide faint planets in its photon noise.