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 collecting area. The luminosity ratio between a terrestrial planet and its star ($10^{-10}$) is such an ambitious goal that a thorough study needs to be carried out. We started with a simple analysis of the fundamental limitations for the detection of extraterrestrial planets with ELTs. Here, we considered an extreme adaptive optics device upstream of a perfect coronagraph. Even with high Strehl ratios, the coronagraphic halo level is only $10^{-6}$ to $10^{-7}$ 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 simple but 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 itself. Numerical simulations prove that, after the calibration, the limitations are set by the static aberrations which cannot be neglected anymore.

Direct detection and characterization of Earth-like planets from the ground is a very challenging issue. Among the projects, the Extremely Large Telescopes are very promising to improve the angular resolution and to increase the total number of collected photons. We studied this type of instruments in a very optimistic case to evaluate what level of aberrations limits fundamentally the detection. For that purpose, we considered a perfect coronagraph coupled with an extreme adaptive optics device. Even with a Strehl ratio of more than 96%, it only provides a contrast of $10^{-6}-10^{-7}$ at $30\lambda/D$. A calibration system downstream the coronagraph is therefore mandatory to reach the contrast of $10^{-10}$ between a terrestrial planet and its star in the near infra-red. We modelized a very general system taking into account dynamic aberrations left uncorrected by the adaptive optics system, static aberrations of the system and differential static aberrations due to the calibration channel. Numerical simulations demonstrate that the static aberrations are becoming very limitative and must not be neglected. Indeed, to achieve a contrast of $10^{-10}$, with common aberrations of 5 nm on a 100 meter telescope, the differential aberrations must be controlled at the level of 200 picometers. We also compare this speckle noise to the limitation due to the photon noise.

The first images of Titan at 1.3 micron were obtained with the PUEO adaptive optics system at the CFHT, on October 26 1998. We have also obtained images of Titan at 1.6 micron. Our diffraction-limited images (about 10 independent resolution elements or 20 pixels on the disk) pertain to Titan’s leading hemisphere. We have used narrow-band filters centered outside (1.29, 1.6 µm) and in the wings (1.18, 1.64 µm) of the methane bands, so as to be able to subtract the atmospheric contribution from our data. Images of Titan’s surface show the large equatorial region to be bright at both wavelengths. Combined with previous spectroscopic and imaging data, our findings are compatible with the presence of a mountainous plateau, covered with bright ice (perhaps methane), near the equator of Titan and with hydrocarbon deposits in the darker areas.