The European Space Agency's Darwin and NASA's Terrestrial Planet Finder (TPF) are among the most challenging space science missions ever considered. Their principal objective is to detect Earth-like planets around nearby stars and to characterize their atmospheres. Darwin and TPF-I are currently conceived as nulling interferometers with free-flying telescopes. Within the frame of the Darwin program, the ESA and the European Southern Observatory (ESO), supported by European industries and scientific institutes, have performed two parallel Phase A studies of a ground-based nulling interferometry experiment (GENIE) at the site of ESO's Very Large Telescope Interferometer (VLTI) in Paranal, Chile. GENIE will demonstrate several key technologies required for the Darwin mission. Its science objectives include the detection and characterization of dust disks and low-mass companions around nearby stars. These studies have established detailed instrumental designs, in which GENIE will operate in the L' band around 3.8 microns as a single Bracewell nulling or constructive interferometer, using either two Auxiliary or two Unit Telescopes. The studies were supported by detailed numerical simulations which indicated the possibility of detection and low-resolution spectroscopy in nulling mode of extra-solar giant planets (EGPs) with atmospheric temperatures down to 700 K, provided that a proper calibration of instrumental effects is applied. Detection of circumstellar exo-zodiacal (EZ) dust clouds is possible down to 0.5 mJy, with interesting prospects for the characterization of planet-forming disks.

In this laboratory experiment, we study the possibility of producing an apodization of the pupil of a telescope using a classical Michelson interferometer. To simulate the star, we successively used a Laser source, a source of spectral light and a source of white light. Our goal is to study the performance of the assembly with polychromatic light. We present the results of experiments carried out with a rectangular aperture using a HeNe Laser and Na spectral light sources.

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.

In the context of extrasolar planet direct detection, we evaluated the performance of differential imaging with ground-based telescopes. This study was carried out in the framework of the VLT-Planet Finder project and is further extended to the case of Extremely Large Telescopes. Our analysis is providing critical specifications for future instruments mostly in terms of phase aberrations but also regarding alignments of the instrument optics or offset pointing on the coronagraph. It is found that Planet Finder projects on 8m class telescopes can be successful at detecting Extrasolar Giant Planets providing phase aberrations, alignments and pointing are accurately controlled. The situation is more pessimistic for the detection of terrestrial planets with Extremely Large Telescopes for which phase aberrations must be lowered at a very challenging level.

We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and its role as a calibration point for theoretical tracks. We argue for a new 70$\pm$30 Myr age estimate and present two additional detections of C with the Simultaneous Differential Imaging (SDI) camera. Our improved analysis (Nielsen et al. 2005) confirms our spectral type of M8 ($\pm$1) and mass of 0.090 $\pm$0.003 M$_{\hbox{\it \scriptsize sun}}$ for AB Dor C. However, Luhman & Potter (2006) argue for a hotter spectral type (M6$\pm$1). Here we adopt the consistent spectral range of M7$\pm$1 in this paper as a final spectral type. Plotting AB Dor C (and all other young (${<}0.12$ Myr), low-mass ($0.3-0.05 M_{\hbox{\it \scriptsize sun}}$) objects with accurate dynamical masses) on the HR diagram suggests a trend where current evolutionary models tend to over-predict the temperature (or under-predict the mass) for young low-mass stars and high-mass brown dwarfs. With our uncertainties, there is a $\sim$90% chance that the mass of AB Dor C is underestimated by the DUSTY tracks in the HR diagram.

We report preliminary results from our search for massive giant planets (6–12 Jupiter masses) around the known seven single white dwarfs in the Hyades cluster at sub-arcsec separations. At an age of 625 Myr, the white dwarfs had progenitor masses of about 3 solar masses, and massive gaseous giant planets should have formed in the massive circumstellar disks around these ex-Herbig A0 stars, probably at orbital separations similar or slightly larger than that of Jupiter. Such planets would have survived the post-Main-Sequence mass loss of the parent star and would have migrated outward adiabatically to orbital separations of about 25 AU. At the distance of the Hyades (45 pc) this corresponds to angular separations of about 0.5 arcsec which can be resolved with NICMOS/HST; the expected contrast in the J and H bands amount to 7.5 $\pm$ 1.5 mag.

Evaluation of our NICMOS data set did not reveal any evidence for planetary mass companions with masses down to about 10 Jupiter masses nor brown dwarfs around any of the seven white dwarfs for separations larger than 0.5 arcsec. However, we detected a low-mass, probably stellar, companion to a field white dwarf (WD1847-223J, distance $\sim$50 pc, age $\sim$1 Gyr; separation $\sim$0.5 arcsec, $\Delta$H $\sim$ 2.5 mag), using the NACO adaptive optics system at the VLT.

SAO has set up a testbed to study coronagraphic techniques, starting with Labeyrie's multi-step speckle reduction technique. This technique expands the general concept of a coronagraph by incorporating a speckle corrector (phase and/or amplitude) in combination with a second occulter for speckle light suppression. The correction function is derived applying the phase diversity method on images taken in focus and slightly out-of-focus. The occulter masks for the testbed will initially be produced lithographically. However, in a parallel program we are studying a new manufacturing method. This method utilizes focussed ion beams and will directly mill the mask shape into absorbing material deposited on a transparent substrate.

As high contrast imaging surveys are being designed and carried out to directly detect extrasolar planets around young, nearby stars it is important to carefully evaluate the criteria for selection of target stars, as well as the predicted success of such a survey based on the sensitivity of the AO system used. We have developed a routine to simulate a large number of planets around each potential target star, and determine what fraction can be reliably (5$\sigma$) detected using an AO system's predicted or observed sensitivity curve (the maximum flux ratio between the parent star and a detectable planet as a function of projected radius). Each planet has a randomly assigned semi-major axis, mass, and eccentricity (following extrapolations of detected radial velocity planet power laws), as well as random viewing angles and orbital phase. The orbital parameters give a projected separation for each planet, while the mass is converted into a flux ratio in the appropriate bandpass of the detector using the models of Burrows et al. (2003); this allows the simulated planets to be directly evaluated against the system's sensitivity curve. Since this method requires basic parameters (age, distance, spectral type, apparent magnitude) for each target star, a target list can be constructed that maximizes the likelihood of detecting planets, or competing instrument designs can be evaluated with respect to their predicted success for a given survey. We are already employing this method to select targets for our Simultaneous Differential Imaging (SDI) surveys (Biller et al. 2004), now underway at telescopes in the northern (MMT) and southern hemispheres (VLT).

The Terrestrial Planet Finder Coronagraph (TPFC) is a National Aeronautics and Space Administration (NASA) exploration mission to directly detect and characterize terrestrial exoplanets at visible and near-infrared wavelengths. The TPFC mission is currently in a “pre-formulation” stage where requirements and designs are traded. TPFC must distinguish a planet that is more than 10 orders of magnitude fainter than its parent star at a separation of 62 mas ($\lambda = 600$ nm). Coronagraphic detection requires a large aperture telescope to resolve the exoplanet from its parent star, and great system (wavefront) stability during detection and characterization. This paper discusses the design considerations, trade studies and analysis leading to the current, “reference” design for the TPFC telescope. We present the salient features of the design and the most significant structural, thermal and optical analysis results. We also discuss the planned model validation and performance verification approach.

The Lyot Project near-infrared JHK coronagraph achieved first light on the Advanced Electro-Optical System (AEOS) in March 2004. Optical pupil plane imaging at video rates from this coronagraph provides data on atmospheric scintillation and quasi-static pupil intensity variations. We examine the effect of these variations on coronagraphic performance. Early simulations suggested Strehl ratio reductions of the order of 2–3% due to residual uncorrected phase aberrations in H-band. We find that static or quasi-static pupil illumination non-uniformity in I-band reduces Strehl by $\sim$2%. A lower bound on the effects of dynamic illumination variation over the pupil is also $\sim$2% in I-band. Some of the static intensity variations in the pupil are due to pinned deformable mirror (DM) actuators. We simulate the effects a pinned actuator has on the coronagraph. The resultant speckles in simulated coronagraphic images show similarities to some Lyot Project PSFs. This highlights the importance of knowledge of the pupil in next-generation extreme AO coronagraphs in order to realize the predicted photometric dynamic range of their images.

Three-dimensional common-path interferometer is proposed to obtain achromatic nulling for the on-axial source; the off-axial source remains detectable. The 3D interferometer involves $\pm 90^{\circ }$ polarization rotations in each interferometer arm. That results in the achromatic 180$^{\circ}$ phase shift, so that the on-axial source interferes destructively. Depending on the source axial position, the light energy is split by different ratios between the Bright and the Nulled interferometer outputs. For the linearly polarized on-axial source, all the energy at nearly 100% is directed to the Bright port. For the off-axial source, the light is split by the ratio at nearly 50%/50% between the Bright and the Nulled ports. The common-path scheme compensates effectively the optical path difference (OPD) and it remains stable to mechanical vibrations. Theory, simulations and preliminary breadboard experiments are shown to be in reasonable agreement.

The goals of the Navigator Program at NASA are to find Earth-like planets around nearby stars, to determine if they are habitable, and to search for signs of life. Three strategic missions are planned to carry out this program: the Space Interferometer Mission Planetquest (SIM), the Terrestrial Planet Finder Coronagraph (TPF-C), and the Terrestrial Planet Finder Interferometer (TPF-I). These missions, along with the PI-class Kepler project, will each discover unique knowledge about extrasolar planets, synergistically building on the other missions.

Radial velocity surveys provide evidence that giant extrasolar planets are common, but their detection space is limited to only a few astronomical units from the stars. In order to close this gap, the adaptive optics assisted NIR imager NAOS-CONICA (NACO) at the VLT was used for a deep (15-20 minutes exposure time per target) L-band survey of a sample of closeby young stars. All stars are members of the Tucana and $\beta$ Pictoris moving groups apart from the the somewhat older star HIP 71395 that has a radial velocity trend suggesting a massive planet in large orbit. The chosen observation wavelength is very well suited for very high contrast imaging of close companions at this age and makes this survey unique. The goal was to detect substellar companions to these stars at distances as close as 5-20 AU and ultimately to detect giant extrasolar planets down to a few Jupiter masses, to measure their frequency, and - by comparison with models - determine their physical properties. This paper presents the results obtained on a subsample of 12 stars that have been observed during ESO P73.

The search for life on extrasolar planets is based on the assumption that one can screen extrasolar planets for habitability spectroscopically. The first space born instruments able to detect as well as characterize extrasolar planets, Darwin and terrestrial planet finder (TPF-I and TPF-C) are scheduled to launch before the end of the next decade. The composition of the planetary surface, atmosphere, and its temperature-pressure profile influence a detectable spectroscopic signal considerably. For future space-based missions it will be crucial to know this influence to interpret the observed signals and detect signatures of life in remotely observed atmospheres. We give an overview of biomarkers in the visible and IR range, corresponding to the TPF-C and TPF-I/DARWIN concepts, respectively. We also give an overview of the evolution of biomarkers over time and its implication for the search for life on extrasolar Earth-like planets. We show that atmospheric features on Earth can provide clues of biological activities for at least 2 billion years.

The Planet Detection Testbed is designed to simulate the architecture and operation of a space-borne four-telescope nulling interferometer. Constructed in the form of a dual chopped Bracewell interferometer together with star and planet sources, it reproduces the principal features of the flight beam combiner designed at JPL for the proposed TPF-I Formation Flying Interferometer. The aims of the testbed are to demonstrate stable four-beam nulling and planet detection at representative star-planet contrast ratios. In the flight design, starlight between 7 and 17 micron wavelength is to be nulled, and 2 to 3 micron starlight is used for fringe tracking and phasing the interferometer. There are also metrology systems and alignment systems which are required for deep and stable nulling. The testbed reproduces these features; 2 to 3 micron light from a thermal source is used for fringe tracking, and nulling and planet detection is performed at 10 microns. The testbed also incorporates laser metrology and other systems enabling continuous control of beam alignment. The ultimate goal is to simulate planet detection at star-planet contrast ratios of order $10^{-7}$ during full rotations of the telescope array using the phase chopping method. The latest results from the testbed are presented including four-beam nulling experiments at null depths of $10^{-5}$ and planet signal detections at similar contrast ratios.

The effects of photon noise, aliasing, wavefront chromaticity and scintillation on the PSF contrast achievable by ground-based adaptive optics (AO) are evaluated for different wavefront sensing schemes. I show that “classical” AO (sensing in the visible, imaging in the near-IR) is limited to about $10^5$ PSF contrast in the central arcsecond because of scintillation chromaticity.

This comparative study shows that a focal-plane based wavefront sensor (WFS), combining wavefront sensing and scientific imaging on the same detector is optimal for high contrast imaging. This approach combines high WFS sensitivity, immunity to aliasing and non common path errors and optical design simplicity. Its theoretical performance is compared to commonly used WFSs, illustrating the advantages of this technique.

I show that such a system can be efficiently used as a second stage after a low-order AO system. Control and data reduction algorithms are presented, as well as possible optical designs incorporating a coronagraph. A laboratory demonstration of this technique is currently being done at Subaru Telescope.

Direct detection of exoplanets is a topic of increasing interest since the first exoplanet discovery by indirect methods. It represents a formidable task because of the small angular separation and large contrast ratio between planet and parent star. We present a novel family of stellar coronagraphs based on the standard coronagraph design but transformed to perform optical differentiation. The proposed coronagraphic masks are used to perform the first or the second derivative of the incoming field. This concept offers a new method to detect exoplanets providing both, deep starlight extinction and high angular resolution. To perform optical differentiation the coronagraph's occulting disk is replaced by an especially designed mask. A further improvement on the coronagraph performance is made by adding a gaussian profile to the differentiation mask in order to reduce the amount of diffracted light. The theoretical rejection rate of our coronagraph is infinite. Computer simulations carried out for the ideal case show that it achieves deep starlight reduction corresponding to a gain of at least 37 mag ($10^{-15}$ light intensity reduction). To take full advantage of the capabilities of our coronagraph atmospheric distortions must be reduced by the use of extreme adaptive optics systems or by its use on space telescopes.

We present the current performances of the AMBER / VLTI instrument in terms of differential observables (differential phase and differential visibility) and show that we are already able to reach a sufficient precision for very low mass companions spectroscopy and mass characterization. We perform some extrapolations with the knowledge of the current limitations of the instrument facility.

We show that with the current setup of the AMBER instrument, we can already reach $3\sigma = 10^{-3}$ radians and have the potential to some low mass companions characterization (Brown dwarves or hypothetical very hot Extra Solar Giant Planets). With some upgrades of the VLTI infrastructure, improvements of the instrument calibration and improvements of the observing strategy, we will be able to reach $3\sigma = 10^{-4}$ radians and will have the potential to perform Extra Solar Giant Planets spectroscopy and mass characterization.

The achievable contrast level for space-based detection of exo-planets will be limited by the stability of the optics. As a consequence, active amplitude and phase compensation will be needed. High order aberrations can arise from both non linearity and Fresnel propagation, their wavelength dependence is studied here and it is concluded that $\lambda$ independent errors are dominant. The behavior of a Michelson interferometer equipped with two deformable mirrors under chromatic light is then presented, and an achromatization method based on a dispersive element is suggested.

We discuss the evolution of the habitable zone around low mass and intermediate mass stars as they evolve off the main sequence. This work shows that this class of stars should be included in the search for life because if planets could be found in their habitable zones, they will allow to test different hypothesis concerning the conditions of life formation. For instance if these planets show traces for life, it is possible to empirically determine a lower limit to the timescale for the formation of life. This time scale is not well determined from the Earth's study, as life formation initially occurred during a period of heavy bombardment from comets and asteroids during the formation of the solar system. We will summarize (to the best of our knowledge) some of the recent results concerning the early signs of life on Earth. We will present some of our work in progress in which we evaluate the effects of stellar evolution on the habitable zone changes.