Imaging Of Extrasolar Planets Research Articles

Notch‐Filter Masks: Practical Image Masks for Planet‐finding Coronagraphs

An ideal coronagraph with a band-limited image mask can efficiently image off-axis sources while removing identically all of the light from an on-axis source. However, strict mask construction tolerances limit the utility of this technique for directly imaging extrasolar terrestrial planets. We present a variation on the basic band-limited mask design--a family of ``notch filter'' masks--that mitigates this problem. These robust and trivially achromatic masks can be easily manufactured by cutting holes in opaque material.

Phase-induced amplitude apodization of telescope pupils for extrasolar terrestrial planet imaging

In this paper, an alternative to classical pupil apodization techniques (use of an amplitude pupil mask) is proposed. It is shown that an achromatic apodized pupil suitable for imaging of extrasolar planets can be obtained by reflection of an unapodized flat wavefront on two mirrors. By carefully choosing the shape of these two mirrors, it is possible to obtain a contrast better than 109 at a distance smaller than from the optical axis. Because this technique preserves both the angular resolution and light gathering capabilities of the unapodized pupil, it allows efficient detection of terrestrial extrasolar planets with a 1.5 m telescope in the visible.

Extrasolar Planet Finding via Optimal Apodized‐Pupil and Shaped‐Pupil Coronagraphs

In this paper we examine several different apodization approaches to achieving high-contrast imaging of extrasolar planets and compare different designs on a selection of performance metrics. These approaches are characterized by their use of the pupil's transmission function to focus the starlight rather than by masking the star in the image plane as in a classical coronagraph. There are two broad classes of pupil coronagraphs examined in this paper: apodized pupils with spatially varying transmission functions and shaped pupils, whose transmission values are either 0 or 1. The latter are much easier to manufacture to the needed tolerances. In addition to comparing existing approaches, numerical optimization is used to design new pupil shapes. These new designs can achieve nearly as high a throughput as the best apodized pupils and perform significantly better than the apodized square aperture design. The new shaped pupils enable searches of 50%-100% of the detectable region, suppress the star's light to below 10-10 of its peak value, and have inner working distances as small as 2.8λ/D. Pupils are shown for terrestrial planet discovery using square, rectangular, circular, and elliptical apertures. A mask targeted at Jovian planet discovery is also presented, in which contrast is given up to yield greater throughput.

Extra-solar planets

The discovery of the first extra-solar planet surrounding a main-sequence star was announced in 1995, based on very precise radial velocity (Doppler) measurements. A total of 34 such planets were known by the end of March 2000, and their numbers are growing steadily. The newly discovered systems confirm some of the features predicted by standard theories of star and planet formation, but systems with massive planets, having very small orbital radii and large eccentricities, are common and were generally unexpected. Other techniques being used to search for planetary signatures include accurate measurement of positional (astrometric) displacements, gravitational microlensing and pulsar timing, the latter resulting in the detection of the first planetary mass bodies beyond our solar system in 1992. The transit of a planet across the face of the host star provides significant physical diagnostics, and the first such detection was announced in 1999. Protoplanetary disks, which represent an important evolutionary stage for understanding planet formation, are being imaged from space. In contrast, direct imaging of extra-solar planets represents an enormous challenge. Long-term efforts are directed towards infrared space interferometry, the detection of Earth-mass planets, and measurement of their spectral characteristics. Theoretical atmospheric models provide predictions of planetary temperatures, radii, albedos, chemical condensates and spectral features as a function of mass, composition and distance from the host star. Efforts to characterize planets occupying the `habitable zone', in which liquid water may be present, and indicators of the presence of life are advancing quantitatively.

The Four‐Quadrant Phase‐Mask Coronagraph. I. Principle

ABSTRACTWe describe a new type of coronagraph, based on the principle of a phase mask as proposed by Roddier and Roddier a few years ago but using an original mask design found by one of us (D. R.), a four‐quadrant binary phase mask (0, π) covering the full field of view at the focal plane. The mutually destructive interferences of the coherent light from the main source produce a very efficient nulling. The computed rejection rate of this coronagraph appears to be very high since, when perfectly aligned and phase‐error free, it could in principle reduce the total amount of light from the bright source by a factor of 108, corresponding to a gain of 20 mag in brightness at the location of the first Airy ring, relative to the Airy peak. In the real world the gain is of course reduced by a strong factor, but nulling is still performing quite well, provided that the perturbation of the phase, for instance, due to the Earth’s atmosphere, is efficiently corrected by adaptive optics. We show from simulations that a detection at a contrast of 10 mag between a star and a faint companion is achievable in excellent conditions, while 8 mag appears routinely feasible. This coronagraph appears less sensitive to atmospheric turbulence and has a larger dynamic range than other recently proposed nulling techniques: the phase‐mask coronagraph (by Roddier and Roddier) or the Achromatic Interfero‐Coronagraph (by Gay and Rabbia). We present the principle of the four‐quadrant coronagraph and results of a first series of simulations. We compare those results with theoretical performances of other devices. We briefly analyze the different limitations in space or ground‐based observations, as well as the issue of manufacturing the device. We also discuss several ways to improve the detection of a faint companion around a bright object. We conclude that, with respect to previous techniques, an instrument equipped with this coronagraph should have better performance and even enable the imaging of extrasolar giant planets at a young stage, when coupled with additional cleaning techniques.

Refined laboratory simulations of dark-speckle coronagraphy

Dark-speckle coronagraphy with large telescopes is expected to image extrasolar planets. We present new results about the dark-speckle coronagraphic camera, obtained in the laboratory with a simulated atmosphere and an adaptive optics bench. In good seeing conditions, and with an accurate subtraction of a reference frame, we show that our instrument allows high-resolution, high-contrast imaging, more effectively than classical long exposure, for magnitude differences larger than 5 at visible wavelength. With the present capability of the adaptive optics system used, on a 1.5 m telescope pupil, we should expect to detect stellar companions 10 4 fainter than the primary star. Instrument concepts and processing techniques are discussed.

Snapshots of Alien Worlds--The Future of Interferometry

The recent discovery of extrasolar planets illustrates the difficulties astronomers are faced with when trying to identify and characterize distant, faint objects. Optical and infrared interferometry have the potential for providing images of such objects, and may also show fine structure on distant stars, but are difficult to implement. Several international projects, ground-based and in space, are now underway for implementing such interferometers, and an even more ambitious concept - the hyper-telescope in which many segments, spaced kilometers apart but only meters in diameter, form a sparse giant mirror spanning hundreds or thousands of kilometers - is receiving increasing attention, bringing images of extrasolar planets and stars into the realm of possibility.

Imaging of extrasolar advanced terrestrial planets

The gravitational lens effect of the Sun would allow, by using a detector at one of its foci, to obtain a “telescope” with gigantic amplification and resolution powers opening extraordinary perspectives for the detailed study of extrasolar planets, particularly technologically advanced ones. But, astronautical challenges are raised by the necessity to align precisely and put in an efficient tracking and scanning mode the detector, necessarily modest in size compared to the dimensions of the planet images and ranges of orbital and rotational motions. In the frame of the FOCAL space mission submitted to ESA, we present the dynamical geometry of the images for two typical cases of observational wavelengths: 10 centimeters (radio) and 10 micrometers (infrared), for a solar-type stellar system 10 parsecs away. Plasma thrusters could provide interesting solutions for the control of the detector for tracking and scanning the focal images.

Halo characteristics and their influence on companion searches at the Starfire Optical Range

To image extrasolar planets at their large contrast, high-resolution adaptive optics (AO) is needed to correct atmospheric seeing. The 1.5-m AO system at the Starfire Optical Range was used to confirm theoretical models. Halo levels were reduced by a factor of 4, on average, from 0.5 to 3.0 arc sec radius, which when combined with the increased Strehl ratio improved the gain by a factor of 80. Speckle lifetimes ranged from 5 to 30 ms at 0.3 arc sec, which is much longer than the 0.6-ms AO update time. These results show good agreement with predictions for current technology and reveal no limitations, in principle, to the detection of planets by use of AO systems with higher speeds and resolutions.

Atmospheric Intensity Scintillation of Stars. III. Effects for Different Telescope Apertures

ABSTRACTStellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Atmospheric turbulence causes “flying shadows” on the ground, and intensity fluctuations occur both because this pattern is carried by winds and is intrinsically changing. Temporal statistics and time changes were treated in Paper I, and the dependence on optical wavelength in Paper II. This paper discusses the structure of these flying shadows and analyzes the scintillation signals recorded in telescopes of different size and with different (secondary‐mirror) obscurations. Using scintillation theory, a sequence of power spectra measured for smaller apertures is extrapolated up to very large (8 m) telescopes. Apodized apertures (with a gradual transmission falloff near the edges) are experimentally tested and modeled for suppressing the most rapid scintillation components. Double apertures determine the speed and direction of the flying shadows. Challenging photometry tasks (e.g., stellar microvariability) require methods for decreasing the scintillation “noise.” The true source intensity I(λ) may be segregated from the scintillation component ΔI(t,λ,x,y) in postdetection computation, using physical modeling of the temporal, chromatic, and spatial properties of scintillation, rather than treating it as mere “noise.” Such a scheme ideally requires multicolor high‐speed (≲10 ms) photometry on the flying shadows over the spatially resolved (≲10 cm) telescope entrance pupil. Adaptive correction in real time of the two‐dimensional intensity excursions across the telescope pupil also appears feasible, but would probably not offer photometric precision. However, such “second‐order” adaptive optics, correcting not only the wavefront phase but also scintillation effects, is required for other critical tasks such as the direct imaging of extrasolar planets with large ground‐based telescopes.

Dark-lens diffracting telescope: novel concept for direct extrasolar planet imaging

Current narrow-band optical filters for professional solar astronomy applications are primarily based on Lyot filters.1 However, the Lyot filter is severely limited in operation at UV wavelengths because of the lack of suitable UV hightransmission polarizers. In addition, the Lyot filter is comprised of over 50 elements ~over 30 cm long and weighing several kilograms! that require precise alignment and temperature stabilization for all of the elements.

La percée attendue des télescopes géants interférométriques

Recent theoretical advances regarding the analysis of image formation in diluted interferometric arrays of N apertures show that the observing performance can be higher than previously expected. Technical progress in the way of active mirrors and telescope mounts announce the construction of such instruments in the form of 10 m telescopes, arrayed on a platform spanning several kilometers. Such instruments can in principle produce resolved images of extra-solar planets.

Detecting Interstellar Migrations

AbstractInterstellar Migrations may occur when a civilization’s star enters the Red Giant phase, thereby dooming the life-bearing planet(s). Ecologically self-contained «world ships», massing billions of kilograms and propelled by hyperthin, space-manufactured solar sails thousands of kilometers in diameter unfurled near the home star are possible vehicles to transfer a threatened civilization to a neighboring star. Consideration of the nearest Red Giants reveals that Pollux is the nearest formerly solar-type Red Giant. Known stellar neighbors of Pollux are surveyed to determine likely directions for an interstellar migration departing Pollux. Such migrations might consist of many world ships launched over millenia on voyages of about 1000 terrestrial-year duration; discovery of such events will be serendipitous. The difficulties of observing solar-sail star ships near Pollux are considered. A facility dedicated to imaging extrasolar planets within 10 parsecs might be capable of detecting these large spacecraft.

Resolved imaging of extra-solar planets with future 10-100 km optical interferometric arrays

In the recent years, interferometric arrays of optical telescopes have reached sizes of the order of 100 m, but they have yet to produce high-resolution images. The analysis of image formation now shows that such images are theoretically obtainable directly in the recombined focal plane, if there are enough telescopes. Resolved images of extra-solar planets are in principle obtainable with 10 km ground-based arrays.

Multiresolution‐element imaging of extrasolar Earthlike planets

It has been suggested recently that imaging of extrasolar Earthlike planets should be considered as a possible future goal of the NASA space program. As an aid to discussing what would be required in order to undertake imaging, a partial design is described for a separated spacecraft interferometer which could achieve images in the visible with 10 resolution elements across the planet. Between 15 and 25 large collector telescopes or clusters of telescopes spread out over roughly a 200‐km baseline in solar orbit at 1 AU from the Sun or possibly in high Earth orbit are required. A very preliminary approach to the use of multispectral remote sensing techniques is discussed also. Finally, the generalization of this approach to larger numbers of resolution elements across the planet is considered. Clearly, imaging is possible only if the truly staggering problem of avoiding the 1010 times stronger scattered light from the nearby star can be overcome. However, even if this can be done, the amount and precision of the required hardware for even poor‐quality images appears to present an obstacle to such a program which would be extremely difficult to overcome.

Image analysis of two point sources

In this paper the images of two point sources are considered. The effects of diffraction are discussed, and techniques for reducing the effects of a very bright point source on the image of a much fainter point source are suggested. The implications of these ideas on the prospects of obtaining images of extra solar planets are also discussed.

Optimization and Performance of Adaptive Optics for Imaging Extrasolar Planets

A recent study by Angel (1994) using simplified analytical models indicated the feasibility of imaging extrasolar planets from the ground, making use of adaptive optics correction of a large telescope. We have performed detailed simulations of the method using computer codes that model propagation through atmospheric turbulence, adaptive correction, and broadband imaging. We confirm that high-resolution correction at the limit of photon noise errors reduces the halo intensity to 10-6 of the peak star flux. Our work shows how to avoid systematic errors. Thus, we find that time delays between sensing the wave front and its detection lead to persistent structure in the stellar halo, which uncorrected would prevent rapid averaging of the residual halo speckle structure. A local wave front reconstructor that extrapolates ahead in time has been devised to remove this problem. We find the chromatic differences in wave front structure are small enough that the signal-to-noise ratio can be improved by wave front sensing and imaging in separate adjacent bands. We verify that correction of amplitude scintillation is needed and the optimum level of clipping is derived. A simulated image of a twin of the solar system at 8 pc is presented for the new 6.5 m telescope and the measured turbulence at the Multiple Mirror Telescope site. The Jupiter twin shows up at the 5 σ level in a 5 hr integration.

Ground-based imaging of extrasolar planets using adaptive optics

The detection of extrasolar planets by direct imaging presents an extraordinary technical challenge. They must be identified against background light scattered from a star close by and about a billion times brighter. It has been supposed that a near-perfect space telescope would be required to avoid atmospheric blurring. But by using adaptive optics operating at fundamental performance limits, the new generation of large ground-based telescopes has the potential to detect planets orbiting nearby stars.

Direct imaging of extra-solar planets with stationary occultations viewed by a space telescope

The feasibility of detecting planets outside the solar system through imaging at optical wavelengths by a telescope in space is considered. The “black” limb of the moon can be used as an occulting edge to greatly reduce the background light from the planet's star. With this technique and if certain other technical requirements can be realized, a hypothetical Jupiter-Sun system could be detected at a distance of 10 pc. For this system, a signal-to-noise ratio of 9 could be achieved in less than 20 min with a 2.4-m telescope in space. For diffraction limited optics, the required integration time is inversely proportional to the fifth power of the telescope aperture. An orbit for the telescope is described that could achieve a stationary lunar occultation of any star that would last nearly two hours, providing six times more integration time than required by the hypothetical Jupiter-Sun example.