International Conference on Space Optics 4-8 October 2010 IMAGING EXOPLANETS Oak Grove Drive, Pasadena, CA 91109, USA

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1 I. INTRODUCTION IMAGING EXOPLANETS Peter R. Lawson 1 1 Jet Propulsion Laboratory, California Institute of Technology, MS , 4800 Oak Grove Drive, Pasadena, CA 91109, USA The world in which we live is the only planet we know that harbours life. Is our planet unique? We have not yet found life on Mars, despite ample evidence of the existence of water, nor have we found evidence of life anywhere else in our Solar System. A tantalizing possibility is that life may yet exist under the ice of the moons of Jupiter yet there is no proof. Is there life elsewhere in the Universe? If we were able to image planetary systems around neighbouring stars, and beyond that, characterize the surfaces and atmospheres of constituent planets, we would be one step closer to answering this question. This paper is a brief tutorial introduction to the challenges of imaging extrasolar planets, with particular attention to the technology and optics required for starlight suppression. Additional background material is available at the website of Jean Schneider s Exoplanet Encyclopaedia [1], and elsewhere: For a more mathematical treatment of the planet detection techniques the reader is referred to the excellent tutorial paper by Perryman [2]; Biosignatures and the search for life are described in the 2010 special issue of Astrobiology [3]; A more in-depth description of coronagraph techniques is provided in the review by Oppenheimer and Hinkley [4]. II. EXOPLANET SCIENCE The study of planets around other stars, exoplanet science, is a rapidly developing field. It had its start in the early 1990s, first with the discovery of pulsar planets [5], and in 1995 with the discovery of the first Hot Jupiter planets [6]. To date, almost 500 planets have been detected around other stars through a combination of radial-velocity techniques (93%), transit experiments (being about 1/5 th of the planets also detected by radial velocity measurements), microlensing (2%), timing experiments (2%), and imaging (3%). Low-resolution spectra of three planets have also been detected using the Hubble Space Telescope, the Spitzer Space Telescope, and even ground-based observatories; in almost all cases, the planets have been objects unlike anything in our Solar System, being mostly Jupiter-like planets in Mercury-like orbits, or planets in wider orbits but with masses larger than Jupiter. All but 3% of planets were detected by measuring subtle changes in the intensity or spectrum of starlight, not by the detection of planet light. Radial velocity measurements detect changes in the wavelength of emission lines in the stellar spectrum that indicates the presence of a companion pulling the star toward or away from the observer. Transit measurements detect changes in the intensity of the starlight as a planet passes in front of or behind a star. Microlensing measurements detect an increase in the intensity of a background star, as it is lensed and modulated by a foreground planet and its host star. Timing experiments detect a slow modulation in the period of millisecond pulsars due to the presence of an orbiting companion essentially a radial velocity shift in the pulsar period due to the presence of a planet. Only nine planets (3% of the total) have been imaged as separate objects, primarily using large ground-based telescopes equipped with adaptive optics and coronagraphic masks. (This count is partly limited by the definition of what qualifies as a planet. Any stellar companion with a mass greater than 13 Jupiter-masses is no longer considered a planet but a Brown Dwarf.) The imaged planets are all larger than any planets in our solar system, with an average mass of about 8 Jupiter masses and with some having orbits as close-in as Jupiter s but some extending 100 times further out. What is know today of exoplanets may rapidly be overtaken by new discoveries in the coming year. The Kepler mission (NASA) is studying data from 706 stars that show evidence of planetary transits [7]. The publically released data from Kepler shows that smaller planets are much more common than could be inferred from ground-based data alone. Analysis of the first year of Kepler data should reveal many more surprises and possibly the detection of the first Earth-sized exoplanet in an Earth-like orbit. Spectra have been obtained of three exoplanets [8,9,10]: HD b, HD b, and HR 8799c, the latter being one of the exoplanets detected by imaging, the first two having been detected by transit measurements. Water vapour, carbon monoxide, carbon dioxide, and methane have been detected. Some of these detections appear marginal in their signal-to-noise ratio, which in each case is limited by the presence of starlight. Were it possible to suppress the starlight without also diminishing the planet light, more detail could

2 be extracted from the spectrum of a much larger number of exoplanets. The remainder of this paper will consider the design of future telescopes and the different techniques of starlight suppression that could be applied to the measurement of exoplanetary spectra. Fig. 1. (Left) Image of Jupiter taken with the Hubble Space Telescope, with an inset simulated image of Earth shown for relative size. The image of the Earth is expanded (right) to better illustrate the surface resolution of ~160 km available in the Hubble image of Jupiter. III. TELESCOPES TO IMAGE EXOPLANETS As a thought experiment, consider the telescope size that would be needed to produce images of exoplanets comparable to the images in Fig. 1 (Courtesy NASA). Figure 1 shows an image of Jupiter taken with the Hubble Space Telescope, where the surface resolution at Jupiter is about 160 km. An inset (simulated) image of Earth with the same surface resolution is shown, and the same image is expanded (right) so that the limited surface resolution is more apparent. If we wanted to make similar images of exoplanets around the closest stars, we would need to resolve angles with the same surface resolution of 160 km, but on a target that is 15,000 times further away ( Centauri being 4.4 light-years distant, or 1.3 parsec). Our exoplanet telescope would then need to be roughly 15,000 times larger than the 2.4-m Hubble, which is to say approximately 36 km in diameter. That is about the diameter of Cape Cod Bay. (For reference also note that Mount Everest rises about 9 km above sealevel and that commercial passenger airplanes fly at an altitude of about 10 km.) If this observatory were part of a larger program, we might want to scale it up by a factor of about 10, so that it could image planets around the 100 nearest stars not just our nearest neighbour. It would then require a primary mirror diameter of 360 km. However, suppose that our goal was simply to resolve planets as separate from each other in the nearest systems. This would allow us to perform spectroscopy and determine the planets atmospheric constituents. We might then require a spatial resolution of only about 0.2 AU, or one fifth the distance of the orbit of the Earth around the Sun. The relative distance of planets in our Solar System is illustrated in Fig. 2 (Courtesy NASA). A resolution of 0.2 AU would make it possible to image a planet like Earth as a single spot of light that was separate from the spots of light from planets similar to Venus or Mars. Our telescope would then need to be only about 0.9 m in diameter. Note that the 36-inch refractor at Lick Observatory (completed in 1888) is slightly larger. Such a small telescope seems implausible to image planets from the ground or space, and as you might guess, we are neglecting a few important issues yet to be addressed. Consider the image of the Brown Dwarf, Gliese 992 b, shown in Fig. 3 (Courtesy Caltech/JHU/NASA). The resolution of the Hubble is indicated by the spot size of the unresolved Brown Dwarf (centre-right); however the image is dominated by the glare of unsuppressed scattered starlight and bright diffraction artifacts. The star itself is also unresolved, but because it is so much brighter than its companion, its scattered light and the wings of its diffraction pattern saturate the image in that quadrant. For faint planets to be detected close-in to a star, the residual starlight must not only be suppressed, but diffraction through the telescope must be shaped to provide a usable search area as close to the star as possible. Even if the diffraction is carefully controlled, the useable search area, is limited by an inner working angle (IWA), and is never closer to the star than several times the telescope resolution, ie. 2 3 /D. For a telescope to have access to a reasonable number of target stars to look for exoplanets, the telescope must have a primary mirror that is slightly oversized otherwise the planets would be hidden in the glare. Mission concepts to look for Jupiter-like planets typically propose space telescopes with primary mirrors at least m in diameter.

3 ICSO 2010 International Conference on Space Optics 4-8 October 2010 Fig. 2. Illustration showing the relative distances of the planets in our Solar System (not to scale). A space telescope with the ability to resolve feature of 0.2 AU at distances corresponding to those of the nearest stars would allow light from similar nearby exoplanets to be resolved as separate spots of light. Fig. 3. Gliese 229 b (shown here, centre right) imaged with the Hubble Space Telescope. As mentioned earlier, some form of starlight suppression is required to increase the number of detectable exoplanets seen through any telescope. The simplest method of starlight suppression is to place an optical stop at an intermediate focus in the internal optical path of the telescope and centre the stop on the image of the star. If the stop is sized correctly it will remove most of the starlight. However, diffracted starlight around the stop may still overwhelm the image of a faint planet. The addition of a subsequent mask in the pupil of the recollimated light (a Lyot stop) then removes much of the remaining diffracted light [11]. Images with contrasts as great as 1 part in 100,000 have been obtained this way. The essential problem is that Earth-like exoplanets are estimated to be 100,000 times fainter still at visible wavelengths; roughly 1 part in 10,000,000,000, or at a contrast of [12].

4 The solution to higher dynamic range lies in the application of new methods in Fourier optics and in imaginative applications of diffraction theory. Diffraction of light at hard edges causes bright fringes to appear in diffraction patterns, so part of the solution is to avoid sharp discontinuities in amplitude (or phase) in any mask that is placed in the beam path. Designs of advanced masks take advantage of the Fourier transform relationship between waves in a pupil and the phase and amplitude distribution in the image plane. For example, rather than blocking starlight with an image stop that is a hard disk, if the stop is partly transparent in the form of a circular fringe pattern, it can theoretically provide full attenuation of light at the subsequent Lyot stop [13]. Another approach is to use two mirrors to reshape the amplitude and phase distribution of the incoming light in the telescope so it has more of a Gaussian intensity distribution, thus tapering the sidelobes in the diffraction pattern [14]. An altogether different approach is to use a starshade. A simple design would be an opaque disk placed some distance in front of the telescope that would block starlight from entering the telescope directly, but around which planet light would be detectable. The starshade must be dimensioned so that it is much larger than the telescope itself, and at a distance such that it blocks an angle on the sky smaller than the apparent orbit of the furthest planet to be studied. In this way no complicated optics within the telescope are needed to suppress starlight, because the starlight never enters the telescope in the first place. Note that an occulting disk would produce a diffraction pattern with a bright spot in its centre, a Poisson spot, because all edges of the disk diffract light toward the centre [15]. New designs for starshades [16,17], therefore have petal-like edges that diffract light tangentially, away from the centre, thus suppressing the Poisson spot and making the detection of close-in planets possible. In typical designs the starshade might be 50-m in diameter and located 50,000 km from the telescope. IV. SYNTHESIS IMAGING TECHNIQUES A major drawback of all coronagraph designs is that their angular resolution, and thus the number of accessible exoplanets, is strictly limited by the diameter of the primary mirror of the telescope. The limitations in angular resolution of a single telescope can however be overcome if instead multiple telescopes are used simultaneously as an interferometer in a synthesis array. This provides an increase in resolution proportional to the telescope-telescope separation, not simply the telescope diameter. Since the late 1950s, radio astronomers have used arrays of radio telescopes for synthesis imaging, realizing that it would never be possible to build steerable telescopes larger than about 100-m (such as the National Radio Astronomy Observatory s Green Bank Telescope in West Virginia), nor fixed telescopes larger ~300-m (the extreme example being Cornell s Arecibo telescope in Puerto Rico). Combining signals from separated telescopes is relatively straightforward at radio and millimetre wavelengths, because radio receivers with adequate phase stability and phase references are readily available. 1 At optical and infrared wavelengths the problem is significantly more difficult, because of the increased stability requirements at these shorter wavelengths. Nonetheless, this approach promises in the longterm to be the ultimate path to imaging other planetary systems and finding life on other worlds. An optical or infrared telescope array in space is a formidable technical and engineering challenge. Because none of the telescopes in the array would themselves resolve the star and planet system, starlight must be suppressed through the control of interference fringes. Although separate telescopes need only be controlled relative to each other by several tens of centimetres, or perhaps metres, the fringes themselves must be controlled to a few nanometres using optical delay lines. Despite the apparent challenge, the required starlight suppression of a factor of 100,000 [18] and achievable contrast of 10-million (in the mid-infrared) has been demonstrated in the lab [19,20]. Many of the optical components needed have also been demonstrated. Further work is needed in cryogenic engineering of starlight suppression for the mid-infrared. In space a mid-infrared telescope array would be very large, with telescope separations of up to 400-m needed to survey the nearest 150 or so stars. The telescopes would need to be operated cooperatively as a formationflying array, as it would be impracticable to build a structure so large. This approach was for many years the baseline design of NASA s Terrestrial Planet Finder (TPF) mission [21]. Although experiments in space have demonstrated rendezvous and docking of separate spacecraft, no synthesis array has yet been flown. There is no precedent for a mission like TPF. Work is still ongoing in the development of technology for the detection of exoplanets. The following section describes some of the technical challenges and possible solutions. 1 Examples of radio arrays include NRAO s Very Large Array, and the Atacama Large Millimeter Array (ALMA), being built with the European Southern Observatory.

5 V. SCIENCE AND TECHNOLOGY IN THE DECADE A. Transit, Microlensing, and Astrometry Technology The technology already exists to implement several classes of missions to detect exoplanets. For example, microlensing missions and transit missions require no new technology. Their designs would be based on those of conventional space telescopes, with the most stringent requirements being placed on the format of the detector arrays. Depending on the instrument and telescope design, it may be possible with such a mission to measure spectra of the brightest exoplanets, in cases where the star/planet contrast is favourable and starlight suppression is not necessary. The implementation of these missions would be relatively straightforward [22]. The technology is also ready to implement astrometry missions based on interferometry techniques, developed as part of the technology program for NASA s Space Interferometry Mission (SIM). Although SIM may not be implemented, similar missions would be possible. B. Coronagraph Technology Mission concepts that will enable the measurement of spectra of Earth-like exoplanets impose the greatest technical challenges. In these cases, the missions require starlight suppression of a factor of ~10 10 at visible wavelengths to suppress the glare of starlight. As shown in Fig. 4, starlight suppression within a factor of a few of this goal has already been demonstrated with bandwidths of up to 10%. This level of suppression is sufficient to now enable a mission to measure spectra of gas giants and Super-Earths around Sun-like stars. Fig. 4. Achieved contrast of starlight suppression experiments at visible and mid-infrared wavelengths. The contrast requirements in each case are different because at visible wavelengths planets are detected in reflected sunlight (contrast of ~10-10 ) and in the mid-infrared the planets are detected by their thermal emission (contrast of ~ 10-7 ). In each regime, experiments have obtained results within a factor of a few of the goal.

6 For coronagraph missions, only evolutionary advances are needed in instrument design to be capable of detecting Earth-like planets. The most mature designs for coronagraphs use Lyot masks [23]. Other techniques include the use of vector vortex masks [24], phase-induced amplitude apodization, or visible nulling interferometry. Each has its relative merits [25]. Continued improvements are yet needed in broadband wavefront control to increase science throughput of these missions. Deformable mirrors (DMs) current exist in 32x32 pixel format and are being developed in larger array sizes. To the author s knowledge, no DM has ever been flown in space, and so further work needs to be done to flight-qualify these devices. The Electric Field Conjugation algorithm [26] represents the state-of-the-art in wavefront-control algorithms. Improvements in the speed of convergence are expected through the use of coherent speckle detection. Minor advances are needed in the performance of low-light detectors. Candidate detector arrays exist, such as E2V s L3CCDs, but require further improvements in quantum efficiency and noise properties. The same need of improved detectors also exists for starshade concepts. C. Starshade Technology The deployment and shape control of starshades needs to be demonstrated. The shape of petal edges of starshades must be controlled to millimetre or sub-millimetre accuracy [27]. Candidate control methods exist, but need to be verified and validated. System modelling will be especially important for a starshade-telescope observatory. The full observatory cannot be tested on the ground, and performance estimates will need to rely on advanced system modelling. Preliminary error budgets and models exist but need to be matured. Only evolutionary advances are needed in formation control. Although demonstrations are needed, the technology for guidance, navigation and control of starshade and telescope does not appear too challenging. Additional development will be needed in the areas of alignment sensors and possibly propulsion. D. Large Telescope Technology Advances in technology for optical-wavelength coronagraph missions, described previously, will directly enable a large-class (ie. 8-m primary mirror, or larger) optical mission. The same approaches to starlight suppression and wavefront control would be used, but in the environment of a physically larger observatory necessitating more advanced mirror and structure technology, as well as more advanced passive/active thermal control. Advanced mirror development is needed for monolithic or segmented designs. Mirrors will need to be lightweight yet provide the largest possible angular resolution. Advanced coatings will be also needed for high throughput and broadband operation. Further improvements in large-scale deployable structure technology is needed. Innovative deployment methods for structures and telescope systems will be required to optimize the designs, despite the mass and volume constraints of launch. Advanced vibration damping and the mitigation of control-structure interaction may also be needed. A large-scale mission will necessitate new actuators, sensors, and control strategies. A large-class mission will require the phasing and control of mirrors across the full aperture as well as across any segment discontinuities. Innovative, time-stable, and precise metrology will be required to interact with state of the art actuators. Integrated modelling and model validation will be challenging. The observatory performance under zerogravity will be difficult to predict prior to launch and will need to rely on advanced optical-thermal-mechanical modelling. New integration and test methods and facilities may need to be developed. Strategies for integration and test will need to be developed early; it will not be possible to test the full observatory on the ground, and the mission will likely need facilities that exceed the current state of the art. E. Infrared Interferometry Technology Future infrared exoplanet missions would very likely be based on methods of nulling interferometry. Because these observatories will need to be larger than any conventional telescope to resolve the same exoplanetary

7 systems, their physical dimensions may exceed what could be built on a single structure. Formation flying technology would enable the concepts with the highest angular resolution even exceeding the resolution available through a large optical mission. Fig. 5. (Left) High-Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory. The HCIT is a vacuum testbed that includes a 32x32 Xinetics deformable mirror and is used to demonstrate coronagraph technology. Experiments have demonstrated contrasts at visible wavelengths of between 10-9 and (Right) Planet Detection Testbed (PDT) at the Jet Propulsion Laboratory. The PDT is a mid-interferometry testbed to demonstrate observations with a 4-telescope interferometer. Contrasts of between 10-6 and 10-7 have been demonstrated at mid-infrared wavelengths. Cryogenic testing would be needed for components, subsystems and systems. Although much of the technology has already been proven at room temperature, the same technology will need to be validated in a cryogenic vacuum environment. This would include the development of cryogenic spatial filters, adaptive nullers, and larger system testbed. Cryocooler development may be needed to suit a low-vibration environment that interferometers require. Integrated modelling and model validation will be challenging for both probe and large-scale infrared missions. The observatory performance under zero-gravity will also be difficult to predict prior to launch and will need to rely on advanced system modelling. In-space testing of formation flying will be needed prior to a large-class infrared mission. The observatory performance will be difficult to predict prior to launch and will need to rely on advanced system modelling. Ground-based tests [28] and experiments in space have produced very encouraging results. I & T methods and facilities will be needed for a large-class mission. Strategies for integration and test will need to be developed early; it will not be possible to test the full observatory on the ground, and the mission will likely need facilities that exceed the current state of the art. VI. STRATEGY AND PLANS FOR NASA AND ESA What does the future hold for exoplanet science? The Kepler and Corot missions are still ongoing and will no doubt continue to produce new detections. In particular, the large number of planets being studied by the Kepler team should be made public by early GAIA (ESA) will be launched around 2012 with the ability to detect Neptune-mass planets by astrometry. The James Webb Space Telescope (NASA/ESA) should launch around mid-decade with the ability to image bright gas giants. Ground-based observations by radial velocity techniques continue to detect ever smaller planets at longer periods; advances in laser-comb frequency standards should help push the detection limit so that the fundamental noise will be limited by oscillations in the stars themselves. If the trend continues, Earth-sized planets in Earth-like orbits should be detectable by transit measurements within a few years. NASA is very likely to follow-up Kepler with a microlensing mission to be launched toward In August of 2010, the U.S. National Academy of Sciences released their Decadal Survey in Astronomy and Astrophysics for the decade [29]. Their recommendations included, as their highest priority for space sciences, the development of a mission called WFIRST (Wide Field Infra-Red Survey Telescope) that would be capable of dark energy observations along with exoplanet microlensing measurements and infrared survey work. The

8 Survey also recommended continued development of starlight suppression technology through a New Worlds Technology Program, with a mission concept to be selected for a more focused effort sometime after middecade. The Space Interferometry Mission was not endorsed for continuation. At the time of writing, NASA s response to the Survey is still being formulated. The European Space Agency has one exoplanet mission under study. One of the highest priority science themes in the ESA Cosmic Vision [30] is the search for life around other stars. A transit survey mission called PLATO was selected for study in the first round of calls in 2007 and is currently in competition with other concepts for a launch. ESA s Exoplanet Roadmap Advisory Team (EP-RAT) is preparing their recommendations for future exoplanet missions [31] as guidance for the next Cosmic Vision call in late ACKNOWLEDGMENTS Section V of this paper is partly based on recommendations by Brandon Florow and Julien Lamamy at JPL, in collaboration with Richard Capps, Jennifer Dooley, Marie Levine, Douglas Lisman, Howard MacEwen, Joseph Pitman, and Stuart Shaklan. The author is grateful for their guidance. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Government sponsorship acknowledged. California Institute of Technology. REFERENCES [1] J. Schneider, Extrasolar Planets Encyclopaedia, Observatory of Paris, [2] M. A. C. Perryman, Extra-solar planets, Rep. on Prog. Phys., vol. 63, , [3] M. Fridlund, et al., The astrobiology primer, Astrobiology, vol. 10. pp. 1-4, [4] B. R. Oppenheimer and S. Hinkley, High contrast observations in optical and infrared astronomy, Ann. Rev. Astron. Astrophys., vol. 47, pp , [5] A. Wolszczan, and D. A. Frail, A planetary system around the millisecond pulsar PSR , Nature, vol. 355, , [6] M. Mayor and D. Queloz, A Jupiter-mass companion to a solar-type star, Nature, 378, , [7] W. J. Borucki, Characteristics of Kepler planetary candidates based on the first data set: The majority are found to be Neptune-size and smaller, Astrophys. J., in press, [8] M. R. Swain, G. Vasisht, G. Tinetti, et al., Molecular signatures in the near-infrared dayside spectrum of HD b, Astrophys. J., vol. 690, L114-L117, [9] M. R. Swain, G. Tinetti, G. Vasisht, et al., Water, methane, and carbon dioxide present in the dayside spectrum of the exoplanet HD b, Astrophys. J., vol. 704, , [10] M. Janson, C. Bergfors, M. Goto, W. Brandner, D. Lafrenière, Spatially resolved spectroscopy of the exoplanet HR 8799 c, Astrophys. J., vol. 710, L35-L38, [11] B. Lyot, Étude de la couronne solaire en dehors des eclipses, Zeitschrift fuer Astrophysik, vol. 5, 73-95, [12] D. J. Des Marais, M. O. Harwit, K. W. Jucks, et al., Remote sensing of planetary properties and biosignatures on extrasolar terrestrial planets, Astrobiology, vol. 2, , [13] M. J. Kuchner and W. A. Traub, A coronagraph with a bandwidth-limited mask for finding terrestrial planets, Astrophys. J., 570, , [14] O. Guyon, Phase-induced amplitude apodization of telescope pupils for extrasolar terrestrial planet imaging, Astron. Astrophys. 404, , [15] A. Fresnel, Mémoire sur la diffraction de la lumière, Mémoires de l Académies des Sciences (Paris), vol. 5, , (See page , Note I. Calcul de l intensité de la lumière au centre de l ombre d un écran et d une ouverture circulaires éclairés par un point radieux. ) [16] W. Cash, Detection of Earth-like planets around nearby stars using a petal-shaped occulter, Nature, vol. 442, pp , [17] R. J. Vanderbei, E. Cady, and N. J. Kasdin, Optimal occulter design for finding extrasolar planets, Astrophys. J., vol. 665, pp , [18] R. D. Peters, O. P. Lay, and P. R. Lawson, Adaptive nulling for the detection of Earth-like exoplanets, Pub. Astron. Soc. Pac., vol. 122, pp , [19] S. R. Martin and A. J. Booth, Strong starlight suppression sufficient to enable direct detection of exoplanets in the habitable zone, Astron. Astrophys., vol. 511, pp. L1, [20] S. R. Martin and A. J. Booth, Demonstration of exoplanet detection using an infrared telescope array, Astron. Astrophys., in press, [21] C. A. Beichman, N. J. Woolf, and C. A. Lindensmith, eds., The Terrestrial Planet Finder (TPF), Jet Propulsion Laboratory, Pub. 99-3, Pasadena, California, USA, 1999.

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