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Abstract
Detection and characterization of exoplanets pose formidable observational challenges, as planets are orders of magnitudes fainter, less massive and smaller than the stars they orbit. Multiple detection techniques exist, all relying on the high accuracy measurement of stellar light (flux, spectra, position or wavefront). Astrophotonic technologies for light creation, shaping and detection are essential to meet these challenges. While current detection and characterization techniques are suitable for giant (Jupiter-like) planets, potentially habitable Earth-sized rocky planets are considerably more challenging to observe. Recent advances in astrophotonics hold the key to characterization of these smaller planets, and to the remote identification of biomarkers indicative of life.
© 2017 Optical Society of America
1. Introduction
Since the first detection of an exoplanet around a Sun-like star [1], identification of exoplanets has experienced an exponential growth: thousands have been detected, and in recent years Earth-size planets have been identified. Notable findings include:
- Identification of a likely rocky planet in the habitable zone of the closest star to the solar system, Proxima Cen [4]
2. Detecting habitable exoplanets
2.1. Detection techniques
Exoplanet detection techniques (Fig. 1) can rely on the planet’s effect on its host starlight (indirect detection) or direct detection of light emitted by the planet. The small mass, size and brightness of the planet relative to its host star requires high accuracy measurements (microlensing is an exception to this rule). Earth-mass planets, while abundant, are generally significantly more challenging to detect than giant Jupiter-like planets, so most currently known exoplanets are gas giants. The detection bias toward gas giants is becoming less severe as detection techniques improve.
The nature of the habitable exoplanet detection challenge varies with the approach:
- In Transit photometry, a small drop in stellar apparent brightness is detected as the planet passes in front of its host star. The planet orbit needs to be viewed nearly edge-on for transits to occur (a 0.5% probability for an Sun-Earth system). The transit depth (relative drop in stellar brightness) is challengingly small: 8e-5 for an Earth-Sun system. The transit measurement for a Sun-Earth system at 10 pc is equivalent to watching a HD TV screen (1920×1080 pixel) from a distance of 100km, and detecting a 13×13 pixel area going dark for 12hr once a year.
- In microlensing, the planet acts as a gravitational lens amplifying a background star’s apparent brightness. While the brightness increase is large (≈ 2×), the background star (usually a bulge star in the Milky Way), exoplanet and observer need to be aligned to a few microarcsecond, so microlensing events are extremely rare. As viewed from Earth, a single bulge star experiences a lensing event by a foreground star approximately every 100,000 yr. Assuming each of the lensing stars hosts a single Earth-mass planet, corresponding microlensing events occur once every ≈ 50 million year per bulge star. The technique has become a powerful exoplanet detection tool thanks to automated telescope surveying a large number of stars.
- In radial velocity, the periodic motion induced by the planet on the star is detected thanks to the wavelength Doppler shift. The velocity amplitude is 10cm.s−1 for a Sun-Earth system, inducing wavelength shift of one part in 3e9.
- In astrometry, the same planet-induced stellar motion is measured as a periodic modulation of the star position on the sky. For a Sun-Earth system at 10 pc, the amplitude is 0.3 micro-arcsecond, equivalent to the thickness of a human hair at 30,000 km distance.
- In nulling interferometry, multiple beams, each containing both star and planet light, are coherently mixed to induce a destructive interference of starlight. Thanks to the angular separation between the star and the planet, the planet light is not entirely nulled, allowing detection. The fractional amount of residual starlight at the dark output of a 2-beam interferometer is sin(2πdp)2, where dp is the phase error in wave. The star-to-planet contrast, which varies from 1e7 (thermal IR) to 1e10 (visible light), directly translates into an optical pathlength matching requirement between ≈ 1/10, 000 and 1/1, 000, 000 of a wave.
- Coronagraphic Imaging uses carefully designed amplitude and/or phase masks in pupil and focal planes to cancel on-axis starlight while preserving light from the planet. The high star-to-planet contrast (typically 1e6 to 1e10) requires exquisite wavefront control and highly optimized coronagraph masks delivering near-total starlight suppression at small inner working angle.
Jupiter-mass gas giants, which are more representative of currently identified exoplanets, are considerably easier to detect. Jupiter’s mass is 318 times Earth’s mass, and the amplitude of both the astrometry and the radial velocity signals are proportional to planet mass. Transit signal amplitude and reflected light flux both scale as the square of planet diameter, a 120x factor in favor of Jupiter-sized planets. For thermal imaging, the gain is even more significant thanks to giant planets’ internal heat. Microlensing also favors more massive planets, as the Einstein radius (the lens diameter) scales linearly with planet mass, making small planet lensing event more rare.
2.2. Photonics technology needs
Table 1 lists, for each detection technique, the main photonics challenges, discussed in more details in this paper.
Table 1. Photonics technology challenges.
3. Lucky alignments: microlensing and transit
3.1. Detecting low probability events with large pixel arrays
Exoplanet transits and microlensing events are low-probability events, so a large number of stars need to be followed. Most ground-based transit surveys use large format visible light CCD with telephoto lenses or small wide field telescopes to follow thousands of stars in each image [7–12]. These blind surveys have instantaneous field of view ranging from a few degrees to a few tens of degrees on a side, and are pointed at low galactic latitude fields to maximize the number of stars in each field. The Kepler space mission [13] followed approximately 100,000 stars over several years and identified several thousand planets. The Kepler focal plane (Fig. 2) illustrates the challenges associated wide-field imaging to follow a large number of stars: 42 large CCD arrays were assembled to cover the telescope’s curved focal plane. A complementary strategy is to target a small number nearby stars [14, 15]; this targeted search mode does not require large focal plane arrays, but relies on automated telescope(s) to observe a number of targets at high temporal cadence.
Fig. 2 Left: the Kepler transit space mission focal plane array consists of 42 CCDs, each 2200×1024 pixel (image credit: NASA, Kepler Team). Right: Sample image, showing Comet Siding Spring (image credit: NASA Ames/W Stenzel; SETI Institute/D Caldwell).
Microlensing surveys rely on deep wide-field images of distant starfields [16–19] to identify stellar and exoplanet microlensing events. Ongoing stellar microlensing event can be followed up at high temporal cadence to search for shorter exoplanet microlensing events [20].
3.2. Future directions
Transit and microlensing surveys will continue to benefit from advances in detector technologies, combined with large wide-field telescopes. The prime example is the 2.4m WFIRST space telescope [21], which will include a large near-IR focal plane array [22] consisting of 18 4kx4k HgCdTe near-IR detectors covering 0.28 square degree. It will offer unprecedented sensitivity thanks to (1) atmosphere-free high accuracy photometry, (2) high sensitivity to bulge stars, which are heavily absorbed at visible wavelength due to interstellar dust, (3) wide field of view.
The space-based TESS mission [23], which will use red-sensitive deep-depletion CCD arrays for an all-sky transit survey, will be very sensitive to planets in the habitable zones of M-type stars, and will identify prime targets for follow-up by the 6.5m diameter space-based JWST.
Techniques for exoplanet characterization with the transit and microlensing approaches are continuously improving. Transit follow-up observations include transit timing variations to measure mutual gravitational perturbations between planets (now one of the prime methods to measure planet masses), and transit spectroscopy. Microlensing observations from multiple locations (for example a ground-based telescope and a space-based telescope observing the same microlensing event) as well as high angular resolution adaptive optics imaging of the lens can help constrain the microlensing geometry and mass parameters.
4. Gravity pull: astrometry and radial velocity
Astrometry and radial velocity are measuring the gravitational pull of the exoplanet on the host star: both objects orbit the system center of mass, so the star is moving along a scaled down copy of the planetary orbit. In radial velocity (RV), high accuracy spectroscopy is used to measure the velocity of the star along the line of sight. In astrometry, the star position is measured on the sky. Figure 3 shows the RV and astrometry signal amplitude for simulated Earth-mass planets in the habitable zones of nearby stars. Both signals are extremely small, and challenging to measure: RV amplitudes are in the ≈ 10 cm/s to ≈ 1 m/s range, while astrometry amplitudes are in the ≈ 0.1 μarcsec to ≈ 1 μarcsec range. The measurements are complementary in sensitivity: astrometry favors hotter stars while radial velocity favors cooler stars. Astrometry also strongly favors the nearest stars, as the signal amplitude is proportional to the inverse distance to the system.
Fig. 3 Astrometry and radial velocity signal amplitudes for simulated habitable planets orbiting nearby stars. A simulated Earth-like planet has been added to each star within 5pc. Circle color indicates stellar temperature. Circle diameter is inverse proportional to system distance.
4.1. Radial velocity
Current visible-light RV instrument operate at the ≈ 10cm/s precision thanks to extreme wavelength stability and calibration. The HARPS [24] and ESPRESSO [25] spectrographs are pushing to cm/s level precision using laser frequency comb reference. Precision RV instruments on 30-m class telescopes will offer greater sensitivity [26, 27].
As shown in Fig. 3, low-mass M-type red stars are particularly suitable for RV, as the signal from an Earth-mass habitable zone planet is in excess of 1 m/s. Near-IR RV instruments are well optimized for such stars, which emit most of their flux in the near-IR. Several near-IR RV instruments are currently under development or early operation: IRD on the 8.2m Subaru Telescope [28], Spirou on the 3.6m Canada-France-Hawaii Telescope [29], CARMENES on the 3.5m Calar Alto Telescope [30], and HZPF on the 10m Hobby Eberly Telescope [31].
A significant source of noise in RV measurements is due to interactions between the time-variable stellar PSF and the spectrograph. In fiber-fed spectrographs, this issue can be solved by scrambling the starlight transported in a multimode fiber linking the telescope to the spectrograph. A cleaner solution to this challenge is to use single mode fibers, as they filter all beam aberrations, at the cost of a low throughput for highly disturbed wavefronts. Photonic-based integrated optics spectrographs with single-mode fiber input can also be designed to be compact and stable [32]. On large telescopes, injecting light in a single mode fiber requires high performance upstream adaptive optics correction to maintain useable throughput [33]. The stability and compact spectrograph benefits offered by single mode fiber coupling can extend to multimode fiber coupling by using optics photonic lanterns [34, 35], which efficiently couple a multimode fiber into a set of single mode fibers.
Accurate and stable wavelength calibration, which used to be provided by a gas absorption cell or a gas emission lamp, can now be produced by a laser comb offering a large set of regularly (in wavelength) spaced emission lines. This new approach is particularly attractive in near-IR [36], where the conventional gas absorption / emission approach is more challenging.
4.2. Astrometry
Astrometric detection of exoplanets requires sub-μarcsecond position measurement. The Space Interferometry Mission [37] (SIM) mission concept proposed to achieve the required accuracy using an interferometer measuring angular separation between pairs of stars. High-precision laser metrology was used to calibrate instrumental terms. Recently, it has been recognized that the required astrometric precision could be reached with a single well-calibrated aperture as opposed to an inteferometer. Laser metrology can calibrate astrometric distortions induced by optical elements and detectors [38] by providing a “ruler” consisting of interference fringes. The diffractive pupil [39,40] concept implements a starlight-based calibration “ruler” imprinted by a diffraction pattern etched on the telescope optics. These single-aperture approaches are currently under development in preparation for future space-based mission(s).
The space-based GAIA astrometric mission currently in operation is performing astrometric measurements of more than a billion stars. While its astrometric precision does not allow detection of habitable planets, it is expected to detect thousands of massive exoplanets [41, 42].
Astrometry can also be performed from the ground using large interferometric baselines. Precise measurement of the angular separation between the two components of a close binary (differential astrometry) can be done at the 10μas level required to identify gas giant planets, as demonstrated on the Palomar Testbed Interferometer (PTI) [43, 44]. Differential astrometry was also deployed on the Very Large Telescope interferometer with the PRIMA instrument [45]. The most significant current effort in ground-based astrometry, the GRAVITY instrument on the Very Large Telescope interferometer, is primarily aimed at observing the immediate surrounding of the supermassive black hole at the Milky Way’s center, offering 10uas-level astrometry [46].
5. Direct detection: isolating and analyzing the planet’s light
With direct detection, the exoplanet light can be physically separated from starlight for spectroscopic analysis. While fundamentally very powerful (high signal-to-noise spectra could be acquired without suffering from stellar photon noise), the technique is also probably the most challenging way to observe exoplanets. The challenge is that potentially habitable exoplanets are both significantly fainter than their host stars, and also located very close (angularly) to the much brighter host stars. Figure 4 shows the reflected light contrast values for simulated habitable planets within 5 pc: contrast values range from ≈ 1e-6 for M-type stars to ≈ 1e-11 for hot stars.
Fig. 4 Reflected light contrast of simulated habitable planets orbiting nearby stars. A simulated Earth-like planet has been added to each star within 5pc. Circle color indicates stellar temperature. Circle diameter indicates habitable zone angular size.
Direct detection can measure starlight reflected by the planet or thermal emission from the planet. While Fig. 4 shows that reflected light detection of habitable planets around sun-like stars requires ≈ 1e-10 contrast in visible light, the contrast is much milder in thermal infrared. The thermal emission peak for temperate habitable planets is around 10μm wavelength, with a corresponding contrast between 1e-6 and 1e-7 around a sun-like star. Such detections will be possible around nearby stars with future large telescopes [47–50], and is within reach of current large telescopes for temperate habitable zone planets in the Alpha Centauri system [51].
5.1. Coronagraphy
At the 1e-6 to 1e-11 contrast level, a conventional imaging system is not suitable, as the faint exoplanet image would be lost in the much brighter image plane diffraction features (Airy rings) of the central star. First, photon noise from the starlight would exceed the exoplanet signal. Second, coherent mixing between the star’s diffraction features and speckles due to small wavefront errors would swamp the exoplanet signal.
A coronagraph, or optical device(s) optimized to remove diffraction from the central star, is required for the observation. A large range of coronagraph options have been developed and associated fundamental limits are relatively well understood [55]. Practical implementation, especially at the ultra-high contrast level (1e-9 to 1e-11) required for space-based imaging of habitable planets around Sun-like stars, remains challenging. Masks offering fine control of transmitted (or reflected) amplitude and phase are essential to coronagraphs. Examples are shown in Fig. 5.
Fig. 5 Top: PIAACMC focal plane mask [52]. Vector Vortex Coronagraph (VVC) focal plane mask for infrared use: the phase vortex is induced by a sub-wavelength grating [53]. HLC mask for WFIRST coronagraph mission [54].
5.2. Wavefront control
Wavefront control is usually the most challenging part of a high contrast imaging system, especially on ground-based telescopes. The performance of high contrast optimized wavefront control is mostly driven by the input disturbance temporal speed and amplitude, as well as the wavefront sensor sensitivity [56]. On ground-based systems, chromaticity effects can also be significant, as sensing is typically performed at shorter wavelength than scientific image acquisition.
High-performance adaptive optics (AO) systems on current large telescope can observe giant planets in the habitable zones of the nearest stars [57]. Significant recent advances in wavefront control include higher sensitivity optical sensors, and focal-plane based speckle control loops that measure starlight exactly where it needs to be removed: in the final image. Speckle control has been demonstrated on-sky [58] with some success, but high performance operation relies on fast sensitive near-IR / visible detector technology. The photon-counting MKIDS detectors [59] and the electron-multiplied near-IR SAPHIRA [60] cameras will enable speckle control to operate at the ≈ 1 kHz frequency to reach deep imaging contrasts.
5.3. Photonics-based imaging and nulling
Using single mode wave guides, light from a telescope aperture can be filtered and coherently combined in ways that are not possible or feasible with conventional optical elements such as lenses and mirrors [61]. Photonics devices can be designed to process light for optimal imaging or nulling properties. In high angular resolution imaging, coherent waveguides, each coupled to a part of the telescope pupil, can be coherently combined to break the frequency / baseline ambiguities encountered in conventional imaging [62], allowing for high fidelity imaging at small angular resolution [63, 64]. The same approach can also be optimized to produce deep starlight suppression to aid exoplanet imaging, by choosing phase delays and coupling coefficients creating destructive interference output(s).
6. Conclusions
Multiple approaches have been developed to observe habitable planets, each having their own challenges and astrophotonics needs. Much of the recent and future progresses in exoplanet detection are attributable to advancements in detector performance, which has benefited all approaches. Other key technologies are often more specific to a single exoplanet detection technique.
The multiple observation techniques are scientifically complementary, and several approaches are required for characterization of exoplanets. For example, direct imaging is key to measure the exoplanet’s atmospheric composition (spectroscopy), but does not measure exoplanet radius (measured by transit) or mass (measured by radial velocity or astrometry).
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