Performance study of interferometric small-sats to detect exoplanets:Updated exoplanet yield and application to nearby exoplanets

Nulling interferometry is considered as one of the most promising solutions to spectrally characterize rocky exoplanets in the habitable zone of nearby stars. It provides both high angular resolution and starlight mitigation. It requires however several technologies that need to be demonstrated before a large interferometry space-based mission flies. A small-sat mission is a good technological precursor. Based on a Bracewell architecture, this unique satellite can demonstrate some key components (null capability, fiber injection, achromatic phase shifter). Scientific capabilities of such a mission are presented. An exoplanet detection yield is derived, and we show that the detection of exoplanets around nearby stars is feasible

The expected performance of nulling at the VLTI down to 5 mas

While VLTI offers the recombination of four 8-m telescopes with baselines of more than 100m, it has never hosted a dedicated high-contrast nulling beam-combiner. The SCIFY project aims to design, build and commission Hi- 5, the first nulling beam-combiner of the VLTI, optimized for the detection and characterization of young giant exoplanets near the snow line, with spectroscopy up to R=2000 in the L’ band. It will make use of advanced four-beam nulling combination schemes, like double-Bracewell and kernel-nulling implemented in a single-mode photonic device to produce differential nulled outputs with self-calibrating properties. In the wavelength range of interest, both instrumental errors and background noise are significant. In order to estimate the practical performance of these different configurations in the presence of instrumental errors and further optimize the instrumental design, we have developed SCIFYsim. SCIFYsim is an end-to-end simulator geared towards single- mode beam combiners with of a wide variety of instrumental errors, like optical path difference residuals from fringe tracking, wavefront error at the injection, longitudinal dispersion, chromaticity of the combiner chip, and more. In order to evaluate the performance of the combined spectral channels, we use statistical tests based on likelihood ratios, and account for the covariance in the data. In this paper, we present the expected performance of Hi-5 with a few examples and discuss the main technical limitations to reach the contrast required to image young giant exoplanets.SCIF

Asgard/NOTT: L-band nulling interferometry at the VLTI I. Simulating the expected high-contrast performance

Context: NOTT (formerly Hi-5) is a new high-contrast L' band (3.5-4.0 \textmu m) beam combiner for the VLTI with the ambitious goal to be sensitive to young giant exoplanets down to 5 mas separation around nearby stars. The performance of nulling interferometers in these wavelengths is affected both by fundamental noise from the background and by the contributions of instrumental noises. This motivates the development of end-to-end simulations to optimize these instruments. Aims: To enable the performance evaluation and inform the design of such instruments on the current and future infrastructures, taking into account the different sources of noise, and their correlation. Methods: SCIFYsim is an end-to-end simulator for single mode filtered beam combiners, with an emphasis on nulling interferometers. It is used to compute a covariance matrix of the errors. Statistical detection tests based on likelihood ratios are then used to compute compound detection limits for the instrument. Results: With the current assumptions on the performance of the wavefront correction systems, the errors are dominated by correlated instrumental errors down to stars of magnitude 6-7 in the L band, beyond which thermal background from the telescopes and relay system becomes dominant. Conclusions: SCIFYsim is suited to anticipate some of the challenges of design, tuning, operation and signal processing for integrated optics beam combiners. The detection limits found for this early version of NOTT simulation with the unit telescopes are compatible with detections at contrasts up to $10^5$ in the L band at separations of 5 to 80 mas around bright stars

Technical requirements and optical design of the Hi-5 spectrometer

Hi-5 is a proposed L’ band high-contrast nulling interferometric instrument for the visitor focus of the Very Large Telescope Interferometer (VLTI). As a part of the ERC consolidator project called SCIFY (Self-Calibrated Interferometry For exoplanet spectroscopY), the instrument aims to achieve sufficient dynamic range and angular resolution to directly image and characterize the snow line of young extra-solar planetary systems. The spectrometer is based on a dispersive grism and is located downstream of an integrated optics beam- combiner. To reach the contrast and sensitivity specifications, the outputs of the I/O chip must be sufficiently separated and properly sampled on the Hawaii-2RG detector. This has many implications for the photonic chip and spectrometer design. We present these technical requirements, trade-off studies, and phase-A of the optical design of the Hi-5 spectrometer in this paper. For both science and contract-driven reasons, the instrument design currently features three different spectroscopic modes (R=20, 400, and 2000). Designs and efficiency estimates for the grisms are also presented as well as the strategy to separate the two polarization states.SCIF

Large Interferometer For Exoplanets (LIFE): II. Signal simulation, signal extraction, and fundamental exoplanet parameters from single-epoch observations

peer reviewedContext. The Large Interferometer For Exoplanets (LIFE) initiative is developing the science and a technology road map for an ambitious space mission featuring a space-based mid-infrared (MIR) nulling interferometer in order to detect the thermal emission of hundreds of exoplanets and characterize their atmospheres. Aims. In order to quantify the science potential of such a mission, in particular in the context of technical trade-offs, an instrument simulator is required. In addition, signal extraction algorithms are needed to verify that exoplanet properties (e.g., angular separation and spectral flux) contained in simulated exoplanet data sets can be accurately retrieved. Methods. We present LIFEsim, a software tool developed for simulating observations of exoplanetary systems with an MIR space-based nulling interferometer. It includes astrophysical noise sources (i.e., stellar leakage and thermal emission from local zodiacal and exozodiacal dust) and offers the flexibility to include instrumental noise terms in the future. Here, we provide some first quantitative limits on instrumental effects that would allow the measurements to remain in the fundamental noise limited regime. We demonstrate updated signal extraction approaches to validating signal-to-noise ratio (S/N) estimates from the simulator. Monte Carlo simulations are used to generate a mock survey of nearby terrestrial exoplanets and determine to which accuracy fundamental planet properties can be retrieved. Results. LIFEsim provides an accessible way to predict the expected S/N of future observations as a function of various key instrument and target parameters. The S/Ns of the extracted spectra are photon noise dominated, as expected from our current simulations. Signals from multi-planet systems can be reliably extracted. From single-epoch observations in our mock survey of small (R < 1.5 REarth) planets orbiting within the habitable zones of their stars, we find that typical uncertainties in the estimated effective temperature of the exoplanets are ≲10%, for the exoplanet radius ≲20%, and for the separation from the host star ≲2%. Signal-to-noise-ratio values obtained in the signal extraction process deviate by less than 10% from purely photon-counting statistics-based S/Ns. Conclusions. LIFEsim has been sufficiently well validated so that it can be shared with a broader community interested in quantifying various exoplanet science cases that a future space-based MIR nulling interferometer could address. Reliable signal extraction algorithms exist, and our results underline the power of the MIR wavelength range for deriving fundamental exoplanet properties from single-epoch observations.Large Interferometer For Exoplanets (LIFE

De l'analyse de la mission à l'ingénierie des systèmes du nanosatellite OUFTI-Next

OUFTI-Next is the new CubeSat project of the University of Liège. This mission was imagined after the success of OUFTI-1. The goal of this nanosatellite is to detect hydric stress of agricultural fields around the world. It is equipped with a Mid-Wavelength InfraRed (MWIR) detector. It will be a world premiere with such a small satellite (3U or 30 cm × 10 cm × 10 cm). From the data, the temperature of the crop will be extracted and the irrigation status assessed. This satellite is a technology demonstrator for an ambitious project. The final goal is indeed to create a smart irrigation program with a daily revisit over a location. It will provide tools for farmers to improve the irrigation, increase the yield of their fields and spare less drinkable water. With only one satellite, it is unfortunately impossible. OUFTI-Next’s mission is no less important because it will demonstrate that the integration of a MWIR detector is feasible. This master thesis is the continuity of a feasibility study done last year (2016-2017). From the requirements, primordial aspects of the satellite are developed. Orbits, communication, power budget, attitude strategy, ... are typical topics introduced in this work. It offers an overview of the satellite and a link between different subjects addressed in other master theses (the detector’s cooling system, the optical design and the thermal aspect). At the end, some configurations, thought as simple as possible, are introduced and discussed. All subsystems are reviewed with the will to find an optimal configuration. Of course, concessions are done and assumptions made. At this stage of the development, it is natural that some information is missing.OUFTI-NEX

Etude de faisabilité d'un small sat pour l'étude d'exoplanètes par interférométrie

One of the main goals of exoplanet science is to characterize the atmosphere of rocky exoplanets in the habitable zone of nearby stars. A space-based nulling interferometry, observing in the mid-infrared (3-20 μm), is considered to be one of the most promising solutions to tackle this observing challenge. The LIFE project, a free-flying space-based mid-infrared nulling interferometer, would have this capability. However, several key technologies need to be demonstrated before launching such an ambitious mission. A small space-based mission can be considered as a useful prerequisite. In this paper, we consider three small satellite architectures, two CubeSats, and a PROBA-like satellite. Based on a Bracewell architecture and without free-flying, these monolithic satellites can demonstrate some key components like the null capability and its stability on real targets. The achromatic phase shifter needs also to be demonstrated in space. Based on the scientific capabilities and exoplanet detection yield of these architectures, optical constraints are derived (pointing stability, and optical path difference correction). Orbital simulations, exploring a range of classical orbits for such a satellite, are also discussed

Etude préliminaire d'un CubeSat interférométrique dédié à l'exoplanétologie

Every week, new exoplanets are discovered mostly by the transit method (77.1% of all discoveries according to NASA). Even if this method is efficient at detecting planets, it is limited to a small fraction of the whole expected exoplanets population due to the low probability of planetary transit. Therefore, a direct method is needed to detect and characterize exoplanets around the nearest stars. In this case, the planet and the star are angularly separated and photons are distinguished. It leads to the detection of the planet. Moreover, it allows the possible characterization of the planet surface or its atmosphere. One way to detect them through direct method is to use interferometry. With at least two sub-pupils (Bracewell interferometer), coherent light from the target is recombined to form interference patterns. The angular resolution depends on the baseline (distance between the two sub-pupils) and not on the diameter of each sub-pupil. Instead of using a single large telescope (around 60 cm diameter), which does not fit into a CubeSat, one can use two small and well separated apertures (around 10 cm each) to synthesize this large telescope. Therefore, it increases drastically the resolution power of CubeSats. In order to detect an exoplanet and get the direct light coming from it, the light from the star must be mitigated. It is called nulling interferometry. Thanks to a pi phase shift induced in one arm of the interferometer, destructive interferences are produced on the line-of-sight in order to suppress the light of the star. The exoplanet, which is on constructive interferences (white fringes), is unveiled. The Centre Spatial de Liège of the University is developing a space-based interferometer with a CubeSat. Goals are twofold: observe the nearest stars and demonstrate this technology in space, which will be a premiere. It is the first step towards a future large interferometry space-based mission which has the ambition to spectrally characterize Earth-like planets. The CubeSat will demonstrate light injection to optical fibers, recombination of the two beams, control of the delay-lines and detection. CubeSats offer low-cost demonstrator capabilities with a fixed baseline and with no free-flying concept. Aside the technical challenges, the second part of our researches is focused on the detection possibilities with this type of nanosatellite. We estimate by numerical simulations the possible science return for such an instrument.Study of a CubeSat nulling interferometer to detect exoplanet

Exoplanet detection yield of a space-based Bracewell interferometer from small to medium satellites

Space-based nulling interferometry is one of the most promising solutions to spectrally characterize the atmosphere of rocky exoplanets in the mid-infrared (3 to 20  μm). It provides both high angular resolution and starlight mitigation. This observing capability depends on several technologies. A CubeSat (up to 20 kg) or a medium satellite (up to a few hundreds of kg), using a Bracewell architecture on a single spacecraft could be an adequate technological precursor to a larger, flagship mission. Beyond technical challenges, the scientific return of such a small-scale mission needs to be assessed. We explore the exoplanet science cases for various missions (several satellite configurations and sizes). Based on physical parameters (diameter and wavelength) and thanks to a state-of-the-art planet population synthesis tool, the performance and the possible exoplanet detection yield of these configurations are presented. Without considering platform stability constraints, a CubeSat (baseline of b  ≃  1  m and pupils diameter of D  ≃  0.1  m) could detect ≃7 Jovian exoplanets, a small satellite (b  ≃  5  m  /  D  ≃  0.25  m) ≃120 exoplanets, whereas a medium satellite (b  ≃  12.5  m  /  D  ≃  0.5  m) could detect ∼250 exoplanets including 51 rocky planets within 20 pc. To complete our study, an analysis of the platform stability constraints (tip/tilt and optical path difference) is performed. Exoplanet studies impose very stringent requirements on both tip/tilt and OPD control

Development of the 4-telescope photonic nuller of Hi-5 for the characterization of exoplanets in the mid-IR

A small-footprint 4-telescope photonic beam combiner is at the heart of the Hi-5 instrument, the high-contrast VLTI visitor instrument focusing on the detection and characterization of young exoplanets in the mid-infrared L’ band. Hi-5 implements the technique of nulling interferometry to efficiently suppress the strong stellar radiation of the central source and enhance the detection of the nearby faint planetary signal. Based on the “Double Bracewell” architecture, the photonic nulling beam combiner is designed around three cascaded achromatic directional couplers with 50/50 coupling ratios. This allows the nulled signals of the first two couplers to be cross-combined with a third central combiner, which produces two conjugated asymmetric transmission maps projected onto the sky. Each individual telescope beam passes first through a side-step to suppress uncoupled stray-light. The corresponding flux is then sampled by an asymmetric Y-junction to provide a simultaneous photometric channel for the estimation of the self-calibrated nulls. We report here on the prototyping phase of the Hi-5 4-telescope photonic beam combiner that is manufactured by ultrafast laser inscription in a Gallium- Lanthanum-Sulphide (GLS) glass substrate, which exhibits high transparency in the L’ band of interest. Using our 2-beam spectro-interferometric lab bench, we measure the throughput of the beam combiners, the chromatic and broadband coupling ratios in the 3.6-3.9 μm range for the couplers and the Y-junctions, as well as the broadband interferometric properties of these 4-telescope mid-infrared photonic beam combiners.SCIF