# Very High Precision Astrometry for Exoplanets and Dark Matter with the Habitable Worlds Observatory

Fabien Malbet, J. Amiaux, F. Ardellier-Desages, E. Doumayrou, P.-A. Frugier, R. Goullioud, T. Greene, L. Labadie, P.-O. Lagage, M. Lizzana, et al.

## ► To cite this version:

Fabien Malbet, J. Amiaux, F. Ardellier-Desages, E. Doumayrou, P.-A. Frugier, et al. Very High Precision Astrometry for Exoplanets and Dark Matter with the Habitable Worlds Observatory. Towards the Habitable Worlds Observatory: Visionary Science and Transformational Technology, Lee, Janice, Jul 2025, Washington DC, United States. [hal-05304671](https://hal.science/hal-05304671v1)

HAL Id: hal-05304671

https://hal.science/hal-05304671v1

Submitted on 20 Oct 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Very High Precision Astrometry for Exoplanets and Dark Matter with the Habitable Worlds Observatory

Fabien Malbet,<fnref n="1" /> Jérôme Amiaux,<fnref n="2" /> Florence Ardellier-Desages,<fnref n="2" /> Éric Doumayrou,<fnref n="2" />

Pierre-Antoine Frugier,<fnref n="2" /> Renaud Goullioud,<fnref n="3" /> Thomas Greene,<fnref n="4" /> Lucas Labadie,<fnref n="5" /> Pierre-Olivier

Lagage,<fnref n="2" /> Manon Lizzana,<fnref n="1" />,<fnref n="6" />,<fnref n="7" /> Alain Léger,<fnref n="8" /> Thierry Lépine,<fnref n="9" /> Gary Mamon,<fnref n="10" /> Jérôme

Martignac,<fnref n="2" /> Fabrice Pancher,<fnref n="1" /> Thibault Pichon,<fnref n="2" /> Aki Roberge,<fnref n="11" /> Samuel Ronayette,<fnref n="2" /> Hugo

Rousset,<fnref n="1" /> Sébastien Soler,<fnref n="1" /> Alessandro Sozzetti,<fnref n="12" /> Thierry Tourette<fnref n="2" />

Astrometry, one of the oldest branches of astronomy, has been revolutionized by missions like Hipparcos and especially Gaia, which mapped billions of stars with extraordinary precision. However, challenges such as detecting Earth-like exoplanets in nearby habitable zones and probing the influence of dark matter in galactic environments require sub-microarcsecond accuracy.

With a 6–8 meter large-aperture telescope operating across at visible wavelengths, the Habitable Worlds Observatory by NASA can combine astrometry and direct imaging to detect rocky exoplanets within 10 parsecs and study their atmospheres. We consider here the scientific requirements and present a concept for a dedicated astrometric instrument on HWO. It is capable to produce diffraction-limited images of large fields, achieving a point-spread function (PSF) precision of 20 milliarcseconds. Equipped with a detector calibration system, HWO can perform high precision astrometry, and, detect and measure the orbit of Earth-mass planets in the habitable zone of Nearby Solar-type stars. HWO can dramatically improve current constraints on the self-interaction cross section of heavy dark matter particles (WIMPs) and on the masses of ultra-high dark matter particles, through the study of stellar motions in galactic environments.

The visible channel of the instrument features a large CMOS-based focal plane with stitched pixel arrays, enabling a large field of view. The “Detector Calibration Unit” system uses interferometric laser fringes to calibrate pixel positions. Using differential astrometry and pointed observations with a stable telescope design enables extended integration times, enhancing sensitivity to sub-microarcsecond precision for detecting exoplanets and studying dark matter through stellar motion.

## 1. Introduction

Astrometry is one of the most fundamental techniques in astronomy, tracing its heritage back to the earliest stellar catalogs. Over the past decades, space missions such as Hipparcos and Gaia have delivered a revolution in our knowledge of stellar positions, parallaxes, and proper motions. Gaia in particular has measured over two billion stars, opening new windows on stellar populations, galactic dynamics, and planetary systems. Yet, even Gaia's exquisite precision (down to tens of microarcseconds) is insufficient

for detecting Earth-mass planets in the habitable zones of nearby stars or for probing subtle dark matter signatures in dwarf galaxies and galactic halos.

The Habitable Worlds Observatory (hereafter HWO), despite initially not being designed to address these challenges, has the potential to achieve measurements with accuracies approaching 0.3 µas by incorporating a dedicated high-precision astrometric instrument. This instrument will enable the detection of true Earth analogs at 10 pc and the dynamical mapping of dark matter distributions on both

<footnote n="1">¹Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, France</footnote>

<footnote n="2">²Univ. Paris-Saclay, CEA, Saclay, France</footnote>

<footnote n="3">³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA</footnote>

<footnote n="4">⁴IPAC, California Institute of Technology, Pasadena, CA, USA</footnote>

<footnote n="5">⁵Univ. of Cologne, Cologne, Germany</footnote>

<footnote n="6">⁶Univ. Paris-Saclay, CNRS, Institut d'astrophysique spatiale, Orsay, France</footnote>

<footnote n="7">⁷Centre National d'Études Spatiales, Toulouse, France</footnote>

<footnote n="8">⁸Pyxalis, Moirans, France</footnote>

<footnote n="9">⁹Institut d'Optique & Hubert Curien Lab, Univ. de Lyon, Saint-Etienne, France</footnote>

<footnote n="10">¹⁰Sorbonne Université, CNRS, Institut d'Astrophysique de Paris, Paris, France</footnote>

<footnote n="11">¹¹NASA Goddard Space Flight Center, Greenbelt, MD, USA</footnote>

<footnote n="12">¹²Obs. Torino/INAF, Pino Torinese, Italy</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1.— Exoplanets known around nearby solar-type stars. Left panel : number of nearby stars ($D \le 20$ pc) with (in red) or without (in blue) known exoplanets extracted from the exoplanets encyclopedia at the date of 25 July 2025. Only 12% of nearby stars have known exoplanets so far and mostly gaseous ones! Right panel : number of exoplanets found around nearby stars of different spectral type around nearby stars (data from The Extrasolar Planets Encyclopedia).</figcaption></figure>

small and large scales. This article develops the scientific
rationale, instrumental principles, anticipated performance,
and synergies with alternative methodologies.

## 2. Science objectives

The U.S. decadal survey on astronomy and astrophysics for the 2020s *Pathways to Discovery in Astronomy and Astrophysics for the 2020s* (National Academies of Sciences, Engineering, and Medicine 2023) identified the detection and characterization of Earth-like exoplanets and the study of dark matter as top priorities for the coming decades. Similarly, the European Space Agency's long-term science plan *Voyage 2050* (Tacconi et al. 2021) emphasized the search for habitable worlds and the understanding of the nature of dark matter as central science themes.

In particular, Section 3.1.7 of the ESA Voyage 2050 report stresses that the next steps in space astrometry should either aim at improving relative astrometric accuracy by an order of magnitude or extend global astrometry to the near-infrared domain. The report highlights that high precision relative astrometry at the sub-microarcsecond level (down to 0.2 µas) would enable two transformative breakthroughs: probing the distribution of dark matter in galactic environments, and detecting and fully characterizing the orbital architectures of planetary systems hosting habitable planets around the nearest stars. Achieving such capabilities requires however substantial advances in detector technology, metrology, and spacecraft stability.

## 2.1. Exoplanet census of nearby FGK stars

A central motivation for HWO astrometry is the discovery and mass determination of rocky planets in the habitable zones of the nearest Sun-like stars. Only about 12% of nearby FGK stars currently have known exoplanets, mostly giant planets (Fig. 1). The low-mass, temperate planets remain elusive, yet they are the most compelling targets for biosignature searches. Astrometry provides complete orbital solutions and absolute masses, overcoming limitations of radial velocity (RV) techniques (Meunier et al. 2020; Meunier & Lagrange 2022). It is especially powerful for planets on longer orbits, inaccessible to transit methods.

To meet the Decadal Survey goal of obtaining ~ 25 atmospheric spectra of potentially habitable exoplanets, the HWO strategy combines a large space telescope equipped with a high-contrast coronagraph, an optimized target list of about 100–160 nearby FGK stars, and yield models based on occurrence rates (~ 0.24 Earth-like planets per star in the habitable zone). By directly imaging planets around these stars and allocating sufficient integration time with repeated visits, the mission can reach the signal-to-noise needed for spectral characterization. Achieving this requires maturing technologies for starlight suppression, wavefront stability, and ultra-stable detectors, while precursor observations refine target selection and reduce risk. Together, these elements are designed to ensure a robust sample of ~ 25 spectra of rocky, temperate exoplanets suitable for habitability studies. Such a target list would significantly reduce the time spent observing for the spectroscopic study.
<figure><img src="image_3.png" /><figcaption>Fig. 2.— The low-mass planetary components that is currently missing. Left panel: Kernel density estimates of four synthetic planetary populations (from Burn & Mordasini 2024). Right panel: mass versus semi-major axis parameter space for known exoplanets in function of the detection technique. The nearby exoplanets ($D \le 20$ pc are displayed in clear above the full sample of known exoplanets at the date of 25 July 2025 (data from The Extrasolar Planets Encyclopaedia).</figcaption></figure>

An astrometric survey targeting 100–200 of the nearest FGK stars can yield dozens of Earth-mass planets. These detections will not only establish occurrence rates but can also deliver the prioritized target list for HWO's coronagraphic and spectroscopic follow-up. Stellar jitter, often a limiting factor for RV, is less problematic for astrometry at these scales, typically contributing < 0.1 µas for Sun-like stars at 10 pc. This places the exoplanet detection threshold securely within reach for Earth analogues.

The combination of astrometry, RV, and direct imaging is transformative. RVs supply high cadence but ambiguous $M \sin i$ estimates, resolved by astrometry. Direct imaging reveals atmospheric spectra but requires orbital ephemerides for efficient scheduling, provided by astrometry. Together, the three approaches enable the most complete and accurate characterization of exoplanetary systems.

## 2.2. Probing dark matter with proper motions

Dark matter (DM) dominates the matter content of the Universe, but its nature remains unknown, in particular the mass of the DM particle and the possible cross-section for DM annihilation through self interaction. DM halos play a central role in shaping the dynamics of galaxies and clusters. Their internal density profiles and subhalo populations remain key open questions in cosmology. Dwarf spheroidal galaxies, being strongly dark-matter dominated, provide fundamental laboratories to test whether halos are cored or cuspy, thus directly probing predictions of the cold

dark matter (CDM) paradigm and its alternatives, such as self-interacting or fuzzy dark matter. CDM simulations also predict the existence of numerous dark subhalos devoid of stars, which may nevertheless be revealed through their gravitational perturbations on stellar streams and the kinematics of stars in the Milky Way disk.

High-precision astrometry with HWO, reaching sub-km/s proper-motion accuracy at the distances of dwarf galaxies, offers the means to distinguish cusp from DM core profiles, detect the dynamical signatures of otherwise invisible subhalos, and constrain the overall shape of the Milky Way's halo using tracers such as hypervelocity stars. Together, these three avenues—inner density slopes, sub-halo detection, and halo shape—illustrate the transformative potential of microarcsecond astrometry with HWO for advancing our understanding of dark matter. HWO would provide a major improvement on the constraints on the fuzzy DM particle mass if DM is ultra-light, as well as on the cross section of self-interaction if DM is heavy.

## 3. Measurement principle and instrument concept

The astrometric capability envisioned for the Habitable Worlds Observatory relies on the principle of high-precision differential astrometry (left part of Fig. 3). In practice, the science target is placed at the center of the field of view, and its angular position is measured relative to an ensemble of reference stars distributed across the detector. By repeating these measurements at multiple epochs over the
<figure><img src="image_4.png" /><figcaption>Fig. 3.— Principle of relative differential astrometry. Left panel: this panel presents an image of the 0.5-degree field of view of the exoplanet stellar host *iota Persei* (from DSS). The arrows show the distances between the target central bright star and some reference stars. Right panel: instrument principle which includes a telescope which is diffraction-limited allowing apparent angle measurements, a detector calibration unit that projects interferometric fringes created by the interferences between laser light onto the focal detector.</figcaption></figure>

mission lifetime, it is possible to disentangle the different contributions to the observed motion (parallax, proper motion, and orbital reflex motion induced by exoplanets) with microarcsecond precision. The differential approach minimizes common-mode errors introduced by the telescope optics or spacecraft instabilities and is therefore particularly well-suited to HWO space-based observations.

To enable such accuracy, the instrument concept (right part of Fig. 3) centers on a large visible-light CMOS-based focal plane composed of stitched pixel arrays (Fig. 4), providing both high stability and a wide field of view. The point-spread function (PSF) is expected to be sampled at a the diffraction-limited resolution of roughly 20 milliarcseconds, ensuring Nyquist sampling in the blue optical regime where photon fluxes from Sun-like stars are maximized. Achieving long-term stability requires a thermal and mechanical design of the telescope optimized to suppress slow drifts and minimize variations in the optical train that could bias the astrometric solution.

A important element of the concept is the Detector Calibration Unit (DCU). This subsystem projects interferometric laser fringes across the focal plane, enabling sub-pixel calibration of detector geometry and intra-pixel response. By continuously monitoring pixel positions at the nanometer scale, the DCU ensures that systematic uncertainties associated with the detector remain below the mission noise floor. Combined with stable spacecraft pointing and advanced wavefront control, this calibration strategy allows the focal plane to operate at the level of a few tenths of a microarcsecond over years of integration.

Finally, the observing strategy itself contributes to the robustness of the measurements. By alternating the orientation

of the telescope on the sky, sampling different reference star ensembles, and revisiting each target multiple times, systematic errors can be averaged down. The combination of instrumental design, calibration techniques, and optimized observing cadence is what makes it possible to achieve the sub-microarcsecond performance required to detect Earth-mass planets at 10 parsecs and to measure the subtle proper motions associated with dark matter studies.

Further information regarding the astrometry instrument that has been proposed for HWO can be found in this volume: Amiaux et al. (2025) present a system analysis for an instrument on-board HWO dedicated to high-precision high-accuracy differential astrometry with the main science driver to detect Earth-like planets signal around the closest Sun-like stars; and Lizzana et al. (2025) present a characterization of the 46-megapixel Gigapyx detector from Pyxalis, which is an appropriate detector for an astrometric instrument, explain how to use a testbed to measure pixel positions, and, present a method for adjusting the telescope optical distortion which was tested using an optical bench developed at IPAG in France.

## 4. Expected performance and noise budget

To detect an Earth-mass planet at 10 pc requires sensitivity at the 0.3 µas level. For other planets the astrometric signal scales as :

$$ \alpha = 0.33 \left( \frac{M_P}{1 M_\oplus} \right) \left( \frac{a_P}{1 AU} \right) \left( \frac{M_*}{1 M_\odot} \right)^{-1} \left( \frac{D}{10 \text{ pc}} \right)^{-1} \mu\text{as} $$

where $M_P$ is the planet mass, $a_P$ is the planet semi-major axis, $M_*$ is the star mass and $D$ is the distance from the observer.
<figure><img src="image_5.png" /><figcaption>Fig. 4.— A possible Focal Plane dedicated instrument for HWO. Upper left panel: HWO telescope configuration for Exploratory Architecture Concepts) EAC1. Upper right panel: location of the focal plane and the detector Calibration unit in the EAC1 case. Bottom left panel: location of the focal plane and the detector Calibration unit in the EAC3 case. Bottom right panel: possible focal plane concept with 4 GigaPyx 220M detectors and a central GigaPyx 14M detector.</figcaption></figure>

Achieving this unprecedented precision of 0.3 µas required for the Habitable Worlds Observatory (HWO) demands a rigorous understanding and control of all potential sources of error. The astrometric error budget for HWO is a complex interplay of instrumental, astrophysical, and environmental factors, each contributing to the overall uncertainty in the measurements. At the heart of the instrumental errors are optical aberrations, which arise from imperfections in the telescope's optics and misalignments in the optical path.

To achieve the detection of Earth-mass exoplanets with a signal-to-noise ratio (SNR) of 6, the Habitable Worlds Observatory (HWO) must attain the residual error of 0.3 µas after averaging controlled at the level of 0.05 µas. This stringent requirement is met by accumulating approximately 100 individual measurements, each with an uncertainty of 0.5 µas, over the mission's 3- to 4-year duration.

However, this approach assumes that random errors dominate and these errors are uncorrelated between measurements. Systematic errors—such as long-term instrumental drifts, calibration biases, or residual optical distortions—must be independently controlled to avoid degrading the final precision. HWO addresses these challenges through interferometric calibrations and repeated observations of stable reference star fields to minimize systematic biases.

Table 1 summarizes the expected contributions of each error source for HWO and for Theia, which is a 0.8-m telescope dedicated to astrometry that is proposed to ESA (Malbet et al. 2022). Theia is a project for a dedicated astrometry telescope. Its advantages include more exposures and exposure time, and, a much wider field of view. However, it also has disadvantages, such as lower sensitivity and poorer spatial resolution. The table demonstrates how the mission's
<figure><img src="image_6.png" /><figcaption>Table 1: Astrometric error budget for bright targets with Theia (0.8-m telescope with 0.5 deg FoV, in orange color, see Malbet et al. 2022) and HWO (7-m telescope with $3 \times 2$ arcmin$^2$ FoV, in blue color).</figcaption></figure>

design accounts for each potential uncertainty. This comprehensive approach to error management will be validated through a combination of laboratory tests, simulations, and in-flight calibration. Pre-launch tests will characterize instrumental errors, while simulations will model astrophysical and environmental contributions. In-flight calibration, using known star fields (globular clusters) and interferometric techniques, will verify the error budget and ensure that HWO meets its precision goals.

Astrophysical errors, such as those caused by stellar activity, binarity, and reference star instability, add another layer of complexity. Stellar activity, including starspots and flares, can cause apparent shifts in a star's photometric center, leading to astrometric errors.

The stability of reference stars is another critical factor in differential astrometry. Any proper motion, parallax, or variability in the reference stars can introduce errors in the measured positions of the science targets. To minimize this, HWO will carefully select reference stars based on their stability and known proper motions, and will use a large ensemble of reference stars to average out individual variations.

Detector noise is a significant source of instrumental error. The Pyxalis CMOS GIGA220M detectors, while designed for low readout noise and high quantum efficiency, still introduce uncertainties due to dark current, pixel gains and readout noise. Calibration inaccuracies also pose a challenge, as errors in the pixel scale or geometric dis-

tortion corrections can propagate into the final astrometric measurements. These inaccuracies can introduce systematic biases if not properly managed. HWO's approach to this issue involves a combination of laboratory calibration, in-flight calibration using known star fields, and interferometric techniques. These methods ensure that the calibration remains accurate and stable throughout the mission, reducing the risk of systematic errors.

Environmental errors, such as thermal fluctuations and mechanical vibrations, further complicate the error budget. Thermal fluctuations can cause expansions and contractions in the telescope structure, leading to misalignments and changes in the optical path. These effects can introduce errors if not controlled. HWO will mitigate thermal fluctuations using active thermal control systems and low thermal expansion materials, ensuring that the telescope maintains a stable operating temperature. Mechanical vibrations, caused by spacecraft operations or reaction wheel activity, can introduce jitter in the telescope's pointing.

In conclusion, the astrometric error budget for HWO is a critical component of the mission's design and operation. By meticulously characterizing and mitigating each source of error, HWO astrometric instrument can achieve the precision necessary to detect Earth-like exoplanets and investigate the nature of dark matter.
**5. Conclusions and perspectives**

Astrometry with HWO represents a natural continuation of the Hipparcos and Gaia legacy, pushing precision into the sub-microarcsecond domain. It uniquely enables the detection of true Earth analogues around nearby stars and delivers essential constraints on dark matter structure in galaxies. The required technology is challenging but achievable, and the scientific rewards, from identifying habitable worlds to probing the fundamental properties of dark matter, are profound. A dedicated astrometric instrument aboard HWO should therefore be considered a strategic priority.

**Acknowledgements**

We acknowledge the HWO community and the many contributors to concept studies that motivated this work. FM thanks collaborators and institutions listed in the author affiliations for support and discussions.

With regard to the funding of our research, we also acknowledge the support of the LabEx FOCUS ANR-11-LABX-0013 and the CNES agency. ML would like to acknowledge the support of her PhD grant from CNES and Pyxalis.

This research has made use of NASA Astrophysics Data

System Bibliographic Services and of data obtained from or tools provided by the portal exoplanet.eu of The Extrasolar Planets Encyclopaedia.

**REFERENCES**

Amiaux, J., Ardellier-Desages, F., Doumayrou, E., et al. 2025, in Towards the Habitable Worlds Observatory: Visionary Science and Transformational Technology, ed. J. Lee, in this volume

Burn, R., & Mordasini, C. 2024, Planetary population synthesis, ed. H. J. Deeg & J. A. Belmonte (Springer Nature Switzerland), 143–2

Lizzana, M., Malbet, F., Léger, A., et al. 2025, in Towards the Habitable Worlds Observatory: Visionary Science and Transformational Technology, ed. J. Lee, in this volume

Malbet, F., Labadie, L., Sozzetti, A., et al. 2022, SPIE, 12180, 121801F

Meunier, N., & Lagrange, A.-M. 2022, A&A, 659, A104

Meunier, N., Lagrange, A. M., & Borgniet, S. 2020, A&A, 644, A77

National Academies of Sciences, Engineering, and Medicine. 2023, Pathways to Discovery in Astronomy and Astrophysics for the 2020s (Washington, DC: The National Academies Press)

Tacconi, L., Arridge, C., Buonanno, A., et al. 2021, Final recommendations from the Voyage 2050 Senior Committee (ESA)