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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, A. Léger, T. Lépine, G. Mamon, J. Martignac, F. Pancher, T. Pichon, A. Roberge, S. Ronayette, H. Rousset, S. Soler, A. Sozzetti, T. Tourette

TL;DR

This paper argues that the Habitable Worlds Observatory can reach sub-$\mu$as astrometric precision with a dedicated visible-light instrument, enabling the detection of Earth-mass planets around the nearest stars and the dynamical mapping of dark matter through stellar proper motions. The approach centers on differential astrometry using a large, stable CMOS focal plane and a Detector Calibration Unit that projects interferometric laser fringes to calibrate pixel geometry to nanometer scales, achieving long-term stability sufficient for $0.3\,\mu\mathrm{as}$ measurements. A practical error budget, including instrumental, astrophysical, and environmental sources, is developed and contrasted with the Theia concept, with a goal of approximately 100 measurements per target over 3–4 years to realize Earth analog detections at 10 pc. The anticipated science impact includes an exoplanet census of nearby FGK stars, formation and architecture insights, and stringent constraints on dark matter properties from precise stellar motions in galactic environments. Overall, the work presents a feasible pathway to a major leap in astrometry, with significant implications for exoplanet science and fundamental DM physics.

Abstract

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.

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

TL;DR

This paper argues that the Habitable Worlds Observatory can reach sub-as astrometric precision with a dedicated visible-light instrument, enabling the detection of Earth-mass planets around the nearest stars and the dynamical mapping of dark matter through stellar proper motions. The approach centers on differential astrometry using a large, stable CMOS focal plane and a Detector Calibration Unit that projects interferometric laser fringes to calibrate pixel geometry to nanometer scales, achieving long-term stability sufficient for measurements. A practical error budget, including instrumental, astrophysical, and environmental sources, is developed and contrasted with the Theia concept, with a goal of approximately 100 measurements per target over 3–4 years to realize Earth analog detections at 10 pc. The anticipated science impact includes an exoplanet census of nearby FGK stars, formation and architecture insights, and stringent constraints on dark matter properties from precise stellar motions in galactic environments. Overall, the work presents a feasible pathway to a major leap in astrometry, with significant implications for exoplanet science and fundamental DM physics.

Abstract

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.
Paper Structure (7 sections, 1 equation, 4 figures, 1 table)

This paper contains 7 sections, 1 equation, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Science objectives
  3. Exoplanet census of nearby FGK stars
  4. Probing dark matter with proper motions
  5. Measurement principle and instrument concept
  6. Expected performance and noise budget
  7. Conclusions and perspectives

Figures (4)

  • Figure 2: Exoplanets known around nearby solar-type stars. Left panel : number of nearby stars ($D\leq 20\,\hbox{pc})$ with (in red) or without (in blue) known exoplanets exxtracted 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 Encyclopaedia).
  • Figure 3: The low-mass planetary components that is currently missing. Left panel : Kernel density estimates of four synthetic planetary populations BurnMordasini24. Right panel: mass versus semi-major axis parameter space for known exoplanets in function of the detection technique. The nearby exoplanets ($D\leq 20\,\hbox{pc}$ are displayed in clear above the full sample of known exoplanets at the date of 25 July 2025 (data from The Extrasolar Planets Encyclopaedia).
  • Figure 4: 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.
  • Figure 5: 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 detectur 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.