Exomoon search with VLTI/GRAVITY around the substellar companion HD 206893 B

Direct astrometric detection of exomoons remains unexplored. This study presents the first application of high-precision astrometry to search for exomoons around substellar companions. We investigate whether the orbital motion of the companion HD 206893 B exhibits astrometric residuals consistent with the gravitational influence of an exomoon or binary planet. Using the VLTI/GRAVITY instrument, we monitored the astrometric positions of HD 206893 B and c across both short (days to months) and long (yearly) timescales. This enabled us to isolate potential residual wobbles in the motion of component B attributable to an orbiting moon. Our analysis reveals tentative astrometric residuals in the HD 206893 B orbit. If interpreted as an exomoon signature, these residuals correspond to a candidate (HD 206893 B I) with an orbital period of approximately 0.76 years and a mass of $\sim$0.4 Jupiter masses. However, the origin of these residuals remains ambiguous and could be due to systematics. Complementing the astrometry, our analysis of GRAVITY $R=4000$ spectroscopy for HD 206893 B confirms a clear detection of water, but no CO is found using cross-correlation. We also find that AF Lep b, and $β$ Pic b are the best short-term candidates to look for moons with GRAVITY+. Our observations demonstrate the transformative potential of high-precision astrometry in the search for exomoons, and proves the feasibility of the technique to detect moons with masses lower than Jupiter and potentially down to less than Neptune in optimistic cases. Crucially, further high-precision astrometric observations with VLTI/GRAVITY are essential to verify the reality and nature of this signal and attempt this technique on a variety of planetary systems.

💡 Research Summary

The paper presents the first application of ultra‑high‑precision astrometry with VLTI GRAVITY to search for moons orbiting a substellar companion, specifically the brown‑dwarf‑like object HD 206893 B. The authors monitored the relative positions of HD 206893 B and its inner companion c over eight epochs in 2024 and a final epoch in 2025, using the four 8.2 m Unit Telescopes in dual‑field on‑axis mode. The observations were carried out at a spectral resolution of R≈4000 in the K‑band, allowing simultaneous acquisition of astrometric data and medium‑resolution spectra. Data reduction employed the ESO GRAVITY pipeline (v1.6.4b) together with custom Python tools, with careful treatment of baseline self‑subtraction by varying polynomial orders (3, 4, 6) in the post‑processing. The resulting astrometry reaches a precision of roughly 10 μas, as reflected by high Pearson correlation coefficients (ρ≈0.9–0.95) due to the elongated VLTI geometry.

The authors model the expected astrometric wobble induced by a satellite of mass Mₘ orbiting a primary of mass Mₚ at a distance aₘ. Using the relation Δ = 2 a_ast/d (where Δ is the angular amplitude, a_ast = q aₘ with q = Mₘ/Mₚ, and d is the distance to the system), they translate the measured residuals (≈0.1 mas) into a mass ratio q≈1.7 % and a satellite mass of ~0.4 MJup. The inferred orbital period of the candidate satellite is ~0.76 yr (≈277 days). This signal appears in both short‑term (days‑to‑months) and long‑term (yearly) residuals, suggesting a coherent wobble.

However, the authors caution that systematic effects cannot be ruled out. The residuals show a dependence on the polynomial order used in the reduction, and the timing of the residuals correlates with the observing cadence, raising the possibility of instrumental or atmospheric systematics (e.g., optical path fluctuations, variable PSF). The high correlation among baselines also indicates that the measured wobble could be partially driven by the geometry of the interferometer rather than a true astrophysical signal.

In parallel, the paper delivers a medium‑resolution (R≈4000) K‑band spectrum of HD 206893 B, constructed by averaging all epochs after scaling each epoch to mitigate ~10 % flux variability caused by atmospheric turbulence and instrumental effects. The combined spectrum clearly shows water (H₂O) absorption features, confirming the object’s L/T transition nature with an effective temperature around 1300 K. Cross‑correlation searches for carbon monoxide (CO) yielded no detection, consistent with previous low‑resolution studies.

Sensitivity analysis, based on Monte‑Carlo simulations of astrometric wobble detection, indicates that GRAVITY’s 10 μas precision could, in principle, detect satellites with mass ratios down to q≈10⁻³ (≈0.02 MJup, i.e., Neptune‑mass) for favorable orbital configurations and sufficient temporal sampling. The authors emphasize that achieving such limits requires dense, multi‑epoch coverage and careful control of systematic errors.

The study also identifies two promising targets for future GRAVITY+ observations: AF Lep b and β Pic b. Both are bright, relatively nearby companions where the expected astrometric signal from a massive moon would be larger and more readily separable from instrumental noise.

In conclusion, the work demonstrates that high‑precision interferometric astrometry can probe the dynamical signatures of massive exomoons around substellar objects. While the tentative signal around HD 206893 B is intriguing, it remains ambiguous pending further observations. The authors advocate for continued, high‑cadence GRAVITY monitoring, refined data‑reduction pipelines, and expanded target lists to ultimately achieve the detection of moons with masses comparable to those of Solar System satellites, potentially down to Neptune‑mass or even lower in optimistic scenarios.