Abstract
Brown dwarfs occupy the mass regime between planets and stars. In systems containing both a star and a substellar companion (be it a brown dwarf or an exoplanet), a third object orbiting the companion can be called an exosatellite. Exoplanet satellites can be easily described as exomoons, but it is not clear if satellites of brown dwarf companions can be called the same, as the term lacks a formal definition. Despite more than 6,000 exoplanets being discovered1, no exomoon has ever been confidently detected. Although there are candidates, they lack confirmation and remain controversial2,3,4,5. In this work, we present evidence of an exosatellite orbiting the directly imaged brown dwarf companion CD-35 2722 B. Applying radial velocity analysis, the same technique used to discover the first exoplanet around a Solar-type star6, to VLT/CRIRES+ spectra of this brown dwarf, we find what appears to be the periodic signal of at least one orbiting satellite. This is the first time, to our knowledge, this technique has produced evidence of satellites around a companion brown dwarf. Our best-fitting model includes a satellite with a minimum mass of about 0.9 Jupiter masses and a period of around 170 days. Although it is uncertain whether this exosatellite will fulfil the presently undefined criteria for qualifying as an exomoon, it is a marked step towards that first uncontroversial detection, as advancing technology will allow the same method to be applied to less massive targets.
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Data availability
All reduced data necessary to replicate this work can be found at https://github.com/kevinmhoy/Nature_Hoy-2026. The raw data can be accessed from the ESO archive with IDs 0112.C-2053(A), 0114.C-2039(A) and 116.2AP9. Source data are provided with this paper.
Code availability
This work is largely produced with publicly available codebases, which can be sourced from their respective citations. All specific utilizations of those codes, and those few original codes used, can be found on GitHub (https://github.com/kevinmhoy/Nature_Hoy-2026).
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Acknowledgements
This study was based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 0112.C-2053(A), 0114.C-2039(A) and 116.2AP9.
Funding
We acknowledge funding from ANID–Millennium Science Initiative Program–Center Code NCN2024_001. A.Z. acknowledges funding from the Fondecyt Regular grant no. 1250249. J.K. acknowledges financial support from grant HA 3279/15-1 from the Deutsche Forschungsgemeinschaft (DFG). S.D. and C.L acknowledge support from the ‘Programma di Ricerca Fondamentale INAF 2023’ of the National Institute of Astrophysics (Large Grant 2023 NextSTEPS). S. Petrusʼs research was supported by an appointment to the NASA Postdoctoral Program at the NASA-Goddard Space Flight Center, administered by Oak Ridge Associated Universities under contract with NASA. V.d.O. acknowledges support from INAF Minigrant 2024 MUGS.
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K.H. wrote the manuscript, analysed the data and obtained the results. A.Z. contributed to the writing of the article, conceived the idea and led the survey. P.A.P.R. ran the EMPEROR analysis. J.K. assisted with the viper analysis. S.D., R.G. and C.L. worked on the observations and analysis. S.P. contributed target-specific insights and contextualization. F.R. and J.S. provided instrument-specific insights. V.d.O., I.C. and I.G. contributed to the observations execution. All the authors reviewed the article and provided feedback.
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Extended data figures and tables
Extended Data Fig. 1 Contamination Estimation in SV Camera Images.
Selected SV images associated with our spectra, fit with Moffat model functions. The contrast curve in the rightmost panels shows the log-scale H-band flux ratio (ΔLogH) between the host star and CD-35 B as a function of angular distance from the star, specifically marking the position of the slit. This is based on the relative diminishment of the model and the contrast of the star and companion given in the discovery paper7. The first row is the night with the worst observing conditions, and shows a maximum contamination of \( \sim \)15%. The other two selected epochs are under better weather conditions. The dark pixels in the center of the real PSF are due to saturation of the detector. These points are masked when fit, but not having central pixels does complicate the fit, as can be seen in the residual ring for the two good weather observations. This effect should not significantly affect the results at separations as distant as the slit.
Extended Data Fig. 2 Nodding Spectra Utilization: Individual RV Binning vs Spectral Combination.
Comparison of final RV calculation methods. The blue points are the favored method, in which we calculate separate RVs for both nodding positions, then bin the results. The orange points are based on combining the two nodded spectra before calculating the RV. The precision improvement with the former method is \( \sim \)5%, as seen in the mean error in the legend.
Extended Data Fig. 3 Potential Secondary Period Posteriors.
Period posteriors for the main and windowed 2-satellite model fits. The first panel shows all the periods recovered for the second signals. The other panels are windowed fits for each of the peaks seen in the first panel, and are labeled b-d accordingly.
Extended Data Fig. 4 All High-Evidence 2-Satellite Models.
The different color lines show all of the 2-satellite models which have similar evidences. They are labelled according to the period of the second, inner satellite, as all models include an outer satellite similar to the one from the 1-satellite fit. One can see the reason for distrusting the 14-day model, as these high-frequency oscillations would appear more likely to be an artifact of the sampling, rather than a true signal we could recover from this data.
Extended Data Fig. 5 Posteriors for Gaussian Process Fitting.
The left column shows that we do not successfully recover any signals indicative of activity. The right column shows a lack of a meaningful noise model when combining GPs with Keplerian model fitting.
Extended Data Fig. 6 MEGNO Stability Analysis.
a) MEGNO map showing the regions of stability for the best-fit two-satellite model. For stable systems, the MEGNO converges to 2, while all other values are considered unstable. The x-axis covers the inclination angles of the inner satellite, while the y-axis is the inclination of the outer satellite. The y-axis is limited to a maximum value of 180° because higher values will result in duplicated configurations. b) MEGNO map showing the regions of stability for the two-satellite model, with both satellites on coplanar, counter-rotating orbits. Both satellites share the same longitude, with the outer satellite being inclined 180° to the inclination given by the y-axis. The red dots indicate the orbits which are coplanar with the BD’s orbit around the star. There we see islands of instability, along with strips of instability at nearly face-on orbits. c) MEGNO curve showing the stability of the one-satellite model, depending on the satellite’s initial argument of periapsis. The horizontal line marks the value of maximum stability, while a maximum value has been placed at 10, at and above which orbits are highly unstable, no matter the true MEGNO value. The blue squares are for simulations of 30 BD orbits, the orange circles are for 300 BD orbits, and the black x’s are for 3,000 BD orbits.
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This file contains extra orbital-fitting figures, other potential periodicities and individual-order RVs.
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Hoy, K., Zurlo, A., Peña R, P.A. et al. Planetary-mass exosatellite detected around the substellar companion of a star. Nature 655, 865–869 (2026). https://doi.org/10.1038/s41586-026-10751-w
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DOI: https://doi.org/10.1038/s41586-026-10751-w
