Beta Pictoris

First Astrometric Limits On Binary Planets And Exomoons Orbiting β Pictoris b

By Keith Cowing

Status Report

astro-ph.EP

December 11, 2025

Filed under astro-ph.EP, Beta PIctoris, Exomoon, exoplanet

First Astrometric Limits On Binary Planets And Exomoons Orbiting β Pictoris b — Upper mass limits for a potential lunar companion to β Pic b as a function of orbital period. The color is proportional to the density of MCMC posterior samples, and the solid curves indicate the 68%, 95%, and 99.7% confidence intervals. The upper panel covers longer orbital periods (≲ 1450 days, within the dynamical stability limit), where companions more massive than ∼ 0.5 MJup (∼ 60 M⊕) are excluded. The dashed line marks the Earth–Moon mass ratio scaled to β Pic b, and the solid green curve shows the moon/binary planet boundary from Stern & Levison (2002), which marks the theoretical separation between large moons and binary planetary systems based on mass ratios and orbital stability criteria. The lower panel zooms in on short orbital periods (≲ 50 days), where the constraints are weaker. The dashed line indicates the Pluto–Charon mass ratio, while the solid green curve again shows the Stern & Levison (2002) moon/binary planet boundary. — astro-ph.EP

The search for exomoons, or moons in other star systems, has attracted significant interest in recent years, driven both by advancements in detection sensitivity and by the expanding population of known exoplanets.

The β Pictoris system is a particularly favorable target, as its proximity and directly imaged planets allow for precise astrometric monitoring. We present astrometric constraints on the presence of binary planets and exomoons in the β Pictoris system using archival observations from the GRAVITY interferometer and SPHERE instruments.

We calculate these limits by modeling the motion of the two orbiting planets and introducing an additional perturbation to the model that simulates the astrometric motion caused by an exomoon orbiting the planet β Pictoris b. We find that for short orbital periods (≈50 days), a lunar companion is only allowed if its mass remains below ≈180 M⊕ (0.6 MJup) at 3σ confidence.

At intermediate periods near 300 days, we exclude moons more massive than ≈65 M⊕ (0.2 MJup) at 3σ confidence. At longer orbital periods, we place the tightest constraints, ruling out any potential exomoon above ≈50 M⊕ (0.15 MJup) at 700 days and ≈30 M⊕ (0.1 MJup) at 1,100 days (both at 3σ confidence).

These results place the first astrometric constraints on moons and binary planets in the β Pictoris system and demonstrate the sensitivity of interferometric observations for exomoon studies.

Isabella Macias, Sydney Jenkins, Andrew Vanderburg

Comments: 16 pages, 4 figures, 4 tables. Submitted to AAS Journals. A machine-readable version of Table 4 is available in LaTeX source
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2512.00160 [astro-ph.EP] (or arXiv:2512.00160v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.00160
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Submission history
From: Isabella Macias
[v1] Fri, 28 Nov 2025 19:01:01 UTC (3,309 KB)
https://arxiv.org/abs/2512.00160
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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