On The Compositions Of Interstellar Objects

[astro-ph.EP] As physical samples of distant planetary systems, the compositions of interstellar objects (ISOs) should correlate with the properties of their diverse parent stars.

We combine a chemical partition model with a sample of stellar elemental abundances from APOGEE DR17 to predict the abundances of commonly-observed cometary volatiles in ISOs. We find that the compositions of ISOs vary significantly between different stars.

In particular, we predict a strong correlation between an ISO’s ammonia abundance and its parent star’s metallicity: high-metallicity stars will create ISOs rich in ammonia. This suggests the production rates of NH₃‘s photolytic daughter products are an ideal observational tracer for the origins of ISOs.

We infer that 2I/Borisov formed around a star of near-solar metallicity (−0.4≲[Fe/H]≲0.3), and corroborate the velocity- and isotope-based inferences that 3I/ATLAS formed around a lower-metallicity star (−0.8≲[Fe/H]≲0.0). The production rates of 2I and 3I imply both only contain hypervolatiles such as CO and N2 that are trapped in other less-volatile ices, similar to typical Solar System comets.

Despite this, we argue that true hypervolatile-ice-rich ISOs may exist; they would exhibit high CO and N₂ production rate ratios, possibly similar to those of C/2016 R2 (PanSTARRS). Our predictions provide context for future ISO discoveries, opening a path to link the properties of their origin systems to observable production rates.

Matthew J. Hopkins, Michele T. Bannister, Bertram Bitsch, Chris Lintott, John C. Forbes

Comments: Submitted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2609.30827 [astro-ph.EP] (or arXiv:2609.30827v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.30827
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Submission history
From: Matthew Hopkins
[v1] Fri, 25 Sep 2026 05:11:48 UTC (330 KB)
https://arxiv.org/abs/2609.30827

Astrobiology, Astrochemistry,