On the Compositions of Interstellar Objects
Abstract
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.
Related
- Measuring the Age and Initial Composition of Interstellar Objects
- Sky-Plane Velocity Distributions of Interstellar Objects and Implications for Their Detection
- Sky-plane velocity distributions of interstellar objects and implications for their detection
- Capture of interstellar objects during stellar encounters
- Diversity of planetary origins and compositions traced by stellar abundance differences among co-natal star systems
BibTeX
@misc{hopkins2026compositions,
title = {{On the Compositions of Interstellar Objects}},
author = {Matthew J. Hopkins and Michele T. Bannister and Bertram Bitsch and Chris Lintott and John C. Forbes},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.30827},
eprint = {2609.30827},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.30827},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - On the Compositions of Interstellar Objects
AU - Matthew J. Hopkins
AU - Michele T. Bannister
AU - Bertram Bitsch
AU - Chris Lintott
AU - John C. Forbes
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.30827
UR - https://arxiv.org/abs/2609.30827
AB - 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 NH_3'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 N_2 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 N_2 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.
ER -