The Galactic kinematics of ISOs provide insights into their approximate age and original stellar population (Hallatt and Wiegert, 2020; Hopkins et al, 2025a). Therefore, measuring their incoming kinematics and chemical compositions can provide information regarding the formation of small bodies across time in the Galaxy (Taylor and Seligman, 2025). This is because of correlations between age, composition, and Galactic kinematics of their stars in the Milky Way, which may be passed on to their ISO progeny.
For stellar populations, a correlation exists between age and velocity dispersion (e.g., Wielen, 1977; Nordstr¨om et al, 2004); this partly reflects kinematic heating from large structures such as giant molecular clouds and spiral arms (Sellwood and Binney, 2002; Gustafsson et al, 2016), but may also in part reflect more violent heating for the older populations (e.g., Miglio et al, 2021), or early star formation in a thicker, turbulent gas disc (e.g., Renaud et al, 2021). ISOs should experience the same degree of kinematic heating as stars, as they tend to be ejected with very low velocities within a few km/s of their host, are subject to the same dynamical forcing in the Galaxy (Hands et al, 2019; Pfalzner et al, 2021; Hopkins et al, 2025a), and so the dispersion of their velocity distribution should also increase with age (Seligman and Laughlin, 2018; Forbes et al, 2025).
Spectroscopic surveys of Milky Way stars have shown that their composition varies, with large scatter, with age (Marsakov et al, 2011). While this age–metallicity relation has been fairly flat for the past ∼6 Gyr (e.g., Haywood et al, 2013), older, more dynamically-excited stars tend to have lower overall metallicities but higher abundances of alpha elements10 relative to iron (e.g., Edvardsson et al, 1993; Bensby et al, 2003; Bovy et al, 2012b; Mackereth et al, 2017). At the extreme of this relation is the “thick disc”, a population of low-metallicity, highly-alpha-enhanced stars with a vertical scale height of over 1 kpc (Gilmore and Reid, 1983) (compared to ∼300 pc for the “thin disc” to which the Sun belongs), though its status as a chemodynamically distinct component of the Milky Way is debated (Bovy et al, 2012a). Thick disc stars do host planetary systems, albeit at a reduced rate (Hallatt and Lee, 2025), making the detection of ISOs from this population an interesting possibility, as their composition may on average vary from objects formed around thin disc stars (Lintott et al, 2022). However, the kinematics of the currently-known ISOs are consistent with a thin disc origin for all three, though this has been debated for 3I/ATLAS (Kakharov and Loeb, 2024; Guo et al, 2025; Hopkins et al, 2025b; P´erez-Couto et al, 2025). Note that classification of stars into the thin or thick disc based purely on kinematics results in significant contamination (Alinder et al, 2025), complicating any relation between an ISO’s composition and its kinematics, and estimates of the age of 3I/ATLAS have uncertainties of many Gyr (Hopkins et al, 2025b; Taylor and Seligman, 2025).
Stellar metallicities and abundances also depend on their formation location, as well as age (Chiappini et al, 2001; Nordstr¨om et al, 2004). Therefore, the chemical composition of interstellar objects and the protoplanetary discs in which they form (Cabral et al, 2023) should vary with both age and formation location within the Galaxy. In principle, ISOs with different Galactic velocities should trace the variable
10Alpha elements are those formed by successive additions of helium nuclei, such as oxygen, magnesium, etc.