Interstellar Objects in the Context of the Milky Way’s Thin and Thick Disks

Journal article

The division of the Milky Way’s disk into “thin” and “thick” components is a common practice, but one that is often ambiguously defined. The two ways of dividing stars are not equivalent: many stars belonging to the stellar population at high [α/Fe] (the “chemical thick disk”) do not belong to the larger-scale-height exponential density component of G. Gilmore & N. Reid (the “kinematic thick disk”). Furthermore, the existence of two distinct kinematic components is debated. This issue has surfaced in discussions about the recently discovered interstellar object 3I/ATLAS, which likely originated around an old star, and has been variously classified as a member of both the “thin disk” and the “thick disk” by different works. We illustrate that the origins of interstellar objects should be discussed with care, and relative to specific, identified populations of stars.

Authors: Hopkins, MJ; Lintott, CJ; Bannister, MT; Forbes, JC

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: Research Notes of the American Astronomical Society
• Volume: 10
• Issue: 6
• Pages: 146
• Article number: 146
• Publication date: 2026-06-01
• Acceptance date: 2026-05-27
• DOI: 10.3847/2515-5172/ae73fd
• EISSN: 2515-5172
• ISSN: 2515-5172
• Language: English
• Keywords: Interstellar objects