Galactic Thick Disk
The galactic thick disk is an older, more diffuse stellar population of the Milky Way orbiting above and below the thin disk on inclined paths. Thick disk stars are typically 10–12 billion years old, compared to ~4.6 Gyr for the Sun and the thin disk population where 1i-oumuamua and 2I/Borisov originated.
Key properties
| Feature | Thin disk | Thick disk |
|---|---|---|
| Typical age | 0–10 Gyr | 10–12 Gyr |
| Metallicity | Higher | Lower (older, less stellar processing) |
| Orbital inclination | Low | Elevated, inclined |
| Solar membership | Yes | No |
Significance for interstellar objects
The Milky Way’s thick disk contains stars old enough that comets and planetesimals from those systems carry ancient chemistry — preserved since before our Solar System formed. An interstellar object that formed around a thick-disk star would have:
- Lower carbon-13/carbon-12 ratios (less stellar nuclear processing)
- Elevated deuterium enrichment (pristine irradiation, never thermally reset)
- Volatiles reflecting low-metallicity stellar environments
3I/ATLAS origin
3i-atlas is the first confirmed interstellar object traced to the galactic thick disk via orbital trajectory analysis (Ōtautahi-Oxford Model; src-3i-atlas-oxford-age-2026-06). Its steep approach angle is the key diagnostic — thin-disk objects approach on near-ecliptic trajectories, while thick-disk objects have high inclinations.
This is consistent with JWST isotopic data (src-3i-atlas-jwst-isotopic-age-2026-06) showing ~30× deuterium enrichment and low ¹³C/¹²C, though the two methods give different age estimates (~7 Gyr orbital vs. ~12 Gyr isotopic).