Carbon Isotope Ratio (¹³C/¹²C)
The carbon isotope ratio (¹³C/¹²C) is a chemical clock for astrophysical objects. Carbon-13 (¹³C) is produced through stellar nucleosynthesis — its abundance increases inside stellar systems over time as stars process hydrogen and helium. A low ¹³C/¹²C ratio therefore implies an object formed very early in the universe’s history, before significant stellar carbon processing had enriched the interstellar medium.
As an age proxy
- Solar System comets: carry a ¹³C/¹²C ratio reflecting the ~4.6-billion-year-old Sun’s birth environment
- Low ¹³C/¹²C: indicates formation at a time when stars had contributed little ¹³C — i.e., in the early universe, ~8–12 Gyr ago
- The ratio is not reset by interstellar travel (unlike thermal signatures), making it a robust chronometer for preserved icy objects
Application to 3I/ATLAS
JWST NIRSpec measurements of 3i-atlas detected an unusually low ¹³C/¹²C ratio — the first such isotopic dating of an interstellar object. Combined with ~30× deuterium enrichment (measured separately by MDM+ALMA and confirmed by NIRSpec), this places 3I/ATLAS’s formation during “cosmic noon” (~12 billion years ago), the universe’s peak star-formation epoch.
This method gives a different age estimate (~12 Gyr) than the orbital galactic-thick-disk trajectory analysis (~7 Gyr; Ōtautahi-Oxford Model). The discrepancy is unresolved as of June 2026 and may reflect genuine uncertainty in both methods.
Sources
- src-3i-atlas-jwst-isotopic-age-2026-06 (Nature, June 22, 2026)
- src-3i-atlas-heavy-water-2026-05 (complementary deuterium/D/H measurements)