Isotopic Evidence for a Cold and Distant Origin of the Interstellar Object 3I/ATLAS

TL;DR

This study presents isotopic measurements of interstellar object 3I/ATLAS, revealing its formation in a cold, metal-poor environment over 10 billion years ago, providing insights into early galaxy chemistry and planetesimal formation.

Contribution

It provides the first isotopic analysis of 3I/ATLAS, showing its unique composition and ancient origin, which informs models of galactic chemical evolution and planet formation.

Findings

3I/ATLAS has a high D/H ratio, indicating cold formation conditions.

Its carbon isotopic ratios suggest formation in a metal-poor environment over 10 billion years ago.

The object is a preserved fragment of an ancient planetary system.

Abstract

Interstellar objects provide the only directly observable samples of icy planetesimals formed around other stars, and can therefore provide insight into the diversity of physical and chemical conditions occurring during exoplanet formation. Here we report isotopic measurements of the interstellar comet 3I/ATLAS, which reveal an elemental composition unlike any Solar System body. The water in 3I/ATLAS is enriched in deuterium, at a level of D/H = (0.95 +- 0.06)%, which is more than an order of magnitude higher than in known comets, while its range of 12C/13C ratios (141-191 for CO2 and 123-172 for CO) exceeds typical values found in the Solar System, as well as nearby interstellar clouds and protoplanetary disks. Such extreme isotopic signatures indicate formation at temperatures $\lesssim30$ K in a relatively metal-poor environment, early in the history of the Galaxy. When interpreted with respect to models for Galactic chemical evolution, the carbon isotopic composition implies that 3I/ATLAS accreted roughly 10-12 billion years ago, following an early period of intense star formation. 3I/ATLAS thus represents a preserved fragment of an ancient planetary system, and provides direct evidence for active ice chemistry and volatile-rich planetesimal formation in the young Milky Way.