The Kinematic Age of 3I/ATLAS and its Implications for Early Planet Formation

TL;DR

This study estimates the age, origin, and formation rate of interstellar object 3I/ATLAS, revealing its likely old age, low-metallicity origin, and implications for early galaxy planet formation.

Contribution

It introduces a framework for inferring interstellar object ages and origins from kinematics and metallicity, based on the third known interstellar object.

Findings

3I/ATLAS is likely 3-11 Gyr old.

There is a 12% chance it originated from a low-metallicity star.

Interstellar object formation is efficient at low metallicities and early galaxy history.

Abstract

The recent discovery of the third interstellar object (3I/ATLAS) expands the known census from two to three and significantly improves statistical inferences regarding the underlying galactic population. In this paper, we argue that cometary activity likely significantly contributes to 3I/ATLAS's brightness, since the nuclear size inferred when assuming an asteroidal reflectance implies an untenable interstellar object mass per star. 3I/ATLAS exhibits a high excess velocity of $v_\infty=58$ km/s relative to the Sun, which implies that 3I/ATLAS is relatively old in comparison to previous interstellar objects. Here, we calculate the posterior distribution of ages implied by the kinematics of the interstellar objects and find that 3I/ATLAS is likely $\sim3-11$ Gyr old, assuming that the interstellar object and stellar age-velocity dispersion relations are equivalent. We also calculate the distribution of host star metallicities and find that 3I/ATLAS has a 12% chance of originating from a star with $\text{[Fe/H]}\leq-0.4$. These results show that interstellar object formation is likely efficient at low metallicities and early in the history of the Galaxy. Finally, we estimate the interstellar object formation rate throughout Galactic history implied by these three objects. As future interstellar objects are discovered, the framework presented here can be applied to further refine this calculation. Comparison between the interstellar object and stellar formation histories will provide unique insights into the history of stellar system formation in the Galaxy.