Letter to the Editor

Oort cloud ecology

III. The Sun’s departure from the parent star cluster shortly after the giant planets formed

¹ Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
² Department of Astronomy, Tsinghua University, 100084 Beijing, China

^⋆ Corresponding author.

Received: 29 March 2025
Accepted: 16 May 2025

Abstract

Context. The Sun was born in a clustered environment with ≲10 000 other stars. Given it is an isolated star today, the Sun must have left the nest. We do not directly know when that happened, how violent the ejection was, or how far its solar siblings have drifted apart.

Aims. The mass of the fragile outer Öpik-Oort cloud (between r_inner ∼ 30 000 au and 200 000 au from the Sun) and the orbital distribution of planetesimals in the inner Oort-Hills cloud (between ∼1000 au and ∼30 000 au) are sensitive to the dynamical processes involving the Sun in the parent cluster. We aim to understand the extent to which we can constrain the Sun’s birth environment based on observations of the Oort cloud.

Methods. The approach presented in this work was based on a combination of theoretical arguments and N-body simulations.

Results. We show that the current mass of the Öpik-Oort cloud (between 0.2 M_⊕ and 2.0 M_⊕) is best explained by the assumption that the Sun left the nest within ∼20 Myr after the giant planets formed and migrated.

Conclusions. As a consequence, a possible dynamical encounter with another star, carving the Kuiper belt, the Sun’s abduction of Sedna, and other perturbations induced by nearby stars then must have taken place shortly after the giant planets in the Solar System formed – but before the Sun left the parent cluster. Signatures of the time the Sun spent in the parent cluster must still be visible in the outer parts of the Solar System even today. The strongest constraints will be the discovery of a population of relatively low-eccentricity (e ≲ 0.9) objects in the inner Oort cloud (but 500 ≲ a ≲ 10⁴ au).