Was Planet 9 captured in the Sun's natal star-forming region?

TL;DR

This study uses N-body simulations to evaluate the likelihood that Planet 9 was captured from the Sun's birth environment, concluding the probability is very low, especially considering supernova constraints.

Contribution

It provides the first detailed simulation-based assessment of Planet 9 capture probability within the Sun's natal star-forming region, incorporating environmental constraints.

Findings

Capture probability is only 1-6% under optimal conditions.

Only 0.05-0.1% of planets are captured into suitable orbits.

Probability drops to nearly zero when considering supernova constraints.

Abstract

The presence of an unseen `Planet 9' on the outskirts of the Solar system has been invoked to explain the unexpected clustering of the orbits of several Edgeworth--Kuiper Belt Objects. We use $N$-body simulations to investigate the probability that Planet 9 was a free-floating planet (FFLOP) that was captured by the Sun in its birth star-formation environment. We find that only 1 - 6 per cent of FFLOPs are ensnared by stars, even with the most optimal initial conditions for capture in star-forming regions (one FFLOP per star, and highly correlated stellar velocities to facilitate capture). Depending on the initial conditions of the star-forming regions, only 5 - 10 of 10000 planets are captured onto orbits that lie within the constraints for Planet 9. When we apply an additional environmental constraint for Solar system formation - namely the injection of short-lived radioisotopes into the Sun's protoplanetary disc from supernovae - we find that the probability for the capture of Planet 9 to be almost zero.