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

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