In the hours after New Horizons swept past Pluto on July 14, 2015, the spacecraft turned back toward the dwarf planet and found something no one had predicted: a thin, luminous blue halo encircling a world whose surface is grey and red. The colour belongs not to Pluto itself but to the physics of tiny organic particles suspended in its near-vacuum atmosphere, scattering short wavelengths of sunlight along long slant paths in a geometry that only a departing spacecraft could reveal. It is a reminder that the most profound discoveries often come not at the moment of arrival, but in the act of lo