C. Santamaria

In an atomic nucleus, it's usually crowded: protons and neutrons are packed like commuters on a rush-hour train. But beryllium-11 is different. One neutron keeps its distance from the rest, forming a diffuse cloud — a nuclear halo. This nucleus resembles a planet with a satellite on a highly elongated orbit.

Two tin nuclei collide at about two-thirds the speed of light. For an instant, a hot, dense cloud forms, and protons fly out in all directions. If you catch pairs of protons and measure how often they fly off in the same direction, you can work out the size of that cloud. This method is called femtoscopy: it works like a microscope for nuclear scales.