The research was led by Gaku Nishiyama of the German Aerospace Center. He and his team examined how Mercury’s hot iron core is slowly cooling, which makes the planet contract.

As the core cools, the surface forms wrinkle‑like lines. Scientists count these wrinkles to measure the shrinkage, but many may be hidden because meteoroid impacts have covered them.

To map the wrinkles, the team used pictures taken by NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015.

The European‑Japanese BepiColombo spacecraft is expected to start orbiting Mercury soon. It will test the new measurements and see how much the planet has really shrunk, Nishiyama said.