Mercury is getting smaller. Over its roughly 4.5 billion years of existence, the dense little planet’s diameter could have shrunk by as much as 14 kilometers, a study published today in Nature Geoscience reports.

It might sound strange, but a shriveling Mercury is not unexpected. In fact, theories describing how the planet’s core cools over time predicted that by now, Mercury’s diameter should have lost around 10 kilometers. But until this new analysis, teams had only been able to measure a 7-kilometer contraction at most. Now, scientists have used data gathered by NASA’s MESSENGER probe, which has been orbiting Mercury since 2011, to calculate how much Earth’s odd little neighbor has shrunk.

Unlike Earth’s, Mercury’s lithosphere – essentially, a