Scientists once believed that Mercury hadn’t changed much in the past few billion years. But new evidence from NASA’s Messenger space probe shows that our solar system’s smallest planet is getting even smaller. 

Last fall, the spacecraft captured images of recently formed scarps, or cliff-like structures, on the planet’s surface. Scientists believe these landforms develop as Mercury’s molten core cools and shrinks, causing the surface to adjust its shape. 

“Think of it like an apple that wrinkles and shrivels over time,” says Thomas Watters, a planetary scientist with the Messenger team. The young age of the scarps observed by the probe suggests that Mercury is still tectonically active—meaning that its outer layer, or crust, is cracking and changing. Scientists had believed that as a small rocky planet gets older its core solidifies, stopping the shrinkage of the crust. The discovery “makes us completely rethink the way rocky planets age,” says Watters.