The US space agency NASA has found that the planet Mercury has shrunk since it first formed.
Mercury is about four miles smaller from its centre to the surface today than when its formed over four billion years ago.
Scientists believe that Mercury has a liquid core, like Earth.
As its core has cooled down over millions of years the planet has shrunk a little bit.
This has caused the surface of the planet to scrunch up causing mountain-like areas called scarps.
Article written by Editor