Just like a flat-cap-wearing old man, the planet Mercury, closest to the Sun in the Solar System, is shrinking in size as it ages. Scientists now estimate that Mercury measures 8.6 miles smaller in diameter than it was before it started picking up its pension four billion years ago.
This reduction in size is due to the planet cooling -- despite it being so close to the Sun. All planets cool as they age, but Mercury's unique positioning and environment has had an unusual effect on its geography, causing mountain ranges and cliffs two miles high to form, stretching 1,050 miles over the face of the planet.
NASA's sun-shielded MESSENGER spacecraft has provided the new details, having orbited Mercury since 2011. It debunks earlier estimates that the planet had only shrank between 1.25 to 3.75 miles over the past four billion years, with the planet's current diameter now thought to be 3,032 miles. [National Geographic]