Which is the smallest planet in the Solar System?

Mercury, at about 4,880 kilometres across - larger than our Moon but smaller than two of the moons in the outer Solar System, Ganymede and Titan. It became the smallest planet in 2006, when Pluto was reclassified and the definition of a planet was written down for the first time.

It is also the densest planet after Earth, because it is mostly iron: the metallic core takes up something like 85 per cent of the radius, which is a completely different proportion from anywhere else. The leading explanation is a colossal early collision that stripped away most of the rocky mantle and left the core behind.

It is shrinking. As the core cools it contracts, and the crust has had to wrinkle to fit, producing cliffs called lobate scarps that run for hundreds of kilometres and cut across older craters - the planet has lost several kilometres of radius. It also has ice, in permanently shadowed polar craters where the temperature never rises, on a world whose sunlit surface reaches 430 degrees.