"What surprised us, at the end, was how effective this type of impact could be in explaining Mercury's unusual structure ...

"Small planets are common ... The process could lead to the formation of a terrestrial planet about the size and mass of Earth. But if a large planetary core around 10 times the mass of our ...

Terrestrial planets include Mercury, Venus, Earth, and Mars. These planets are relatively small in size and in mass. A terrestrial planet has a solid rocky surface, with metals deep in its interior.