Because, like Earth, Venus, and Mars, it is largely composed of rock. The giant outer planets: Jupiter, Saturn, Uranus and Neptune are largely composed of gases.
Mercury
Rocky
Mercury is a rocky planet.
It is a rocky planet.
Mercury is a rocky planet. The inner planets (first four planets from the sun) are rocky planets (which means that you could stand on them) and are the smallest. While the outer planets (last four planets from the sun) are gas planets and the biggest.
Mercury is the closest planets to the sun, and is `rocky` so is also the closest rocky (or terrestrial) planet.
Mercury is the smallest "rocky" planet.
Mercury is the smallest rocky planet and closest to the sun in our solar system.
It is a rocky planet.
Mercury .
The element Mercury is liquid at STP. The planet Mercury is a rocky or terrestrial type planet.
Terrestrial planet is another way of saying rocky planet. The inner rocky planets are; Mercury, Venus, Earth and Mars.