because they are made of mostly metal and rock while the bigger planets are mostly made of gas
The most densest planets are Earth and Mercury
Jupiter is the largest and most massive planet of the Solar System -- although only about 18% larger than Saturn in diameter, it contains 2.5 times the mass of all of the other planets combined.Earth is the densest planet. Although its core is much less dense than those of Jupiter and Saturn, most of the volume of the gas giants is atmosphere.
the Earth is, see : http://www.adlerplanetarium.org/cyberspace/planets/density.html
Mercury is the smallest of the eight planets.
Mercury is the smallest of the eight official planets, then there are dwarf planets that are smaller.
Mercury is the smallest of the terrestrial planets.
Of the Jovian (gas giant) planets, Neptune is the smallest while Jupiter is the largest.
Mercury is the smallest planet of all.
The smallest gas planet is Neptune.
Mercury is one of the four inner terrestrial planets, or rocky planets. It is the smallest of the four and closest to the sun. It is also the smallest of all eight planets.
Mercury is the smallest of the terrestrial planets.The others in order after Mercury:MarsVenusEarth
The 4 smallest planets are also the 4 most inner planets: Mercury, Venus, Earth and Mars.