The solar system is host to two broad categories of planets. The four closest to the sun — Mercury, Venus, Earth and Mars — are the terrestrial planets. They have rocky surfaces enclosed by relatively shallow atmospheres. The gas and ice giants — Jupiter, Saturn, Uranus and Neptune — are outliers. They are much larger than the terrestrial planets, but their cores are small and icy. Most of their size is formed by a combination of gases that become denser and hotter as you get closer to the core. Scientists count eight planets overall. Pluto was reclassified as a dwarf planet in 2006.
