The Solar System

The solar system is the collection of the Sun and everything bound to it by gravity, including the planets, their moons, dwarf planets, asteroids and comets. The Sun, a star, contains the overwhelming majority of the system's mass and its gravity holds the other bodies in orbit. The planets travel around it in the same direction along paths that are nearly, but not exactly, circular, all lying close to a common plane.

There are eight planets. The four inner worlds — Mercury, Venus, Earth and Mars — are comparatively small, rocky and dense. Beyond them lie the four giant planets: Jupiter and Saturn, made largely of hydrogen and helium gas, and Uranus and Neptune, which contain more ices. A belt of rocky asteroids orbits between Mars and Jupiter, while comets, made of ice and dust, follow elongated paths that bring them close to the Sun before swinging far away.

Pluto, long counted as the ninth planet, was reclassified as a dwarf planet in 2006 under a definition adopted by the International Astronomical Union; it is one of several such bodies in the cold outer regions. Many planets have moons of their own, ranging from tiny captured rocks to worlds larger than Mercury. The arrangement of these bodies, their relative distances, and their motions form the basic framework for understanding the night sky and the place of Earth within it.

Frequently asked questions

How many planets are in the solar system?

Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto was reclassified as a dwarf planet in 2006.

What is the difference between the inner and outer planets?

The four inner planets are small and rocky, while the four outer planets are much larger and made mostly of gas and ice.

What holds the solar system together?

The Sun's gravity. It contains most of the system's mass and keeps the planets and other bodies in their orbits.