Introduction to the Solar System

The Solar System, formed about 4.6 billion years ago, is made up of all objects that are bound by the Sun’s gravity and orbit around it. All objects that are under the Sun’s gravitational influence are members of the Solar System.

Apart from the Sun at the center, the Solar System includes eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), their moons, dwarf planets, asteroids, comets, meteoroids, and other objects in the Kuiper Belt*The Kuiper Belt is a doughnut-shaped region of icy objects beyond the orbit of Neptune. It is home to Pluto and most of the known dwarf planets and some comets. and beyond, as well as artificial satellites and space probes.

The planets closest to the Sun, in order of increasing distance are the four terrestrial planets – Mercury, Venus, Earth and Mars, collectively referred to as the Inner Planets. Earth and Mars are the only planets in the Solar System where liquid water can exist on the surface.

The outer region of the Solar System includes Jupiter, Saturn, Uranus and Neptune, which is also home to the giant planets (or Jovian planets) and their large moons. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia, and methane.

There are also five officially recognized dwarf planets in our solar system: Ceres, Pluto, Haumea, Makemake, and Eris.

Astronomers use a custom unit to express distances in the Solar System. One Astronomical Unit (abbreviated as AU) is equal to 150,000,000 km and can be defined as the distance from the Sun to Earth. Based on this description, Earth is exactly one astronomical unit away from the Sun. This unit provides an easy way to quickly compare planets' distances from the Sun.