The solar system is the Sun and everything gravitationally bound to it. That includes eight planets, five dwarf planets, hundreds of moons, thousands of asteroids, and billions of comets. It formed about 4.6 billion years ago from a collapsing cloud of gas and dust. The Sun contains 99.8 percent of the mass. Jupiter contains most of the rest.

The planets divide into two groups. The inner planets, Mercury, Venus, Earth, and Mars, are rocky and small. The outer planets, Jupiter, Saturn, Uranus, and Neptune, are large and gaseous or icy. Between Mars and Jupiter lies the asteroid belt. Beyond Neptune lies the Kuiper Belt, and beyond that, the Oort Cloud.

The solar system is not static. Planets migrate, moons get captured, and comets fall inward. The early solar system was chaotic, with giant impacts shaping the planets. Earth's Moon likely formed from a collision with a Mars-sized body. Uranus is tilted on its side, probably from another impact.

We live inside the solar system, so it is easy to take for granted. But it is one of countless planetary systems in the galaxy, and the only one we have explored up close.