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The solar system

Review your understanding of the solar system in this free article aligned to NGSS standards.

Key points:

  • Our solar system formed from a huge cloud of gas and dust about 4.6 billion years ago. It consists of the sun, eight planets and their moons, an asteroid belt, and other celestial bodies such as comets.
  • The four planets closest to the sun—Mercury, Venus, Earth, and Mars—are called the inner planets. These planets have thin atmospheres, solid surfaces, few or no moons, and no rings.
  • The four planets farthest from the sun—Jupiter, Saturn, Uranus, and Neptune—are called the outer planets. These planets have thick atmospheres, no solid surfaces, and many rings and moons.
  • The inner and outer planets are separated by the asteroid belt, a ring-shaped region containing many rocky bodies called asteroids.
  • Comets are dusty, icy bodies that orbit the sun beyond Neptune. Both comets and asteroids are much smaller than planets.
The comet NEOWISE.
The comet NEOWISE was visible in the night sky in 2020. Image credit: Maxime Storn on Wikimedia Commons, CC BY-SA 4.0.

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