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What is a planet?

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Quick answer

A planet is a large, round object that orbits a star, is massive enough for its gravity to pull it into a sphere, and has cleared its orbit of other debris. Earth is a planet, and eight planets orbit our Sun.

Key things to understand

  1. A planet orbits a star (our planets orbit the Sun), not another planet.
  2. It must be big enough for its own gravity to pull it into a roughly spherical shape.
  3. It must have 'cleared its neighbourhood' — swept up or flung away most other objects in its orbit.
  4. Our solar system has eight planets, from rocky Mercury to the gas giant Jupiter.
  5. Pluto is now classed as a 'dwarf planet' because it hasn't cleared its orbit.

People also ask

Why isn't Pluto a planet anymore?
In 2006 astronomers defined a planet as a body that has 'cleared its orbit' of other debris. Pluto shares its region with many icy objects, so it was reclassified as a dwarf planet.
How many planets are in the solar system?
Eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto and similar bodies are dwarf planets.
What's the difference between a planet and a star?
A star produces its own light and heat through nuclear fusion; a planet doesn't — it only shines by reflecting its star's light, and it orbits the star.

Part of the guide: Space & Astronomy →