• Question: what makes a planet a planet

    Asked by Hussain on 5 Apr 2025.
    • Photo: Katherine Shirley

      Katherine Shirley answered on 5 Apr 2025:

      Hi Hussain,

      There is a special organisation called the International Astronomical Union (IAU) that sets the definitions for planetary objects. These definitions can change with new discoveries so are updated over time.

      Currently, the definition for a planet is an object that meets these three criteria:

      1) It must orbit a star (in our cosmic neighborhood, the Sun).
      2) It must be big enough to have enough gravity to force it into a spherical shape.
      3) It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.

      Pluto does not meet the 3rd requirement, so is now called a dwarf planet.

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