The definition of a planet has evolved over time, and the current definition as recognized by the International Astronomical Union (IAU) is as follows:
- A planet is a celestial body that orbits the Sun.
- It must be spherical in shape, meaning it has achieved hydrostatic equilibrium—essentially, it has enough mass to pull itself into a nearly round shape.
- It has cleared its orbit of other debris and objects, meaning it has become gravitationally dominant in its vicinity.
Based on this definition, a planet needs to be nearly round due to its own gravity, achieving a state of hydrostatic equilibrium. However, it's important to note that a planet doesn't have to be a perfect sphere like Earth or Mars. Planets can have variations in their shape, with bulges at the equator and flattening at the poles due to factors such as rotation and internal dynamics.
In the past, there have been cases where a planet was not perfectly spherical. For example, Saturn's moon, Iapetus, has an oblate shape with a prominent equatorial ridge. However, this definition of a planet does not apply to moons or smaller objects. Moons and other bodies that orbit planets, such as Iapetus, have their own distinct classifications.
It's worth mentioning that the IAU's definition of a planet has been a topic of debate and controversy, particularly regarding the status of Pluto, which was reclassified as a "dwarf planet" in 2006. However, the current definition stands as described above.