Question

Why do think we only consider eight heavenly bodies as planets in our
solar system.

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Answer 1

In order to understand why there are only eight planets we need to understand the characteristics of a planet.

Characteristics of a planet

• A planet should revolve around the Sun.
• A planet should have enough amount of mass  so that its self-gravity can be maintained and it can overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
• A planet should have cleared the neighborhood around it.

Considering these characteristics, the eight heavenly bodies were the only one which fell in this criteria and hence they were only considered as planets.

Answer 2

There are only eight planets in our solar system because Pluto is considered a dwarf planet.

This is because:

• In 2006, Internation Astronomical Union demoted Pluto from the "ninth planet" to one of the "dwarf planets" in the solar system.
• The IAU redefined the planet stating three criteria which are:

1. It must orbit Sun
2. It must be a big celestial body and spherical shape
3. The celestial body should be gravitationally dominant and able to clear its neighborhood around its orbit, except its natural satellite.

• The pluto fails to fulfill one of the criteria above listed.
• The pluto is a spherical celestial body but not big enough to exert its gravitational dominance or orbital dominance to clear its neighborhood.
• Hence, Pluto is a dwarf planet and not the ninth planet of our solar system.