Question

Why does planet's mass affect it's speed around sun?

Answer 1

because momentum of system is conserved
higher the mass lower is the velocity

Answer 2

Hey !

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The simple answer is no. Not significantly. The position of a planet in relation to the sun is a matter of velocity. Two planets of the same mass moving at different velocities will orbit at different distances. Similarly, two of the same velocity but different masses will orbit at the same distance.

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Thanks !