Answer
According to Answers, Inertia is defined by Newton's first law,
"Objects in motion stay in motion, objects at rest stay at rest"
If a planet had no inertia it could come to a stop or accelerate to high speeds without any force applied to it. It also would not find an orbit.
Inertia is a fundamental property of the law of conservation of energy.
Without it, the planet, and the universe would be quite, quite different.
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