Without orbiting the planet would fall into the sun because of gravity. The speed perpendicular to the direction of gravity must exactly offset the gravity, otherwise the planet would either move toward or away from the sun. So it's determined by the distance to the sun and the mass of the planet.
Without orbiting the planet would fall into the sun because of gravity. The speed perpendicular to the direction of gravity must exactly offset the gravity, otherwise the planet would either move toward or away from the sun. So it's determined by the distance to the sun and the mass of the planet.
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