Final answer:
The primary cause of planetary migration is due to gravitational interactions, either with a planet's own moons, smaller planets, or the drag from gas and dust in the protoplanetary disk. Other factors like magnetic forces or radiation pressure from the star are not primary causes of migration.
Step-by-step explanation:
The cause of migration of planets within a solar system is primarily due to their gravitational interactions. Of the causes listed:
- Gravitational interaction with the giant planet’s own moons (a) can cause migration due to the gravitational forces between these celestial bodies.
- Drag from gas and dust in the protoplanetary disk (b) can cause a planet to lose energy and spiral inward due to friction-like effects.
- Gravitational interaction with a smaller planet (c) can also lead to migration, as angular momentum is transferred and planets can be slingshot inward.
However, magnetic forces from the star (e) and radiation pressure from the star (f) are not generally considered primary causes of planetary migration.
Understanding the effects of gravity reveals why planets orbit the Sun, just as gravity causes an apple to fall, and the Moon to orbit Earth. It is a fundamental force that acts across the cosmos without physical contact.