Question: What is necessary for a planet to travel in a circular motion around a star?
It is not possible for a planet or anything else to travel in a circle of its own accord.
Newton's first law of motion tells us that all objects will travel in straight lines unless acted upon by unbalanced forces. This is why we cannot throw a ball in a circle. You will learn all about this in Module P3.
So for the planets to travel in circular orbits around the Sun there must be an unbalanced force pulling the planet constantly off a straight line so that the planet will in fact travel around in a circle.
The force that keeps an object travelling around in a circle is called the Centripetal Force.