What is the centripetal force that holds planets in orbit?

Gravity force

If we approximate the orbit of the planets around the Sun to circular orbits with a uniform circular motion, where the velocity is a vector, whose direction is perpendicular to the radius of the trajectory; the acceleration is directed towards the center of the circumference (that's why it's called centripetal acceleration).  

Now, according to Newton's 2nd law, the force is directly proportional and in the same direction as the acceleration:  

 

Therefore the net force resulting from the movement of a planet orbiting the Sun points towards the center of the circle, this is called Centripetal Force which is a central force that in this case is equal to the gravity force.

No it doesnt because it can be any number.

The radiation of the fire causes the saucepan to heat up. the conduction of the saucepan then to distribute the heat over the saucepan and to give heat to the water. the convection of the water distributing the heat within the water, till all the water is boiling