The planets orbit the Sun because nearly all the mass is in one place

1. The Sun contains most of the mass of the Solar System, well over $99\%$ of it, with everything else sharing the scraps.
2. Because gravitational pull grows with mass, that one object dominates every other pull in the system, which is why everything goes round the Sun rather than round each other.
3. Without a resultant force, a moving planet would carry on in a straight line at constant speed and simply leave.
4. The Sun's gravitational attraction supplies that resultant force, acting towards the Sun at every instant.
5. A force at right angles to the motion changes the direction of the velocity without speeding the planet up, so the path curves round instead of straightening out.
6. The force that keeps any object in orbit around the Sun, whether a planet, a dwarf planet or a comet, is named the same way: the gravitational attraction of the Sun.

An orbit is a fall that keeps missing

1. Picture firing a cannonball horizontally from a very tall tower: fired slowly it curves down and lands, and fired faster it lands further away.
2. Fire it fast enough and the ground curves away beneath it as fast as it falls, so it never lands: that is an orbit.
3. An orbiting object needs no engine and no outward force; its own forward motion plus the inward pull is the whole mechanism.
4. There is a resultant force on an orbiting object at all times, pointing towards the centre, which is what makes it accelerate by changing direction.

Further from the Sun means a weaker pull and a slower planet

1. The strength of the Sun's gravitational field decreases as distance from the Sun increases, in exactly the way the field around any massive body does.
2. The orbital speeds of the planets also decrease as distance from the Sun increases, so Mercury races round while Neptune crawls.
3. Set the reasoning out as a chain, because each link is worth writing down.
   1. Distance from the Sun increases.
   2. The Sun's gravitational field strength decreases there.
   3. The gravitational attraction on the planet is therefore weaker.
   4. A weaker inward force can only hold the planet on a curved path at a lower orbital speed.
4. Outer planets therefore take longer to go round for two reasons at once: a longer path and a slower speed along it.

In an elliptical orbit the speed changes, and energy conservation says why

1. An object on an elliptical orbit is not always the same distance from the Sun, so its speed is not constant.
2. It travels faster when closer to the Sun and slower when further away.
3. The reason is conservation of energy: the total energy stays the same while the two stores swap between each other.
   1. Moving inwards, the object loses gravitational potential energy because it is falling towards the Sun.
   2. That energy is transferred to its kinetic energy store, so its speed rises.
   3. Climbing back outwards, kinetic energy is transferred back into the gravitational potential energy store, so its speed falls.
4. A comet shows this most dramatically, tearing past the Sun in weeks and then drifting for decades out beyond the outer planets.