Planets closer to the sun orbit faster than those farther away because of the inverse square law of gravity. This law states that the force of gravity between two objects is inversely proportional to the square of the distance between them. In other words, the farther apart two objects are, the weaker the gravitational force between them.

For planets orbiting the sun, this means that the planets closer to the sun are subject to a stronger gravitational force than the planets farther away. This stronger gravitational force causes the planets closer to the sun to orbit faster than the planets farther away.

To illustrate this, consider the following example. Imagine two planets, Planet A and Planet B, orbiting the sun. Planet A is twice as far away from the sun as Planet B. According to the inverse square law of gravity, the force of gravity between the sun and Planet A is four times weaker than the force of gravity between the sun and Planet B. This weaker gravitational force means that Planet A orbits the sun at a slower speed than Planet B.

In general, the closer a planet is to the sun, the faster it orbits. This is why Mercury, the planet closest to the sun, orbits the fastest, while Neptune, the planet farthest from the sun, orbits the slowest.