* Gravity: The sun's immense mass creates a strong gravitational pull, attracting everything around it, including the planets. This pull is what keeps the planets from flying off into space.

* Inertia: Planets are constantly moving in a straight line due to their inertia, the tendency of an object to resist changes in its motion.

Here's how it works:

1. Initial Velocity: When the solar system was forming, the planets were initially moving in random directions.

2. Gravitational Pull: The sun's gravity attracted these planets, pulling them towards itself.

3. Orbital Path: Instead of crashing into the sun, the planets' initial velocity and the sun's gravity combined to create a curved path – an orbit.

4. Balance: The planets are constantly falling towards the sun due to gravity, but their inertia keeps them moving forward. This perfect balance between gravity and inertia keeps them in orbit.

Imagine a ball on a string:

* Gravity is like the string: Holding the ball towards the center.

* Inertia is like the ball's motion: Making it want to fly off in a straight line.

The string (gravity) prevents the ball from flying off, and the ball's motion (inertia) keeps it from falling straight down. This is essentially what happens with planets orbiting the sun.

It's important to note: The planets' orbits aren't perfectly circular, but are slightly elliptical. This is due to variations in the initial velocities and gravitational pulls from other planets.