Earthed - What force prevents planets from crashing into one another?

What force prevents planets from crashing into one another?

Why doesn't the orbit of planets degrade? Gravity is constant is it not?

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All the planets are orbitting the sun (circling around it) due to the gravitational pull. Depending on their mass and where they are located in our solar system they will follow a oval shapped path (called orbits) around the sun. None of the planets have orbits that overlap each other or come close enough for the planets to collide. (Except for neptune and pluto... but plutos no longer a planet so I dont know)
you need to think of this in the opposite direction. It is true that planet's gravity affect each other, but think of the time.

we already spent nearly 5 Billion years orbiting around the sun. so the "unstable" orbits have a "pretty good chance" that it already crashed into one and another. (ex. there is a theory that the moon was formed this way, with another planet crashing into earth, Also the tilt of Neptune is suggested of been caused by a huge collision with another planet)

also, you have to realize that the planets mass are EVER SO Smaller then the sun, i mean think of it this way, why doesn't the satalites we shoot into space effect each other's orbit (other then the fact that we planned them out).

Another thing you need to realize is the PURE SCALE of the solar system. you may see in your text book, internet or w.e that the planets looks big and they are orbiting around the sun. IN TRUTH, if you were actually looking at the sun with the out side of the picture being the orbit of Pluto, you'd barely able to see the planets.

also don't forget, the gravitational pull of the planets are not exactly in sync all the time. (ex, they don't orbit the sun in the same amount of days every year.) so over long periods of time, the VERY MINUTE pull on the orbits of other planets gets canceled out by it self. (and if they were in sync, like mentioned earlier, probably already occurred in the past 5 billion years just based on probability.
yes, gravity is constant, but the sun's gravity is just strong enough to keep the planets in their rightful place.
An orbit is a freefall.

Think about this...throw a ball. It travels for a distance then hits the ground. Throw it harder and it will go further and hit the ground. Shoot it from a cannon and it will go further and hit the ground. Eventually you could shoot it far enough and fast enough that (neglecting air resistance) as it fell, the earth curved away from it. That is what an orbit is. The object is above the atmosphere so no air resistance, and fast enough that as it falls, it continuously misses the earth.

In relativity, it is traveling a straight line in curved space. It would require more energy to leave orbit and less energy to fall in. I suspect in the past some objects spiraled in and others escaped. Now what is left is pretty stable. Except for the errant Asterioid knocking it off it's path (that extra energy).
gravitational
Nothing prevents planets from crashing into each other. The evidence is that for the first billion years or two our solar system was quite chaotic, with planetary orbits shifting around, and lots of collisions.

Over time, though, all the objects in unstable orbits either crash into something or get flung out of the system.

I'm not sure why you think gravity should cause orbits to degrade, but you are apparently making some erroneous assumptions. In order for an orbit to degrade, it would have to be subject to some force that dissipates energy, such as friction. Gravitational energy is perfectly conserved, so there is no dissipation of energy there.

There are some gradual orbital changes due to energy being dissipated by tidal force (i.e., flexing of planets due to moving gravitational attraction), but the magnitude of this force is very small compared to the inertia of the planets. The current solar system is dynamically fairly robust, so these small changes are not destabilizing.
Force of Gravity depends on the position of the planet. It's always pointed towards the Center of the Orbit, no matter where the planet is, but the direction towards the center, changes as the planet moves around the Sun. Thus resulting in centripetal motion and a stable or relatively stable orbit.
the star's gravity and inertia