# Orbit Question

1. Jun 27, 2010

### neurotikfisch

I've got a pretty simple question, and it may sound dumb, but I'm going to ask it anyway:

First, I'll preface by saying that from what I understand (and please verify or correct me), planetary orbit in a star system decays over time a la net force. I also have come to understand that the planet will either be pulled into the star or take on an elliptical path around the star (again, please correct me if I'm wrong.)

My question is this: if these are true, is there any way that a planet can leave orbit, acted only upon by the gravity of the star it orbits, without any other forces interfering?

2. Jun 27, 2010

### HallsofIvy

I'm not sure what you mean by "ala net force". The only way an orbit can decay is through friction with atoms, molecules, or other objects in its path. Nor do I understand what you mean by "take on an elliptical path". All objects orbiting in the solar system are in elliptical paths.

As for your question, no. As long as the total energy of the object stays the same, the planet will remain in its orbit. And the only way friction can can change total energy is to reduce it- and if an object is already in orbit, it would need greater energy, not less, to leave orbit and escape the gravitational influence.

3. Jun 27, 2010

### Xentrix

As above, no.

To elaborate we need to look at your question again:

Now we have to look at the word planet... A planet is defined as an object that is massive enough to have it's self-gravity form the planet into a near spherical shape, it must orbit in a clear path around it's star and hold the orbit should anything fall in its path.

So it would not be a planet if it was able to leave the orbit without any kind of external force and it would take some force to get a planet out of orbit.

The only way a planet may leave an orbit is if the gravity of it's star is majorly reduced for some reason.

If you want to look at orbital decay in basic form you will be best looking at satellites whose orbits do decay due to their having to battle with forces such as solar winds and atmospheric particles.

4. Jun 27, 2010

### K^2

The Sun's angular velocity is much higher than planetary angular velocity. That means tidal interactions will make planetary orbits rise due to tidal interactions. But I don't expect it to be a very strong effect.