If you could move a star, would its planets follow?

[the_mango55]

Question in title.

If one could, theoretically, move a star to a different location (not like teleporting, but pushing it), would the planets in its orbit stay in orbit?

[GuineaPig]

It's planets, assuming the star was not moved into another solar system, would reassume a new orbit with a larger/smaller radius. It wouldn't just follow like it was attached by a string.

[Baron von Sky Hat]

It would depend how fast you were moving it, and the circumstances.
Our sun is orbiting the galactic centre, so could be counted as moving, but our orbits are still stable, as the planets have a net motion along with the sun whatever part of their orbit they are at.

If you were to move it quickly relative to the planets orbiting, it could cause problems. Firstly depending on where the planet was in it's orbit, the new orbits would likely become very elliptical as the sun moved. They would effectively suddenly have the `wrong' energy to maintain the same orbital distance, so one part of its orbital path would become further in or out than it would otherwise have been. However, if the suns motion is slow enough then the orbits would `want' to change so slowly that they would remain more stable in the long run.

It's almost similar to something I'm working on relating to the mass loss of a star in it's `Asymptotic Giant Branch' phase, as a Red Giant. Mass is thrown out by the star due to the pressures inside it from Helium fusion. This creates a large expanding `envelope' of low density matter flying away from the star. This means that the stars mass is changing, and hence the orbits of any planets around it. If the rate is slow enough, the orbits will simply very gradually migrate outwards, until the whole system exists at the same sort of distance ratios, but further out. However, if you remove mass quickly enough, the orbits change rapidly enough that they are more likely to interact in a way that will cause the system to become unstable. Resulting from the planets entering high eccentricity orbits, where they are more likely to be close enough to interact in a way that means planets end up either `dropped' into the star, or ejected away from the star.

Returning to moving stars, If you moved it so fast that it was well outside the normal orbital radius of all of the planets before the planets have time to `react' (they would initially remain at their current velocity, any changes in force due to a moving central mass would `filter through' almost through acceleration progressively affecting velocity), then they would probably all fall practically straight into the sun. All of the force at that point would be an entirely different direction to `normal' and they likely wouldn't have the tangential velocity to orbit rather than hit.

It's difficult to describe through text, I can picture it visually, and have written enough simulations of particle dynamics in gravitational fields to imagine it, but describing their new orbital paths is more difficult since it varies with what any of the initial masses are, their orbits and positions within those orbits, and how fast the central object is moved. So it's hard to describe one answer.

If you have any programming knowledge, I'd strong recommend writing a very simple particle code to simulate it. Particles just need their masses, velocities and positions defined. Then you incrementally step through time, each step seeing what force would be acting on the particle due to the distance it is from the central mass. That gives you what acceleration should be applied, and you modify the velocity accordingly. Then just have the code rinse and repeat. You'd also want some sort of parameter to define the rate that the central mass is moving, which would also just be a velocity that you apply each time step.

You could have multiple particles representing multiple planets or just starting positions of one orbit. These could either also act on each other (as actual multiple planets would), or only with the central mass (so they are effectively several unconnected simulations running concurrently). If the mass of the central object is assumed to be very much greater than the orbiting ones (as in a sun/planet scenario) you also needn't consider the planets force on the star (which in reality would happen, but should be a small enough effect in this scenario that it can be ignored).

I (or, I presume, other programmers here) would be happy to provide some assistance if it's needed if you wanted to attempt to write it.

[Phalanx300]

Point is that the stars are moving since the universe is expanding, a teacher of my who always like to play a wise guy was talking about how he wondered what would happen after it if it would all get together again.