I know this question may seem a bit too simple, but I still haven't managed to find an answer to it.
So, we have two planets (red and green) moving away from each other at a constant velocity (v). At time t1, the green planet releases a beam of light. Say, this light can draw its path in space (so, like a firework). Since light will travel at the speed of light towards the red planet in the red planet's frame of reference, the start of the light's path will seem stationary to the red planet, while the green planet will keep moving away from it. So there will clearly be a distance between the green planet and the start of the light's path.
In the green planet's frame of reference, though, the green planet will be stationary, and so the start of the light's path will stay with the green planet, while the red planet will be moving away.
So, what we get is that the green planet will see that the light starts its path from the green planet, while the red planet will see that the green planet is moving away from the start of the light's path. In other words, one will see that there is distance between the light's path and the green planet, while the other will see that there is not. Of course, this is not possible - because if there was a substance very close to the green planet which would explode if light was shone on it, for one the substance would explode, and for the other it wouldn't.
I know that there is something VERY wrong with this scenario - it is so basic, and yet there are so many problems with it - but, as long as I have been trying to figure out what, I kept getting to more contradictions.
Can anyone help?