Hey,
i already did some research on that topic as well.
For our game we currently use a particle emitter that spawns one particle every 0.5s so you get a dotted trail behind the planet. I tried around with ribbon particles as well but they usually tend to look not that good if you get really close to it.
Also, I talked ages ago with Christopher Albeluhn (he made a great looking solar system game in udk) about his trails and if I remember correctly he said that he used a torus shaped mesh that’s scaled to fit the different orbits of the planets.
If you have static orbits it could be a decent solution.