draw planet orbit on solar system

maybe with texture 3D?

Agreed. This definitely looks like a model or texture, with a collider, so the orbit and the planet highlight when hovering over the orbit or planet. So if your planets are on a fixed path, then create a model of the orbit path. If your planets are not on a fixed path (ie non-kinematic rigidbodies affected by forces), well then that is going to be a headache.

I don't find anywhere any tutorial for showing "how to". $$anonymous$$ore little helps? :)

You could visualise them with a line renderer too - then you just need to sample some points on the orbit.

http://docs.unity3d.com/Documentation/Components/class-LineRenderer.html

http://docs.unity3d.com/Documentation/ScriptReference/LineRenderer.html

cool thanks a lot =)