After the x coordinate reaches something around 260000 the object simply disappears when I hit Play. It is not like it is not rendered. It is destroyed, because it isn’t visible even in the scene outliner. If I move it below 250000 it starts to work again.
So in my case the limit is around 260000.
If I move the object to 200000 it is 200 times more than it was in the beginning. That means that the velocity of the planet would be increased from 0.0003 to 0.06. Unforunately, this is also the value which triggers sleep mode on my object (see my edit previous post - I edited it when you were typing it seems 
).
A value which should not trigger sleep mode would appear when the object was moved about 1000000 from the center. But as I said before my limit is 260000.