how to: get second camera view out of maincam view ?

hey at all,
is it possible somehow to make the view of a second cam get out of main cam view when the main cam is moving?
my sec. cam has its own layer to render only this one. in this case a unity primitive sphere. that should give the impression of a huge gas planet although the sphere size is just = 1.
i hoped this way i could avoide to make a smooth sphere in blender with thousands of vertices which will be necessary that it still looks smooth in the game when the main cam is near to it. i've made 2 screenshoots to make visible what i mean.
have a look at the sun on the left to saturn and the distance of saturn and my playership now in the sec. pic after i rotated my ship upwards, the sun (left) went down in the view but the saturn(sec. cam view) is still in the same distance to my ship. so its moving accordingly the same way like the main cam does. i 'm searching for a way that this view would also slip out of he main cam view, downwards like the sun did.

i was trying it with using a render texture and displaying that on a plane, also with an ui image at world space but that didnt work really.
sorry for my very basic english (i'm german )

thx previously for your attention and maybe some helpful tips