The cross-product of the normalized `PlanetToObject` and `CameraToObject` directions is perpendicular to both vectors and is, by definition, the normal of a plane that contains both. The cross product of the plane's normal (perpendicular to the plane) and CameraToObject (a vector in the plane) is a second vector, in the same plane, pointing directly toward the PlanetToObject vector.

This was one of the hardest screenshots I've tried to diagram over; I had given up on making it look right but will post it anyway, since I think you'll still get it.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/qmquV.png