I don't know right off hand but you could probably use the E-Z rotate plugin and point your camera towards the center of your planet, move the camera towards the planet's center until collision occurs, rotate your camera back to the original angles. But this would be very jumpy i'm thinking. If I get a chance i'll try to find a way to do it. But no promises. Maybe this will be of some help.
"People don't fail ..... they stop trying." Specs. P4 2.8GHz 800 FSB | 512MB DDR333
GeForce FX 5200 AGP 256MB | Windows XP Pro Full