Walking around 2D planets?

So i have been trying to create a space game in which you walk around a planet... lots of people have done this but all the forum post about it have not worked for me, the closest i got was by using this and adding a character that can walk around however when you get to the edge of the planet you start moving outward into space... by pressing space i moved up and left/right moved me away/towards the sphere

https://gfycat.com/ImmediateGoodEmeraldtreeskink

can anyone help me?