If you don't use solid you can make the player stand perfectly on the sphere like so:
--- player: set angle toward earth.x, earth.y
--- player: rotate 90 degrees counterclockwise
distance(earth.x,earth.y,player.x,player.y) <= radius
--- player: set position to earth
--- player: move radius pixels at angle player.angle+90
Making the platform behavior happy is another issue. Since there is no solid, the platform behavior doesn't know when it's on the ground. You can do the logic manually though. When the second event runs, the player is on the earth, so you can add a sub-event to check for the jump key is pressed and then simulate a jump.
Walking could be done similarly, but for smoothest motion I suppose you can be more clever.