You can make a fine circle collision polygon for the planet. Although it's not recommended to use more than 9 or so points, in reality you can make a circle with 20-30 points and it will still work fine. (unless you have hundreds of planets)
Or instead of the collision polygon you can use "System-> Pick Overlapping point" to detect when the ship touches the surface:
set angle = angle(planet.x, planet.y, ship.x, ship.y)
set pointX=planet.X+planetRadius*cos(angle)
set pointY=planet.Y+planetRadius*sin(angle)
System-> Pick Ship Overlapping point (pointX, pointY)