How to call a function when an element from array is touched?

So I'm trying to do a script that prints something when the circle it's touched by a player. (workspace > folder > model > "Circle") But I still get the error attempt to call a nil value

visible_planets = workspace.planets:GetDescendants()
hidden_planets_folder = game.ReplicatedStorage.planets

circles = {}

for key, model in pairs(visible_planets) do

    if model.ClassName == "Model" then
        circle = model:FindFirstChild("Circle")
        table.insert(circles, circle)
        print(table.getn(circles))
    end
end

function get_planet(x)
    local partParent = x.Parent
    local humanoid = partParent:FindFirstChildWhichIsA("Humanoid")
    if humanoid then
        print('ball touched')
    end
end

for k,v in pairs(circles) do
    v.Touched:Connect(get_planet(v))
end