how to make circular motion with objects?

ive been trying to model the solar system, however i cant get a planet orbit another

picture

--[[
_G.mercx is 100, mercz is 0
]]

xris = false
zris = false
while wait(.05) do
	if _G.mercx >= -100 and xris == false then
		_G.mercx = _G.mercx - 1
	elseif _G.mercx <= -100 and xris == false then
		xris = true
	elseif _G.mercx < 100 and xris == true then
		_G.mercx = _G.mercx + 1
	elseif _G.mercx == 100  and xris == true then
		xris = false
	end
	if _G.mercz > -100 and zris == false then
		_G.mercz = _G.mercz - 1
	elseif _G.mercz == -100 and zris == false then
		zris = true
	elseif _G.mercz <= 100 and zris == true then
		_G.mercz = _G.mercz + 1
	elseif _G.mercz > 100  and zris == true then
		zris = false
	end
	script.Parent.Velocity = Vector3.new(_G.mercx, 0, _G.mercz)
end
--merc is short of mercury, that i am trying to model

its rather an oval than a circle, and not even repeating

Try this:

local self = script.Parent
local MyMoon = workspace.Moon

--// make these different values to make the moon orbit in an oval path, or keep them the same for a perfect circle path!.
local ampX = 20 
local ampZ = 20
local Speed = 0.1
while true do
	local a = (tick() * Speed) * math.pi
	local x = ampX * math.sin(a)
	local z = ampZ * math.cos(a)
	MyMoon.Position = self.Position+Vector3.new(x, 0 ,z)
	wait()
end

Change the self and MyMoon Variables to match yours. Also Change the Speed and ampX/Z for the orbit speed and radius, Keep ampX/Z the same value for a perfect circle or alter them for a more ellipse or oval path and change the Speed where Higher number here means faster orbit speed.

The obvious problem with this script is you cannot have/detect collisions like for instance a asteroid collides with the moon part;The following script

local Moon = script.Parent
Moon.Touched:Connect(function(otr))
	if(otr.Name == "Asteroid") then
		otr:Destroy()
	end
end)

May not work as desired. To get round this you would calculate the distance of the asteroid with the moon or earth part and if the Asteroid distance Magnitude is less than the radius of the moon or the earth then its hit the planet/moon

Something like:

local Asteroid = workspace.Asteroid
while true do
	local EarthRad = script.Parent.Size.Magnitude
	local AstRad = Asteroid.Size .Magnitude
	local Dist = (Earth.Position - Asteroid.Position).magnitude
	if(Dist < EarthRad) then
	--// Boom! we have a collision!
	end
	
wait()
end

Hope this helps!