A question for anyone who develops on Unity and Roblox: How is this code?

I watched a youtube video on some gravity simulation, and they let you see the source code, so I had this idea to recreate it in Roblox, but it was created with C# in Unity. Now I was sort of able to figure out what the code does but I wasn't sure so I did a few google searches if I didn't understand something. Now the thing is SOMETHING isn't right with it because the planets just follow each other and speed up over time until you experience floating-point precision errors, so I want to know, how good is my code? Here's the source: https://github.com/SebLague/Solar-System/tree/Episode_01/Assets/Scripts/Planets Here's my code: (I didn't do gravity object because it appeared to do nothing) Universe: (ModuleScript)

local Universe = {
	gravitationalConstant = 0.0001;
	physicsTimeStep = 0.01;
};

Universe.updateProperties = function(G, T)
	Universe.gravitationalConstant = G
	Universe.physicsTimeStep = T
end

return Universe

CelestialBody: (ModuleScript)

local Universe = require(script.Parent.Universe)
_G.CelestialBody = {}

newCelestialBody = function()
	local this = {}
	
	this.radius = 0
	this.surfaceGravity = 0
	this.initialVelocity = Vector3.new(0, 0, 0)
	this.bodyName = "Unnamed"
	
	this.velocity = Vector3.new(0, 0, 0)
	this.mass = 0
	this.rb = game.ServerStorage.MeshPart:Clone()
	this.rb.Parent = workspace
	local mv = Instance.new("IntValue", this.rb)
	mv.Name = "mass"

	this.Awake = function()
		this.rb.mass.Value = this.mass
		velocity = this.initialVelocity
	end
	
	this.UpdateVelocity = function(allBodies, timeStep)
		for i,otherBody in pairs(allBodies) do
			if otherBody ~= this then
				local sqrDst = (otherBody.rb.Position - this.rb.Position).Magnitude
				local forceDir = (otherBody.rb.Position - this.rb.Position).Unit
				local acceleration = forceDir * Universe.gravitationalConstant * otherBody.mass / sqrDst
				velocity += acceleration * timeStep
			end
		end
	end
	
	this.UpdateVelocity = function(acceleration, timeStep)
		velocity += acceleration * timeStep
	end
	
	this.UpdatePosition = function(timeStep)
		this.rb.Position += (velocity * timeStep)
	end
	
	this.OnValidate = function()
		this.mass = this.surfaceGravity * this.radius * this.radius / Universe.gravitationalConstant
		this.rb.Size = Vector3.new(1, 1, 1)* this.radius
		this.rb.Name = this.bodyName
	end
	
	this.rb.Changed:Connect(this.OnValidate)
	
	this.RigidBody = function()
		return this.rb
	end
	
	this.Position = function()
		return this.rb.Position
	end
	table.insert(_G.CelestialBody, this)
	return this
end

return newCelestialBody

NBodySimulation: (ModuleScript)

local Universe = require(script.Parent.Universe)
local Time = {fixedDeltaTime = 0}

function newNBodySimulation()
	local this = {}
	this.bodies = _G.CelestialBody
	this.NBodySimulation = Instance.new("Folder", workspace)
	
	this.Awake = function()
		this.bodies = _G.CelestialBody
		Time.fixedDeltaTime = Universe.physicsTimeStep
		print("Setting fixedDeltaTime to: "..Universe.physicsTimeStep)
	end
	
	this.FixedUpdate = function()
		for i = 1, #this.bodies do
			local acceleration = this.CalculateAcceleration(this.bodies[i].Position(), this.bodies[i])
			this.bodies[i].UpdateVelocity(acceleration, Universe.physicsTimeStep)
		end
		
		for i = 1, #this.bodies do
			this.bodies[i].UpdatePosition(Universe.physicsTimeStep)
		end	
	end
	
	this.CalculateAcceleration = function(point, ignoreBody)
		local acceleration = Vector3.new(0, 0, 0)
		for i,body in pairs(this.bodies) do
			if body ~= ignoreBody then
				local sqrDst = (body.Position() - point).Magnitude
				local forceDir = (body.Position() - point).Unit
				acceleration += forceDir * Universe.gravitationalConstant * body.mass / sqrDst
				print(acceleration)
			end
		end
		return acceleration
	end
	
	this.Bodies = function()
		return this.bodies
	end
	
	this.NBodySimulation = function(instance) 
		if instance == nil then
			instance = this.NBodySimulation
		end
		return instance
	end
	return this
end

return newNBodySimulation

Heres an example script to demonstrate usage:

local CelestialBody = require(script.Parent.CelestialBody)
local NBodySimulation = require(script.Parent.NBodySimulation)
local planet = CelestialBody()
planet.radius = 2.5
planet.surfaceGravity = 10
planet.bodyName = "New planet"
planet.initialVelocity = Vector3.new(0, 0, 0)
planet.OnValidate()
planet.Awake()
local sim = NBodySimulation()
sim.Awake()

wait(10)

while true do
	sim.FixedUpdate()
	wait()
end