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