You can use the Tag property of the TreeNode objects that you create to store additional information. You can either store the information you want to display, or some kind of reference to the object that holds these information.
For example:
Enclave_Planet_Level = Enclave_Title_Level.Nodes.Add(Form1.GAME.Planet(_Planet).PlanetName)
Can be changed to
Dim newTN As New TreeNode(Form1.GAME.Planet(_Planet).PlanetName)
newTN.Tag = Form1.GAME.Planet(_Planet)
Enclave_Title_Level.Nodes.Add(newTN)
Then in the event handler where you handle the selection of a node you can reuse this reference
Private Sub Enclave_Title_Level_AfterSelect(sender As Object, e As TreeViewEventArgs) Handles Enclave_Title_Level.AfterSelect
If e.Node.Tag IsNot Nothing Then
If TypeOf e.Node.Tag Is Planet Then
'Do something
End If
End If
End Sub
(I assumed the class is called Planet. The Ifs are in place to avoid exceptions).