diff --git a/src/HierarchicalSystem.java b/src/HierarchicalSystem.java
index 3ec93ca..84cbdaf 100644
--- a/src/HierarchicalSystem.java
+++ b/src/HierarchicalSystem.java
@@ -8,8 +8,8 @@ import codedraw.CodeDraw;
 public class HierarchicalSystem implements CosmicSystem {
 
     private final NamedBodyForcePair central;
-    private final CosmicSystem[] orbit;
-    private final CosmicSystem[] all;
+    private CosmicSystem[] orbit;
+    private CosmicSystem[] all;
 
     /**
      * Initializes this system with a name and a central body.
@@ -110,4 +110,21 @@ public class HierarchicalSystem implements CosmicSystem {
 
         return sb.toString();
     }
+
+    /**
+     * Puts the system 'cs' at the first place in the orbit of this system.
+     * Precondition: cs != null
+     */
+    public boolean putFirst(CosmicSystem cs) {
+        CosmicSystem[] old = orbit;
+        orbit = new CosmicSystem[old.length + 1];
+        all = new CosmicSystem[old.length + 2];
+
+        orbit[0] = cs;
+        System.arraycopy(old, 0, orbit, 1, old.length);
+        all[0] = central;
+        System.arraycopy(orbit, 0, all, 1, orbit.length);
+
+        return true;
+    }
 }