diff --git a/main.c b/main.c
index 5b38f570d80c598d1c52677e7d6a796a12591739..cbaf99c27381be9dd0ea7ecd1af29a7fea625f10 100644
--- a/main.c
+++ b/main.c
@@ -20,12 +20,14 @@ int main()
     }
 
     // TODO : create your system
-
+	// end : create system
     while (true)
     {
         gfx_present(ctxt);
         // TODO : draw the current state of your system
+        // end : draw state of system
         // TODO : update your system
+        // end : update system
         gfx_clear(ctxt, COLOR_BLACK);
         if (gfx_keypressed() == SDLK_ESCAPE)
         {
@@ -34,6 +36,7 @@ int main()
     }
 
     // TODO : Free your system
+    // end : free system
     gfx_destroy(ctxt);
     return EXIT_SUCCESS;
 }
diff --git a/planet/planet.c b/planet/planet.c
index 735290653cacaaf39ffdf054fd3d04efa6132b3c..c9b7767708403aa39a7aeb62bba9b7ab4dcf18b6 100644
--- a/planet/planet.c
+++ b/planet/planet.c
@@ -4,4 +4,36 @@
 #define G 6.67e-11
 #define M_SOLEIL 1.989e30
 
-// TODO : magic
\ No newline at end of file
+
+/*
+typedef struct _planet
+{
+    double mass;
+    vec2 pos;      // x(t)
+    vec2 prec_pos; // x(t - dt)
+} planet_t;
+typedef struct _system
+{
+    planet_t star;       // ex. The sun
+    uint32_t nb_planets; // The number of orbiting planets
+    planet_t *planets;   // An array of orbiting planets
+} system_t;
+*/
+
+
+// TODO : complete all implementations
+
+
+planet_t create_planet(double mass, vec2 pos);
+
+
+system_t create_system(double delta_t);
+
+
+void show_system(struct gfx_context_t *ctxt, system_t *system);
+
+
+void update_system(system_t *system, double delta_t);
+
+
+void free_system(system_t *system);
diff --git a/planet/planet.h b/planet/planet.h
index f21984e64616f0d91d3a502c8b6273f49556b172..0b2b87fdefa9709fd7700dc12aa92872afa785a5 100644
--- a/planet/planet.h
+++ b/planet/planet.h
@@ -26,4 +26,4 @@ void show_system(struct gfx_context_t *ctxt, system_t *system);
 void update_system(system_t *system, double delta_t);
 void free_system(system_t *system);
 
-#endif
\ No newline at end of file
+#endif