diff --git a/src/hydro/Planetary/hydro_part.h b/src/hydro/Planetary/hydro_part.h
index 3488c317cd54d82b042a59b215809b3946a427f7..34f7be1d1837a571c70ce612f577e339d17a697a 100644
--- a/src/hydro/Planetary/hydro_part.h
+++ b/src/hydro/Planetary/hydro_part.h
@@ -201,6 +201,9 @@ struct part {
   /*! Time-step length */
   timebin_t time_bin;
 
+  /*! Tree-depth at which size / 2 <= h * gamma < size */
+  char depth_h;
+
   /*! Time-step limiter information */
   struct timestep_limiter_data limiter_data;