Any municipality, in addition to such powers as it has under the provisions of the general statutes or any special act, shall have the power to provide by ordinance for the appointment or election of alternate members to its planning commission. Such alternate members shall, when seated as herein provided, have all the powers and duties set forth in the general statutes or any special act relating to such municipality for such commission and its members. Such alternate members shall be electors and shall not be members of the zoning commission or zoning board of appeals. Such alternates may attend all meetings and executive sessions of said commission. Such ordinance shall provide for the manner of designating alternates to act.