Any incorporated city or town may at any time establish a planning commission under this chapter. A planning commission established hereunder shall consist of not less than 5 nor more than 9 members. Such members shall in cities be appointed by the mayor, subject to confirmation by the city council, and in towns where there is not a mayor shall be elected by the town commissioners. When a planning commission is first established the members thereof shall be appointed or elected for terms of such length and shall be so arranged that the term of at least 1 member shall expire each year and their successor shall be appointed or elected for terms of 2 to 5 years each. Any member of the planning commission so established in a city may be removed for cause after a public hearing by the mayor with the approval of city council; members of the planning commission elected by town commissioners shall be removed by them for cause after a public hearing by a majority vote. A vacancy occurring otherwise than by expiration of term shall be filled for the unexpired term in a city in the same manner as an original appointment and in a town by the town commissioners. Such a planning commission shall elect annually a chairperson and a secretary from among its own number and may employ experts, clerical and other assistants. It may appoint a custodian of its plan and records who may be the city engineer or town clerk.