docs: def a_over_Rs(P,R2,M2,M1=1,R1=1,planet=True):
      """
      Returns a/Rs for given parameters.
      """
      if planet:
          M2 *= REARTH/RSUN
          R2 *= MEARTH/MSUN
      return semimajor(P,M1+M2)*AU/(R1*RSUN)