Cyclic codes have efficient encoding and decoding algorithms over finite fields, so that they have practical applications in communication systems, consumer electronics and data storage systems. The objective of this paper is to give eight new classes of optimal ternary cyclic codes with parameters $[3^m-1,3^m-1-2m,4]$, according to a result on the non-existence of solutions to a certain equation over $F_{3^m}$. It is worth noticing that some recent conclusions on such optimal ternary cyclic codes are some special cases of our work. More importantly, three of the nine open problems proposed by Ding and Helleseth in [8] are solved completely. In addition, another one among the nine open problems is also promoted.