Where, if at all, should one or more commas be added in the following sentence? The planet Mars which is Earth's nearest neighbor has long fascinated scientists as well as science-fiction fans.

The correct option is:

The planet Mars, which is Earth's nearest neighbor, has long fascinated scientists as well as science-fiction fans.

Explanation:

There are defining and non-defining relative clauses. Defining relative clauses provide essential information, whereas non-defining can be removed without altering the meaning of the sentence.

In this case, "which is Earth's nearest neighbor" can be removed without changing the meaning.

"The planet Mars, wich is Earth's nearest neighbor, has long fascinated scientists as well as science-fiction fans."

We use here commas to connect two separated clauses one beign "the planet mars has long fascinated scientists as well as science-fiction fans and the other is planet Mars being Earth's nearest neighbor.