Here's why:
* Gerund: A gerund is a verb that ends in "-ing" and acts as a noun. In this case, "planning" is the gerund.
* Gerund Phrase: A gerund phrase includes the gerund and any modifiers or complements that go with it. In this case, "to succeed" acts as a modifier explaining what kind of planning is happening.
The entire phrase "Planning to succeed" functions as the subject of the sentence.