Why was the hero of Interstellar cheated into space?

Because human beings can't live on the earth, the old professor Brand told the man (in fact, he lied to almost everyone) two plans: one is to solve the gravity equation and immigrate human beings to a new planet; Plan B is to find a livable planet as a colony of human beings and give up the earth and the human beings on it. According to him, Plan B is just a bad plan and an option when Plan A fails.

The hero believed it, so he went into space.

But in fact, the old professor didn't plan to implement Plan A from the beginning, because he lacked data and couldn't solve the gravity equation. However, it is impossible to announce the news, so that people can support him to give up the earth people and use resources to realize plan B, so plan A is needed as a cover.