It’s not just planning. Conspiracy is an agreement among two or more persons to commit an unlawful act, and it requires that at least one of the conspirators commit an overt act to effect the object of the conspiracy.
Under Federal Law, the overt act is not a necessary element.
United States v. Shabani, 513 U.S. 10 (1994), was a decision by the Supreme Court of the United States regarding conspiracy liability under federal statutes. The Court ruled: "...Congress intended to adopt the common law definition of conspiracy, which does not make the doing of any act other than the act of conspiring a condition of liability..." This ruling indicated that conspiracy alone can be criminal.