The market is often cruel to new S&P 500 members.When artificial intelligence (AI)-powered data analyst Palantir Technologies (PLTR -2.50%) joined the S&P 500 on Sept. 23, investors cheered. The stock's price has risen more than 50% since then, including a huge jump the day after the Nov. 5 national election.Can it last?Some investors believe that S&P 500 membership gives stocks a floor: a baseline of support among index funds and other institutional ownership, which helps protect their investment. I wanted ...