Palantir Technologies will be added to the S&P 500 later this month.The S&P 500 (^GSPC -1.73%) measures the performance of 500 companies that meet specific eligibility requirements, including positive earnings over the last four quarters, a market value of at least $18 billion, and a sufficiently liquid stock. The S&P 500 is generally considered to be the best barometer for the overall U.S. stock market.Palantir Technologies (PLTR 0.56%) will join the benchmark index on Monday, Sept. 23, the date of the nex ...