The S&P 500 is going through a bit of a changeup. The stock market index announced on Wednesday, July 2, that it’s adding Datadog (NASDAQ: DDOG), a software company, to its lineup. Datadog is replacing Juniper Networks (NYSE: JNPR), which was acquired by Hewlett Packard Enterprise (NYSE: HPE).  Here’s what you need to know about the S&P 500 change.  What is the S&P 500? Launched on March 4, 1957, the S&P 500, officially known as the Standard and Poor’s 500, tracks the stock price of 500 prominent companies in the United States. As one of the best-known stock market indexes globally, it provides investors with insight into the market over a certain period of time. The S&P 500 aims to cover all U.S. sectors.  Stock market indexes are not the same thing as stock market exchanges, such as the New York Stock Exchange (NYSE). The S&P 500 is one of many stock market indexes across the world. There’s the Dow, Nasdaq-100, S&P Euro, FTSE 100, Nikkei 225, Hang Seng, and more.  Why is Datadog joining the S&P 500? On Wednesday, July 2, HPE completed its acquisition of Juniper Networks after striking a deal with the U.S. Department of Justice (DOJ). Plans for the purchase were first announced in January 2024. As a result, Juniper’s common stock will no longer trade on the NYSE and thus will no longer be part of the S&P 500.  Juniper’s departure from the S&P 500 will leave only 499 companies listed on the stock market index. Datadog is replacing the company to keep the number at 500.  Why was Datadog chosen to join the S&P 500? An index committee chooses which stocks are included in the S&P 500 and takes into account each company’s sector for a broad range. Datadog is a SaaS-based monitoring and security platform for IT and development teams.  The S&P Global lists specific requirements for a company to be selected for the stock market index. These include: Making a profit in its last quarter, and when considering the previous four quarters together Sufficient liquidity Being a large-cap stock (a market capitalization of over $10 billion) Having enough shares available to the public (sufficient “public float”) Contributing to the sector balance  Datadog must have met all of these criteria to have been selected. How unusual is this? The S&P 500 makes changes to its roster throughout the year in order to maintain 500 leading companies.  When will this change take place?  S&P Global announced in a press release that Juniper Networks will be removed from the stock market index before trading begins on Wednesday, July 9. Datadog will replace it that morning.  How have Datadog and Juniper Networks stock prices reacted to the news? Well, Juniper Networks has already been delisted from the NYSE. Anyone holding the stock should receive $40 per share, according to an acquisition filing. Meanwhile, shares of Datadog were up more than 10% in premarket trading on Thursday following Wednesday’s announcement. The stock has struggled this year and was down roughly 6% year to date before Wednesday’s close.