S&P Dow Jones officially announced that $Tesla (TSLA.US)$ will be added to the S&P 500 stock index on December 21 of this year. After that, Tesla stock has jumped nearly 40% since the announcement bringing its year-to-date gain to nearly 580%.
Why this news brings $Tesla (TSLA.US)$ huge effect?
Before answering this question, let's see how S&P 500 counts and the reason why it's influential that a stock is included in the S&P500. 
What is S&P 500?
The S&P 50...