S&P Dow Jones Indices will remove Peabody Energy Corp. from the S&P 500 index, considered a bellwether of the U.S. stock market, effective after the closing of trading on Sept. 19.

St. Louis-based Peabody will then move to the S&P 400 MidCap, an index of smaller public companies, S&P Dow Jones Indices announced Friday.

Peabody Energy first joined the S&P 500 in November 2006, when it had a market capitalization — the value of its outstanding stock — of about $10 billion. The company's market cap stood at $3.9 billion as of Friday.

The size of the company's market cap is a key factor in whether a company becomes part of the S&P 500.

S&P Dow Jones Indices, part of New York-based McGraw Hill Financial, also said Graham Holdings would be removed from the S&P 500, moving to the S&P 400 MidCap. Two companies in that index of smaller companies, United Rentals Inc. and Universal Health Services, will move to the S&P 500; these two companies have a market cap of more than $11 billion.

Removal of Peabody's status as a S&P 500 could create some volatility in its share as mutual funds and exchange-traded funds that match the bellwether index sell their stock in the coal company. Peabody closed Friday at $14.44, unchanged from its previous close.