According to our sources, Emerson's girlfriend discovered his body in their home and described the passing as "sudden." We're told Emerson had plans to tour in Japan next week.
Emerson's death was confirmed on the band's official Facebook page ... though no cause of death was given.
Bandmate Carl Palmer released a statement, saying, "Keith was a gentle soul whose love for music and passion for his performance as a keyboard player will remain unmatched for many years to come."
Emerson, Lake and Palmer was one of the most popular progressive rock bands of the 1970s ... and Emerson's solos are legendary.