Nine singers singing

• Trio of trios
• Three-trio combo
• Many a neo-ska group
• Northern town initially housing a single music group
• The planets, before Pluto's downgrade
• Sizable music combo
• The planets of the Solar System, at one point ('90s kids remember)
• Nine hence unable to play tennis?
• Piece contributing to compilation one thousand

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Nonet \No*net"\, Nonetto \No*net"to\, n. [From L. nonus ninth, like E. duet, fr. L. duo.] (Mus.) A composition for nine instruments, rarely for nine voices.