The Jonisian Nebula, a planetary nebula
A planetary nebula was a type of emission nebula created by matter expelled from a star reaching the end of its life.
The crew of Enterprise NX-01 surveyed a planetary nebula in 2151, at which time they detected a Vulcan survey ship apparently shadowing their activities. This happened three weeks before Enterprise encountered the Suurok-class Ti'Mur. (ENT: "Breaking the Ice")
The writers' second draft script of "Breaking the Ice" referred only to a nebula of unspecified type, though it was mentioned as being "off the Merrium System."