A T-Tauri type was a type of star. It was unusual for a T-Tauri star system to have a planet in the class M range.
Small, extremely unstable wormholes are a phenomenon sometimes encountered near T-Tauri systems – in the century between 2267 and 2367, thirty-nine had been mapped.
In 2367, the USS Enterprise-D discovered a T-Tauri star system within the Ngame Nebula. The system had one planet, a terraformed class M world, home to the xenophobic Paxans. (TNG: "Clues")
According to the Star Trek Encyclopedia, 3rd ed., p. 491, a T-Tauri star was a "young star whose diameter oscillates as that star settles into a stable size."
The type was named after the prototype star T Tauri.