Class T

In planetary classification, class T is used to categorize some types of gas giants.

In 2376, the Delta Flyer came in contact with a dark matter lifeform in a class T cluster and was disabled. Captain Kathryn Janeway took the Flyer to a nearby class T gas giant with orbital rings, including one that was radiogenic, hoping to use those particles to reinitialize the Flyer's warp reaction. She eventually set off a chain reaction inside the rings with the Flyer's phasers to deter more dark matter lifeforms. (VOY: "Good Shepherd")

According to the Star Trek: Star Charts, a class T planet is a gas giant classified as a "large ultragiant", 50,000 to 120,000 kilometers in diameter, with an age of 2 to 10 billion years.