Class Y

In planetary classification, a class Y planet is characterized by a toxic atmosphere, surface temperatures exceeding 500 Kelvin, and thermionic radiation discharges. They are sometimes nicknamed "Demon"-class planets, due to their extreme inhospitability to most humanoid life. Even entering orbit of a class Y planet may be hazardous to starships.

In 2374, the USS Voyager was forced to land on a class-Y planet in the Vaskan sector after detecting desperately needed deuterium on the surface. There, they discovered the deuterium was in fact contained in a biomimetic lifeform called the "Silver Blood", which duplicated the crew and ship. (VOY: "Demon")

A year later, the crew of the mimetic USS Voyager began to deteriorate on their way to the Alpha Quadrant. After discovering their true natures, the crew attempted to take shelter on another class-Y planet but were turned away by a mining concern. The mimetic Voyager disintegrated en route to its original homeworld. (VOY: "Course: Oblivion")

In 2377, the hologram Iden identified a class Y planet as an ideal new home for his group of freed prey holograms. He named the planet Ha'Dara, Bajoran for "home of light". (VOY: "Flesh and Blood")

Information taken from Star Trek: Star Charts indicates class Y worlds have an age between 2-10 billion years and a diameter between 10,000-15,000 km. They can be located anywhere within a solar system.