Class E - Geoplastic
Age: 0-2 billion years
Diameter: 10,000 - 15,000 km
Surface: Molten, high surface temperature
Atmosphere: Hydrogen compunds and reactive gases
Evolution: Cools to become Class-F
Life-Forms: Carbon-cycle (Excalbian)
Class E planets will usually reside in a star's habitable zone. They are comparable to Earth in size, ranging from 10,000 to 15,000 kilometers in diameter. Because they have recently been formed, the surface of a Class E planet is molten, and the atmosphere contains many hydrogen compounds and reactive gases. This combination makes them unsuitable for humanoid life forms. Eventually Class E planets cool to become class F.
Excalbia, the inhospitable volcanic home world of the Excalbians in "The Savage Curtain" (TOS), is Class E. It's surface is molten lava, and the air poisonous to humans.