Class I planets mostly range in age from 2 to 10 billion years old. Their diameters range in size from 140,000 to 500,000 km. They are mostly located in the cold zone region of a star's solar system, although some have been found to exist in a star's hot zone. Their surfaces are tenuous, comprised of gaseous hydrogen and hydrogen compounds. The atmosphere within the planet vary in temperature, pressure and composition. Hydrocarbon-based life has been known to exist in these planets. (Star Trek: Star Charts)

Some young, massive class-I planets have been confused with brown dwarves. (Star Trek: The Stoneship Files: "Super-Jupiter Beacon")

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