In Planetary Classification a Class P planet is a 'Glaciated' planet. They have an age that ranges from 3-10 billion years and a diameter between 10,000-15,000 km. Class P planets are located in the Ecosphere of a Solar System. A Glaciated planet is catagorized by a surface comprised of 80%+ water ice and an atmosphere consisting of nitrogen, oxygen and trace elements. Life-forms found on Class P planets include hardy-vegetation, animal life and occasionally native humanoids.
An example of a Class P planet is Exo III.
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Information taken primarily from Star Trek Star Charts