Class O - Plegaic
Age: 3-10 billion years
Diameter: 10,000 - 15,000 km
Surface: Liquid water covers 80% or more of surface area
Atmosphere: Nitrogen, oxygen, trace elements
Life-Forms: Aquatic vegetation, animal life, humanoids
Examples: Argo, Pacifica
Class O planets typically reside in a star's habitable zone, or Ecosphere. Similar to Earth in size, they range from 10,000 to 15 thousand kilometers in diameter. Like Earth, they have an atmosphere that contains oxygen and nitrogen. However, 80% or more of planetary surface is coverd by water. Water and life forms are typically abundant.
These planets are largely water based in nature, and as a result can support a wide variety of marine lifeforms. Such planets are similar to Class -M's geological constituents in many ways, though land masses are either non-existent or are limited to few small islands where oceanic mountain ranges break the surface. Atmospherically, such worlds posses very dense atmospheres, and extreme tropical heat and humidity. Quite rarely happened upon, they offer research opportunities into planetary science, but their colonization potential is limited.
Argo, a planet in the Alpha quadrant, near the flight path of the Crystalline Entity in "Silicon Avatar" (TNG), is Class O. The beautiful water planet, Pacifica, is also Class O. Known for it's warm blue waters and white beaches, it was seen in "Conspiracy" (TNG) and "Manhunt" (TNT).