Terra

Summary
A terra is a rocky world with an atmosphere that has liquid water on its surface. Earth is a prime example of a natural terra world. Humans are well suited to adapt on these types of worlds.

Continued
Terra's are worlds that have a self-perpetuating tectonic cycle maintained by the planet's inner heat and the lubricating effect of the oceans. The presence of life aids in maintaining the oceans and the atmosphere, just as the oceans and atmosphere allow for the presence of life. These worlds are among the most complex of planetary systemics. The tectonic plates of a terran world form early on in the planet's history, but can significantly change over periods of hundreds of millions of years. Some terran worlds have numerous and small plates, while others have only a few very large plates. These factors often determine the size of the continents and oceans. The atmosphere of a terran world is formed by volcanic out-gassing and cometary inputs, but also maintained and often transformed by the presence of life. While certain elements may vary, for the most part oxygen and nitrogen are the most common and most essential parts of these atmospheres. Life itself can become quite prolific and varied, even producing sapient species. But it is also fragile, and though microbial life is often difficult to eradicate, it does not take much of a global temperature or environmental change to wipe out much of the higher forms.

This type of world consists of a solid mostly iron inner core, a molten liquid outer core, a mostly solid mantle made up largely of peridotite or similar material, and a solid granitic, basaltic, and sedimentary crust, consisting largely of oxygen, silicon and aluminium. Generally some ninety to ninety-six percent of a typical terran geosphere is composed of the elements oxygen, iron, silica, and magnesium. These planets share a common method of geologic, biologic, and atmospheric evolution. There is almost always an ocean of liquid water. Practically all terran type planets support carbon-based life of some sort, usually prokaryotic or equivalent, more rarely metazoan and metaphyta type biota.