Hydrosphere

Portion of a planetThe term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. Click on Term to Read More that is water, including liquid water, ice, and water vapor on the surface, underground, or in the atmosphere.