Why do we call the earth a Blue Planet?
The earth is called the Blue Planet because it has an abundant supply of water on its surface. Water is a rare commodity in our solar system, with no water existing on the sun or anywhere else in the solar system.
Fortunately, the earth has plentiful water, which is an essential component for all life forms that exist over its surface. About 71 per cent of the planetary water is found in the oceans. This vast presence of water makes our planet appear blue from space, hence the name 'Blue Planet'.
Explanation
The textbook explicitly states that water is a rare commodity in our solar system and that Earth fortunately has an abundant supply of water on its surface, which is why it is called the 'Blue Planet'. The context also mentions that about 71 per cent of planetary water is found in the oceans, supporting the explanation.