Question

Why is the earth called the blue planet ?

Answer 1

Because earth is the only planet which has water and on seeing earth from space it looks blue due to the sky's refleation in water making it to look blue & hence earth is called a blue planet. 

Answer 2

Because 3/4th part of the Earth's surface is water. This water is in the form of seas, oceans, lakes, icebergs etc. The rest 1/4th part of it is land. And when viewed from space, it appears blue. That is the reason why Earth is known as the 'Blue Planet'.