Why is our earth known as …

 Earth also called as the “blue planet” or “blue marble”. The earth consists of 71% water in its oceans surrounding the land- the Atlantic Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean and water is also present in other bodies of rivers, lakes, etc. We, the inhabitants of the earth including humans, mammals, birds, other species all need water (H2O) and oxygen to survive and earth is the only planet with all the right conditions to support life as we know it, though there are discoveries being made on finding liquid water on other exoplanets.

Also, most of the water is held by the ocean about 96.5% covered with it. Since it covers such a huge span, the water molecules easily absorb red colours, ultraviolet, and infrared red. "Blue colour" is also one of the last colours to be absorbed by the ocean. This blue colour is reflected when the earth is observed from the space. Thus, the earth is called the "blue planet" or the "water planet".