The planet earth surface is covered by almost 70% of water and rest 30 percent constitute solid surface like continents, islands, mountains.

Hence when we see earth from space it looks like blue planet because the visible light radiation coming from the son contains VIBGYOR means violet,indigo, blue,green, orange and red, they together form white light.

But when white light falls on earth surface it’s water resorvoier absorbe the other wavelengths of light except blue and only blue light reflected back to space, that’s why we see earth as blue planet.