Why we will say that our earth is a unique planet ?

Because it's the only one with life... that we know of.

It's the only planet that we have conclusive proof of the existence of life.

Also the only one that's known to have liquid water in abundance on the surface - essential to the development of life as we know it.

Get Carl Sagan's book: Pale Blue Dot

The opening chapters have really opened my eyes!

YouTube - Carl Sagan - Pale Blue Dot (http://www.youtube.com/watch?v=p86BPM1GV8M)

Because our earth is a waterful planet.