To be similar to earth, a planet must been in a solar system in which the star is the same size and age as our sun, and that planet must orbit this sun from a similar distance and must experience the same gravitational pull from planets closer and farther than it from the sun. This planet must be of similar diameter and shape as ours, and have a similar elemental composition in both the core and the atmosphere. These are the most basic requirements for the development of life as ALL evidence to date points to. If the only example of life is found on the conditions that exist on our planet, what evidence do we have that life can exist outside of these conditions?
How about the water they found on Mars?
What of the creatures on our own ocean floor that live on gases that would instantly kill any other life on the planet?