We can't see gravity, but you concede it exists, right? How do we know it exists? Because we can see its effects. That is EXACTLY how they know these planets exist - through something that is called wobble. Astronomers can detect that wobble, and through some complex calculations (that exceed my ability to explain), the size and distance of the orbiting body can be predicted.
No one is saying these planets resemble earth in every way. But, they are saying they resemble earth in size, and in distance to the star, so that there's a potential for earth-like conditions.