you cannot assume the qualities of space or the laws of the universe are constant.
That is the assumption that we are working under, however; and so far, every thing we see seems to be composed of the same elements, which does suggest the same physics and chemistry.
IMHO it would be difficult to find a planet exactly like Earth, but I think it is extremely likely there are many very similar. Even if there is a 0.000000001% chance per star of having an Earth-like planet, there would still be about 10,000,000,000,000 of them in the observable universe.
Also, we only have an example of life on our planet, it could actually be more likely on other types of planets. Probably no more likely, but even if it can happen on other planets, that only increases the odds.