Sorry to play the grammar nazi, but "seldom" is an adverb. I think "infrequent" is the adjective you were looking for...

The existence of life on Earth is pretty good evidence that the probability is > 0 (how much greater is unknown - but that's the point of the article).

I don't think they're ruining the party. Recent evidence suggests there are upwards of 10 billion terrestrial planets in our galaxy alone. So, even if the probability of any one of those worlds developing life is one in a billion, the aggregate probability that at least one other planet in our galaxy has developed…