How likely it is that life has arisen somewhere else seems impossible to answer to me. I certainly cant see anything special about our planet so my hunch is that it's probably occurred many times elsewhere through the universe. Nonetheless, it is nothing more than a hunch.

The atheist position seems to rely on a spontaneous generation of life at some point through the action of physical laws. To calculate how likely it is that life has arisen elsewhere depends very much on the chance of this spontaneous creation - I've never seen any credible estimate as to how likely this is (though many non-credible claims ranging from zero to one hundred percent)