We know for a fact life exists; we're proof of it. So we can cross "proof of life" off the list and start addressing the probability of life on other planets. We know of 8 planets in our own solar system, and around 1,800 confirmed planets outside our solar system; so far we've only discovered life on one of these (Earth). NASA estimates that there are at least 100,000,000,000 planets in the Milky Way alone, and that is only one galaxy out of potentially 170,000,000,000. If we estimate that we are roughly average in the number of planets our galaxy contains, that gives us 17,000,000,000,000,000,000,000 planets in the universe at minimum. A more likely equation estimates trillions of planets in our own galaxy, so we'll tack an extra zero or two on to that absurdly long number of planets; for simplicity's sake, 1,000,000,000,000,000,000,000,000. It's one thing to be one in a billion or one in a trillion. But one in a septillion? That seems unlikely. Even if we assume that only one in every quadrillion planets will give rise to life, that's billions and billions of planets across the galaxy with life on them. The numbers are just so impossibly huge that it's difficult to comprehend that we'd be so lucky as to be the only planet to get life somehow.