We can no better imagine what will be happening on the moon 500 years from now than Columbus could imagine contemporary Manhattan. Except to say that it will be a place familiar to billions of people.

The central region of the Milky Way, known as the bulge, is stuffed with literally tens of billions of stars. And most of these are old - considerably older than our Sun or its neighbors - because this part of the galaxy formed first. Consequently, bulge stars are generally deficient in heavy elements.

The bottom line is, like, one in five stars has at least one planet where life might spring up. That's a fantastically large percentage. That means in our galaxy, there's on the order of tens of billions of Earth-like worlds.

Recent results from astronomers who study the occasional gravitational lensing of unknown worlds by intervening stars suggest that orphan planets could be at least as numerous as the stars. In other words, there could be hundreds of billions of orphan worlds shuffling through our galaxy.

Even if the Moon didn't exist - even if it had been vaporized billions of years ago by cantankerous Klingons - there would still be (somewhat lower) tides raised by the Sun. For creatures dependent on the oceans' ebb and flow, life could go on.