In round numbers, our Milky Way galaxy alone should hold about a trillion worlds, each and every one of them a unique entity with its own history.

If we add the hundreds of billions of other galaxies within our cosmic bubble, we count about a trillion trillion worlds in our universe, give or take a factor of one hundred. (A geeky comment: It is funny that this is so close to Avogadro’s number, the number of atoms in one gram of hydrogen.)

At this point you might suggest quite reasonably that within this staggering diversity of worlds, almost everything is possible. It may seem so at first sight. But this apparent freedom of very large numbers is not quite as free as it seems. The unity of the laws of physics and chemistry acts as a very powerful constraint on what can and cannot exist in nature.

In science we cannot really rule out what can exist, so long as it satisfies the laws of physics as we know them. But we can use the laws of physics and chemistry to infer what might exist. Case in point: The flying spaghetti monster is quite plausible. We can envision a cousin of the octopus that ventured out of the water some billions of years ago on planet MumbaXX. After millions of years, our creature grew feathers on its tentacles and took flight. Or, if not feathers, some ballooning mechanism using hot air from its digestive tract or from the thermal vents where it feeds.