Triplanetary by E. E. Smith

Two thousand million or so years ago two galaxies were colliding; or, rather, were passing through each other. A couple of hundreds of millions of years either way do not matter, since at least that much time was required for the inter-passage. At about that same time -- within the same plus-or-minus ten percent margin of error, it is believed -- practically all of the suns of both those galaxies became possessed of planets.

There is much evidence to support the belief that it was not merely a coincidence that so many planets came into being at about the same time as the galactic inter-passage. Another school of thought holds that it was pure coincidence; that all suns have planets as naturally and as inevitably as cats have kittens.