Re: How Fast Do Rogue Planets Move?
Spoilers ahead!
The book is far different from the movie. In the book, there are two planets, not a planet and star. Bronson Alpha is a gas giant, roughly the size of Neptune. Bronson Beta is a terrestrial planet/satellite. They were torn from their original orbit presumably by the close passage of another star. The original inhabitants of Bronson Beta saw their doom for centuries, but earth had no such luck. Putting aside the amazing coincidence of these planets coming to our solar system (the characters often ponder if it is divinely-directed, a la the Flood), the motions are fairly well described.
The planets arrive from the southern (and fairly unwatched, in the early 20th century) skies, roughly 90° from the ecliptic. They pass through the solar system on a parabolic(?) course, whipping around the sun. Thus they pass by the earth twice, at six month intervals. On the first passage, Bronson Alpha destroys the Moon. Whether this was a direct impact or if it is torn apart by tidal forces is unspecified. Earth is devastated, of course and civilization collapses. It is on the second pass that the end comes. A few hastily-constructed rockets brave the passage between Earth and Bronson Beta, which has been thawing all this time. Bronson Alpha impacts the earth with a glancing blow, which of course destroys it utterly. The gravitational forces of the masses in question cause Bronson Alpha to continue out into space, while Bronson Beta takes an eccentric orbit about the sun, canted 90° to the ecliptic while varying in distance from the sun somewhat between the distances described by Venus and Mars.
Unlikely premise in the extreme, but again, the characters note this in the book, some becoming extremely religious. It's a great read, and probably my first experience with an Apocalyptic novel.
Keith "Recommends the sequel, too" Curtis