With 5,000+ exoplanets now confirmed to exist, and dozens if not hundreds of still-forming planetary systems observed, we have a pretty good idea in general of how planets form around stars.

In brief, locally dense knots of material in clouds of gas collapse, start spinning, and flatten into a disk. The stuff in the center forms the star, and stuff farther out can coalesce into planets. A lot of the details I’ve skipped over are also pretty well understood, while other are still being figured out.

This is the story for low-mass stars like the Sun, at least. For stars up to about twice the Sun’s mass the system is stable enough to form stars. However, more massive stars cause problems.…