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. Starting around 2.4 times the Sun’s mass, stars tend to blast out a lot of ultraviolet radiation, and this can cause the disk around them to evaporate , breaking up the chemical bonds in molecules and heating the gas so much it cannot form planets.…