The birth of a moon is a cosmic car crash. Two young planets collide, and a cloud of scorching debris swirls around the surviving world. From this disk, like clay on a potter's wheel, a satellite gradually takes shape. That's probably how our Moon came to be. But what if the potter's wheel is being shaken and yanked by the star itself?
Astronomers have now studied exactly this scenario. They modeled giant impacts of rocky planets, but not in empty space — rather in the tight embrace of a star, on orbits with periods from 1 to 300 days. Under such conditions, tidal forces and Coriolis forces (the effect of the system's rotation) come into play. The result is harsh: for planets with periods shorter than 10 days, the star acts like a relentless janitor. At an ultra-short period of 1 day, the disk doesn't survive at all — the debris is simply swept away. At 10 days, only a scrawny, truncated disk remains, with nothing from which a moon can assemble.
This discovery redraws the map of exomoon hunting. It turns out that the most common planets, hot super-Earths on short orbits, most likely lack massive moons. The star simply doesn't let them form. The silence in the search for exomoons now has a simple and elegant explanation: we were looking where moons can't survive.
The Coriolis forces that hinder moon formation around alien stars are the same ones that spin up cyclones on Earth.
In Stanislaw Lem's novel 'Solaris', the ocean-planet itself adjusts its orbit, as if compensating for the destructive tides of a double star.