The hypothesis was based on the idea that a star passed close enough to the sun early in its life to cause tidal bulges to form on its surface, which along with the internal process that leads to solar prominences, caused material to be ejected repeatedly from the sun. Due to the gravitational effects of the passing star, two spiral-like arms would have extended from the sun, and while most of the material would have fallen back, part of it would remain in orbit. This orbiting material would cool and condense into numerous small bodies that they termed planetesimals and a few larger protoplanets. Their theory proposed that as these objects collided over time, the planets and their moons were built up, with comets and asteroids being the leftover debris. The "spiral nebulae" photographed at Lick Observatory were thought to possibly be views of other suns undergoing this process. These nebulae are now known to be galaxies rather than developing solar systems.
