The Chamberlin–Moulton planetesimal hypothesis was proposed in 1905 by geologist Thomas Chrowder Chamberlin and astronomer Forest Ray Moulton to describe the formation of the Solar System.
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.
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.
In 1917, James Hopwood Jeans argued that only a very close approach of a second star was necessary to eject material, instead of requiring solar prominences.