In the early 1900s there was much debate over how our solar system could have formed. One idea was the nebular hypothesis, made popular by Pierre-Simon Laplace in the late 1700s. In this model stars and planets formed together from a primordial cloud of gas and dust.
By the 1900s, the nebular hypothesis had fallen out of favor. Part of the reason had to do with a measure of rotation known as angular momentum. It became clear that most of the solar system’s angular momentum is contained by the planets. But most of the mass is contained by the Sun. At the time there was no clear mechanism that could give angular momentum to the planets while giving mass to the Sun.
An alternative was known as the planetesimal hypothesis. It was proposed by Thomas Chamberlin and Forest Moulton in 1905. In this model planets formed when the Sun had a close encounter with a passing star. The tidal forces of the star would cause solar material to fling out from the Sun. Over time the material would form into the planets.
Interestingly, the observation of galaxies was seen as supporting the planetesimal hypothesis. At the time, there was debate over the distance of galaxies (then known as nebulae), and many thought they existed within the Milky Way. It was thought that spiral nebulae might be solar systems in the making.
But it turns out Laplace was right after all. By the 1920s it became clear that spiral nebulae were in fact distant galaxies similar to the Milky Way, and our growing understanding of gravitational dynamics made the nebular hypothesis much more sensible. More recently, we have now observed the formation of protoplanets near young stars.
Sometimes old ideas turn out to be the best ones after all.