nebular
I think you mean "protoplanet hypothesis". In fact, astronomers usually call it the Nebular Hypothesis A protoplanet is a planet-like object that hasn't fully developed into a planet. Why that hypothesis? It's because it is the best we have to describe the origin of the solar system according to the Laws of Physics.
nebular
a nebular creates stars as it caves in by its own gravity!
That is the correct spelling of "hypothesis" (conjecture, proposition).
Nebular. has written: 'Felines' -- subject(s): Erotic comic books, strips
your second guess other than the first one.
eventually a sentient being with some free time
The answer is Nebular Theory
a cluster of stars.
a controlled experiment
It was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular hypothesis is Solar Nebular Disk Model (SNDM) or simply Solar Nebular Model.