The nebular theory was first proposed by Immanuel Kant in 1755 and later developed by Pierre-Simon Laplace in 1796.

While both are credited with the theory, Kant's original idea was more focused on the formation of the solar system from a rotating cloud of gas and dust. Laplace later refined this theory, providing a more detailed mathematical explanation of how the cloud would collapse and form planets.

So, it's fair to say that both Kant and Laplace played a significant role in the development of the nebular theory.