[AlexanderHowl]

Brown dwarfs are failed stars, our Sun was never a brown dwarf. Now, you could have the parallel being an Earth while the Sun was a protostar, but the atmosphere was likely much more massive before the impact that created the Moon (something similar to Venus). The Earth would have been much hotter, not colder, probably around 1600 K, because protostars are much more luminous than normal stars (while they are powered by gravitational contraction rather than fusion, they are much larger), and the Sun was likely a 1,000 times luminous as a protostar than it is now.