How do scientists think the Earth and atmosphere formed?

Whatever atmosphere was created during the formation of the earth would have been ejected from the earth's atmosphere.

The present atmosphere would have been formed by volcanic activity, emitting water, sulfur and other gases. This means that the atmosphere has always been an oxidizing mixture of gases. This goes against the Miller Urey experiments that postulated a reducing atmosphere necessary for their theory of a biogenesis .

Scientists believe that the Earth and its atmosphere formed about 4.5 billion years ago through a process called accretion, in which small particles of gas and dust in the early solar system started to clump together due to gravity, eventually leading to the formation of the Earth. As the Earth grew in size, it began to differentiate, with lighter materials forming the crust and mantle and heavier materials sinking to the core. The atmosphere formed from gases released by volcanic activity and from comet and asteroidal impacts. Over time, the atmosphere changed due to processes like outgassing, chemical reactions, and biological activity.