As an overview, how did Earth's Moon form? The Moon was leftover debris from the solar nebular that became caught in Earth's orbit, where it coalesced into Moon. A Mars-sized body hit Earth, knocking debris off the impacting body into Earthổs orbit. This debris coalesced to form the Moon. The Moon formed alongside of Earth when Earth was forming. A Mars-sized body hit Earth, knocking debris off Earth into its orbit. This debris coalesced to form the Moon. A Mars-sized body hit Earth and remained intact in the orbit of Earth. This body became the Moon.