How long ago did Earth form?

Step-by-step solution

The universe expanded and hence, the temperature came down. Hydrogen and Helium formed sometime later. The gases condensed under gravitation and formed the galaxies of the present day universe. Oxygen combined with ammonia and methane to form water, CO2 and others. The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and formoceans. Life appeared 500 million years after the formation of earth, i.e., almost four billion years back. In the solar system of the milky way g