Question: how long dose it take for a planet to form ?

Alexander Finch answered on 17 Dec 2015:

A long time by human standards, but it’s a fairly quick process on a cosmological timeline! Although we’ve never seen it happen directly, astronomers think that the planets formed when the sun did, and the process of going from no planet took a few millions years.

But it took far longer to turn into the planet we know today. At first, it was a hot, toxic blob of molten rock, impossible for life to form. It took many more millions of years to cool and become more stable. Eventuallly life appeared and had a profound effect on the chemistry of the atmosphere, making it more like what we see today!