Hello Earth - Origins of Our Planet - Part IV
We can draw pictures of each one, and do our best to lay them out in order from youngest to oldest. By doing this, we can get a pretty good idea of how planetary systems develop. We see that the youngest planets develop around their parent stars in a giant cloud of dust and gas.
As the planets get older, we see that they get larger and larger by collecting dust and gas from that cloud. We see this dust and gas form a giant swirl, like a whirlpool, funneling more and more material into the planet. Over millions of years this dust and gas help the planet grow.
In older planetary systems, we see that the parent stars begins to blow stellar winds outward away from the star. These winds blow much of the dust and gas feeding the planets away, which causes the planets to stop growing.
As the planets age, they start to cool down, and their surfaces become hard. Eventually some are cool enough to allow liquid water to form, creating oceans, lakes, rivers, and streams. Other planets do not form liquid water because they remain too hot, or the liquid water eventually freezes because the planets become too cold. However, for a very small group of lucky planets, the temperatures remain just right to allow their water to remain a liquid.