03. What about planets? We know how the Sun formed, and that hydrogen and oxygen are found in space, but how did the planets form and do they have water? (A K2S Question)
From The Space Library
We believe that smaller clouds of dust far enough away from the large cloud that became the Sun avoided being pulled in by our new Sun's growing gravity, yet were close enough to be caught in its orbit. Slowly they began to draw themselves together the same way that our Sun did. However, because they were so much smaller and less dense than the dust-cloud that formed the Sun, they did not generate the same intense heat and pressure that is needed to create stars. Instead they formed planetisimals that continued to collect dust over a period of about 10,000 years until they were tens of miles across. Then, over the next tens of millions of years, these small planetisimals collided and combined to form the planets we now see before us, including our own Earth. In our solar system, we know for sure that we have water on our own planet. We have strongly confirmed that water exists on Mars, and also believe we have confirmed that water is present on our Moon at the north and south poles. We strongly suspect that water is present on many of the other planets in our solar system, but have not physically confirmed its existence. Because oxygen and hydrogen can both be present on many planets and moons, we believe that water can be found on them as well, if, their atmospheres are cool enough to allow water to form, and thick enough to keep gases like oxygen from sublimating away into space. Although there is some evidence that ice may exist at Mercury's poles in the bottoms of some deep craters, we are generally unlikely to find water or ice on those planets that form close to a sun because they will be very hot and will usually also lack atmospheres that are dense enough to hold hydrogen, oxygen, and other gases around the planet. On those planets, unless water or water-ice is in cold-traps or regions hidden from their sun's energy (usually in deep craters at the planet's poles) it will very likely simply sublimate away into space.
Answer provided by Joe Rhemann
Question and Answer extracted from the book Kids to Space - by Lonnie Schorer
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