Planets

There are many difficult things to conceptualizer about the formation of planets.  The first thing is trying to imagine something the size of the solar system, the Earth is big, Jupiter is huge and the Sun is ginormous.  And then there are the distances the Earth is about one hundred fifty million kilometers from the Sun and Eris is between eighty and one hundred fifty times that away from the Sun.  The best way to imagine it is to dumb every thing down.  Imagine a top the size of a pancake, the center is thicker and the edges are thinner, paper thin.  The sun is in the middle and the earth is just inside the thick part of the pancake.  Wait the solar system does not look like a pancake, right?  True, but in the beginning it did, except the pancake was made up of all the matter that was going to be the solar system.  

Okay, that is the way solar systems looked just before they created planets, before that they were balls of matter that were gravitationally attracted to the center, accelerating as they fell towards the center, causing the ball to spin, and the spin caused the pancake shape. The stuff spinning faster near the outside and the slower spinning stuff close to the inside.  If it was not spinning fast enough it became part of the star and the faster stuff was flung off the disk entirely.  

Gravity is a force, the weakest force, of attraction.  The more matter present the greater the force.  Most of the mass of the solar system is in the Sun, but because of the rest of the mass of the disk, the disk is getting thinner, thanks to gravity.  Matter as it is compressed together heats up, in the center it is fifteen trillion degrees celsius, but in the disk it was much cooler at a few thousand. The next question is what state was this matter in, and that is the most critical question and concept for planet creation.

There are four states of matter, solid, liquid, gas, and plasma.  Super heated gasses change into a different state called plasma, but that is not important here.  One must have an atmosphere for there to be a liquid, all liquids boil away into gas in space, so they are not important here. So we are left with gas and solids and since everything is so hot, everything is gas.  Thinking about gaseous iron blows my mind!  The gasses start to cool down.  The first elements that solidify are stuff like calcium and much later metals like iron and nickel.  When the first solids are formed in the planetary disk, they are very diffuse, atoms of matter separated by kilometers of space and gas.  Some atoms are closer together, some are farther apart, the closer ones find each other and stick together to form clumps of matter.  The first parts of the solar system to form solids were the edges, but there was so little matter out there that clumps took the longest to form, towards the hotter center, there was more matter and so clumps formed faster; the process taking thousands to hundreds of thousands of years.  Then iron solidified and there was more iron than calcium in our solar system, so the process began to accelerate.  Clumps of matter ran into other clumps of matter and these clumps accreted more matter.  The closer to the center of the system, the more mater the was to form clumps, so the bigger the clumps began to become.  Soon there were clumps of matter large enough to have a gravitational fields large enough to affect other clumps of matter,ruses clumps began to grow faster.

Sometime during this rush of activity to create planets something marvelous happened, the ignition of the Sun.  The densest and hottest part of the solar system is at the center, the Sun.  When the nuclear fission occurred, the energy began to press outward on the matter in the star.  The matter pushed back, or rather the gravity pulled back, resisting the push outwards of the nuclear reaction.  The energy made its way slowly to the edge and when it did it pushed outwards with force.  We call this force the solar wind.  Like wind it can push things around when it is strong and when it is weak it will do little.  The solar wind is strongest near the Sun, and weakens the further it is from the center.

After the disk cooled enough for iron to form the other metals solidified too, but it took a very long time for most of the rest to solidify.  Water solidifies at zero, methane, helium, hydrogen, oxygen and all the others that make up most of the weight of our solar system, solidify much cooler than that.  Some parts of the solar system, out near the edge, reached this temperature sooner and in these locations, the metal clumps began to accrete ices.  However, in the center the new solar wind began to blow the volatile gasses out before they could cool and become ice. The clumps of metal could continue to collide and grow in mass, but the majority of the elements of the solar system would not form at their cores.  

At a certain distance the ability of the wind to carry the lighter gasses lessens and these light gasses are not stripped from the disk as fast and the Sun is not heating the clumps above the temperature needed to keep these gases gas and they begin to accrete on the clumps of matter.  When the mass of the clumps exceeds a certain mass, its gravitational field begins to affect objects further and further away, alternately gathering them or pushing them away from its mass.  These planetoids get bigger and bigger accreting more and more clumps and newly formed ices.

The iron bodies in the center of the solar system become large enough that they heat up towards their centers and they form spheroid forms with molten cores while the solar wind strips the lighter gasses away and the light from the star keeps the balls warm.  The most distant objects are cold ice balls, as there was never much matter that far from the center.  Between the two extremes in the cooler but matter rich zones, large gas giants have formed from matter that had enough gravity to resist the wind and hold on to its volatiles and they have grown very large.  Around the edge of the solar system are clumps of ice and planetoids of various sizes many as large as Pluto and Eris and a huge number of lesser objects.