Week 9: An Introduction to Modern Astrophysics Ch23

This chapter of An Introduction to Modern Astrophysics covered planet formation in our solar system and beyond. It overviewed how stars and planets form, metallicities, and a brief overview on exoplanet detection, among other things. One of the topics that I thought was interesting was how the composition of planets in our solar system was determined. I knew that there are both gaseous and rocky planets in our solar system, but I didn't really know why they were ordered in the way they are or how each came to be one or the other.

The most widely accepted theory for planetary formation is that they condensed from the "leftover" material after the Sun collapsed from a cloud of gas and dust. One reason that has been proposed for why the planets are composed of different material from each other and from the Sun is that somehow a gradient formed, both in terms of the composition and the temperature of the protoplanetary disc. This meant that different materials could condense at different distances from the Sun. Jupiter, for instance, was at a distance where it was cold enough that ice particles could condense and add to the growing planet. This region was also close enough to the Sun that the protoplanetary disc was fairly dense. This allowed Jupiter to get even bigger until it was large enough to attract the lower-mass gas particles in the area, ultimately becoming the gas giant we know and love.

Another factor that played into many of our solar system's characteristics is bombardment by smaller objects. A widely accepted theory to explain the formation of our Moon is that an object about the size of Mars collided with the Earth, releasing a substantial amount material into orbit around the Earth. This material ultimately condensed and formed the Moon. Collisions can also explain why some planets (Venus and Uranus) don't rotate in the same way as all the other planets--collisions with other objects in the solar system could have altered their rotations.

The formation of our solar system even included other craziness like ejection of planetesimals from the Sun's orbit, gravitational resonance, and migration of planets inwards--but that's a story for another time.



Sources 
http://acolwell.wikispaces.com/file/view/Inner_Rocky_and_Outer_Gas_Planets.gif/538464050/Inner_Rocky_and_Outer_Gas_Planets.gif
An Introduction to Modern Astrophysics, Carroll & Ostlie 
https://en.wikipedia.org/wiki/Moon
http://www.ucolick.org/~mountain/AAA/aaawiki/doku.php?id=why_do_some_planets_have_reverse_rotation