Porque a Terra é redonda e a Via Láctea é plana?

Vamos ver como a física produz esses dois formatos tão diferentes no universo. 




Veja também:

http://www.askamathematician.com/2014/04/q-why-are-many-galaxies-our-solar-system-and-saturns-rings-all-flat/

https://www.universetoday.com/112805/why-is-everything-spherical/ 

http://curious.astro.cornell.edu/about-us/57-our-solar-system/planets-and-dwarf-planets/orbits/242-why-do-all-the-planets-orbit-in-the-same-plane-intermediate 

https://www.nasa.gov/content/goddard/hubble-eyes-galaxy-as-flat-as-a-pancake 

https://physics.stackexchange.com/questions/93830/why-the-galaxies-form-2d-planes-or-spiral-like-instead-of-3d-balls-or-spheric 

https://www.youtube.com/watch?v=4whKTDzX82A 

https://www.youtube.com/watch?v=Aj6Kc1mvsdo 

https://www.youtube.com/watch?v=tmNXKqeUtJM