How come the planets aren’t arranged spherically around the sun?

the solar system

Instead of being spherical, as seems to be the default shape of things in the universe, the planets are arranged in a more-or-less flat disk — a series of concentric ellipses.

planets and planetessimals

What’s up with that?

Answer: angular momentum + gravity.

The nascent planets (or planetesimals) may have started out spherically arranged around the sun.

But then the spinning sun (or proto-sun) flung its satellites—including Earth—further out into space, like how a merry-go-round flings you off when it spins fast.  So the shape would have become an oblate spheroid, with the major axis being the direction of the sun’s spin.

[an oblate spheroid]

At the same time as centrifugal force pushed out in just one circle, gravity pulled in on the entire sphere.

So the minor axis of the oblate spheroid had nothing pushing it out, only something pulling in, while the major axis was being pushed and pulled.  Eventually the minor axis mostly collapsed and now we humans observe the “flat disk” shape.

SOURCE: Essentials of Geophysics, Part One

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