Why do the planets all orbit the Sun in the same plane?

[left-of-center2012]

Because of the way the Sun formed, explains David DeVorkin, a senior curator in the space history division at the Air and Space Museum.
About 4.5 billion years ago, a massive cloud of dust started contracting as gravity pulled its parts toward the center. This dust cloud rotated slowly at first but sped up as it contracted, in much the same way that a figure skater spins more quickly when she pulls her arms closer to her body. (This law of physics, which makes things spin faster as they contract, is called angular momentum.)

The faster rotation flattened the cloud into a pancake, with the Sun at the center and planets forming within that plane. Planetary systems around other stars tend to form in a similar way.

https://www.smithsonianmag.com/smithsonian-institution/ask-smithsonian-why-do-planets-orbit-sun-same-plane-180976243/

[Arne]

1. They now have an actual picture of this

It is from far far away.
https://www.universetoday.com/145974/more-pictures-of-planet-forming-disks-around-young-stars/

Scroll down

[Salviati]

2. It's the interactions between the elements of the rotating cloud/disk...

(namely crashing into each other and other ways of dispersing energy) that insure that even if they start out as independent objects moving in random orbits around the star, eventually they will coalesce into a disc rotating in the same plane. The conservation of angular momentum is what keeps it all from just collapsing into the star entirely.

Which is why, under the most straightforward models of dark matter, dark matter does not form into a disk in galaxies, as in those simple models, dark matter doesn't interact with itself (or other matter which is the whole point) other than gravitationally (which is very, very weak)

If we do find dark matter disks, then this implies that dark matter may have stronger interactions with itself, and may suggest a whole family of dark particles that may interact with each other. But we haven't observed such a phenomenon in our galaxy...

https://www.quantamagazine.org/deathblow-dealt-to-dark-matter-disks-20171117/

[kurtcagle]

3. Previous supernovae may have also had an impact

When a star sloughs off its hydrogen/helium outer layers as it collapses to a white dwarf, neutron star or black hole, the resulting expanding shell creates shockwaves that move at close to the speed of light through the interstellar medium. Because of the relative low frequency of these waves, when they encounter pockets of hydrogen, the shockwave flattens the resulting medium, as well as usually imparting a certain measure of angular momentum. This is basically what happened to the Sun and the other stars in its immediate neighborhood, with a supernova going off about 13 million years ago that seeded Sol, Alpha Centauri, Gliese, Tau Ceti, and other stars, many of which have now moved well away from the Sun.

Betelgeuse will probably have the same effect when it finally goes nova, likely within the next half-million years. The large red giant would likely produce gamma-ray jets that would extend out of the plane of the ecliptic (and that could potentially sterilize anything in its path), but the primary effect of the shock wave to Sol would be increased solar activity as the magnetic fields interact. Because we're in something of a void in the galaxy (between two arms, on the outer edges of a spur) Betelgeuse probably won't produce many new stars, but it could produce some, and they too would end up being aligned more or less with the ecliptic plane of the Milky Way.

[Talitha]

4. Even before the Pluto 'debate', I long suspected it wasn't a true planet because of its orbit.

It doesn't orbit in the typical 'pancake' flatness, and for a number of years, its orbit is inside that of Neptune. That's not what a true planet does.

Thanks for posting this!!