Additionally, why do all planets revolve in the same direction relative to the sun?
ELI5: Why does Venus rotate in a different direction than all other planets?
Planetary ScienceCould it be an errant planet captured by the sun?
With same orbital plane as the rest of the planets and a circular orbit? That's so unlikely that you have a better chance at getting struck by lightning a few million times
The physics of capturing an extrasolar object are very complex. They come streaking in from deep space with incredible relative velocity so you need a complicated interaction between multiple large bodies to burn off that energy and let the planet get captured.
In a lot of scenarios, for one planet to get captured, another one must get ejected.
That’s certainly possible, but highly unlikely.
The fact that Venus has settled into a nice circular orbit along the normal plane also makes that unlikely - a captured object would start on a bizarre oblong orbit.
Neptune’s moon Triton is believed to be a captured object and it has a funky orbit that’s both backwards and highly inclined.
For the 2nd question, this is about how stars, and orbiting bodies form.
Imagine a huge cloud of dust (a nebula) that slowly shrinks due to gravity. There is random movement in it, that as it shrinks it speeds up (like being on a playground merry-go-round when you get speed up, then pull yourself into the middle)
Over time, particles collide and things, and it all averages out to become a spinning ball of dust. This continues to collapse due to gravity, but because of the spin, it collapses faster at the top and bottom, which leads to a big lump in the center and a disc out to the sides (that all spins the same way)
The planets can then start forming in a very similar way to the star, only from the disc. Bits of the disc collect under more local gravity, and are already spinning in the same way as the disc around the sun.
So all planets default to spin in the same direction as the disc, and orbit in roughly the same plane as the disc.
So why do some planets have more tilted orbits and rotations? Mostly because of random calamities, big cosmic impacts that push them out of their starting orbit, and off-tilt for the rotation axis.
Over time, particles collide and things,
And gravitational interactions tend to make angular momentum vectors converge.
2 missing replies
There's no solid answer to the first question. We have a lot of theories, like a big asteroid changed its rotation, or that Venus used to spin in the other direction before it was tidally locked with the sun and it just gradually slowed down and flipped, but no one knows for sure.
The second question is believed to be because the solar system used to have a massive cloud of gas near its equator and the planets formed from the accretion of this cloud, so they all rotate the same direction.