By that, I mean, why do planets always orbit… horizontally(?) around the sun. Why not vertically? Space is a 3D space, I’d course. So why would the planets not end up going up as well as sideways?
Edit: Space science is a lot more complicated than I thought, and I am here for this rabbit hole. Ty everyone for your answers so far!
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Further adding to op’s question: how self correcting is the orbital plane? During the early stages of the solar system, some major collisions occurred such as Earth and Theia. The extreme axial tilt of Venus may have been caused by a massive impact. Perhaps many of the early protoplanets that suffered these impacts are no longer part of the solar system today, but how much deviation to orbital plane would these events generate and how did the present solar system restore its plane?
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all the planets formed from the same spinning, collapsing cloud of gas that the sun formed from. so the starting conditions made it such that every body that formed from that cloud has the same rotation and orbital plane. the only reason anything is different is the 5 billion years of gravitational interactions and collisions that have happened since.
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It minimises the kinetic/gravitational energy of the system for a given angular momentum. Angular momentum and energy are both conserved, but energy can be (and usually will be, if possible) transformed into heat.
Essentially, after the sun formed there was a 3d cloud of dust around it. Overtime as this dust collided it flattened out into the proto-planetary disk. This was a flat plane that sat at the same inclination the planets now sit at. The reason the planets all orbit in more or less the same plane is because the materials they formed from were orbiting in that plane from the disk. Also angular momentum is very powerful and it requires a lot of energy to change orbital inclination
Planets tend to orbit in the same orbital plane (“horizontally”, as you put it) because they form out of the same spinning disk of dust and gas (called a “protoplanetary disk”).
Here’s an [artistic computer rendering](https://commons.wikimedia.org/wiki/File:Artist%E2%80%99s_Impression_of_a_Baby_Star_Still_Surrounded_by_a_Protoplanetary_Disc.jpg) of this disk around a young a star, and here’s an [actual (radio) telescope image of a real one](https://commons.wikimedia.org/wiki/File:HL_Tau_protoplanetary_disk.jpg) (around the star HL Tauri).
Since this disk is flat and already spinning (why that’s so in a minute), planets that form around the star will tend to 1) have orbits that are close to the plane of this disk, 2) orbit the star in the same direction as the disk spins, and even 3) spin on their own axes in the same “direction” (that is, clockwise or counter-clockwise as seen from “above”).
Of course, perturbations (such as collisions or gravitational influence of other planets) during or after planet formation can sometimes change these things, making them orbit out of this plane, and even spin backward or not at all (like Venus and Uranus — here’s [a nice video](https://www.youtube.com/watch?v=2H5NfcYS3YE) showing planet spin directions and speeds).
This spin-aligning effect is also weaker at greater distances from the star (because gravity is weaker at greater distance), which explains why objects beyond Neptune (including comets, and also Pluto) can have orbits [much more out of the plane](https://assets.coursehero.com/study-guides/lumen/images/towson-astronomy-2/overview-of-our-planetary-system/OSC_Astro_07_01_Orbits2.jpg) of the solar system. It’s also much easier to alter the orbit of a small object, like an asteroid or comet, than that of a planet, which is much more massive.
As to why the initial dust and gas forms into a disk in the first place. Solar systems form out of huge clouds of gas and dust, which are initially roughly spherical. As the cloud start to contract under its own gravity (here’s a [computer simulation](https://www.youtube.com/watch?v=YbdwTwB8jtc)), whatever tiny spin it initially had will get greatly amplified (due to something called “conservation of angular momentum”), making it spin faster and faster. And then the centrifugal force will shape the contracting gas and dust into a disk — a bit like why pizza dough becomes [more flat when you spin it](https://www.youtube.com/watch?v=HWL__9yDu8I).
It’s actually quite interesting and a little tricky to ELI5.
But it has to do with things spinning around in circles and laaaarge timescales. Imagine, if you will, a random star in a vacuum without anything around it.
You, as the omnipotent imaginer, throw a little rock/piece of dust in it’s general direction. Maybe you hit it, but more likely you’ll miss the star. And when you miss the star the little rock/pice of dust starts circling around the star (thats not quite how orbital mechanics works but works for this illustration).
The way your little rock/dust orbits the star is determined by how you missed it. If you threw it “over” the star it’ll start swinging around in a vertical orbit. If you missed it to the right it will start whizzing around in a horizontal orbit. you could imagine it like a string being connected between the star and the rock/dust you threw.
Now you throw another piece. And another piece, and another, and another. You start throwing maaaaaaaaany many little rockts and pieces of dust and ice at the star. with some you almost hit the star and they end up in a very close circle around it. Others end up in a very large circle around the star. Others again end up doing ellipses around the star, being close at some point and far away at another point. And the planes all the pieces orbit in are also all different, because some flew past the star on the left when you first threw them, some on the right, some above, some below, others, somewhere in between.
And you keep throwing little rocks and dust at your star and suddenly the first two little pieces collide. And they most likely don’t collide head on and rather at an angle, where they just combine or create a little debris and the pieces fly of in a different direction.
As you wait and throw more and more little rocks at your star (now surrounded by a cloud of a uncountably many rocks and dust specs) more collisions happen, and some rocks combine into larger groups as they collide.
The larger pieces have a little bit of a stronger gravity and attract more pieces. And slowly the cloud of pieces coalesces into larger clumps which gather up even more dust.
With every collision the pieces involved change the direction a little bit, according to their masses and velocities. So a collection of tiny rocks has the mass of all the pieces and the average direction of and velocity of the pieces (not super accurate phrasing, but it works i guess).
these clumps turn into planets over a looong time as they gather more and more pieces. Since they all collect pieces from the same cloud of dust and rocks they have a very similar direction of travel because it is very close to the average direction of travel of the original cloud. This is the first part of the explanation because eventually all planets can be seen as the averaged remnants of your cloud.
The second part is, because of the planets attracting each other. conceivably you could have all planets orbiting in close to the same plane with one planet being an outlier, orbiting perpendicular to that plane. When that anomalous planet is on its point furthest away from the plane of the other planets the other planets will attract him and try to pull the “rogue” one into the same plane of rotation. This tugging is veeery small, but over eons of years it will eventually pull the odd one out into the same plane as all the others.
Thats how you end up with them all rotating in the same plane. If you want to get into the weeds of it there is a lot of interesting effects with orbital mechanics, angular momentum and gravitational interaction involved.
They don’t. It seems like it but their orbital inclination can be way different (see kerbal space program for a good example of how this works in a relatively simple format.)
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