Why are all planets in the solar system aligned like a pancake, rather than in a sphere surrounding the sun?
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TL;DR: Gravity.
TL;DR2: They’re not technically spheres, but hey, close enough.
TL;DR3: Ok fine, they’re oblate spheres (i.e., spheres that are kinda squished).
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//This has turned out to be a tiny portion of the answer. See other comments for the proper answers. I’m not deleting this though, because combined with the responses gives extra information//
While I can’t claim to be able to keep up in astrophysics, I believe I have learned that one of the reasons is the rotation of the sun.
When the sun rotates, the bit that happens to be closest to…mercury, lets say, very slightly drags Mercury along with it, and as a new spot takes its place, it does the exact same thing, over and over, subtly influencing the orbital path over the many, many years.
But again, I could have horribly misunderstood something, as well as this is probably only part of the answer if it is true. I would *love* to have some correct me though. I think it’s all amazing, I just struggle to keep up with things sometimes:(
Basically, its because of what’s called centrifugal force. When an object (or in this case, a cloud of space dust) begins spinning, parts of the object pull away from the axis (poles) that the object is spinning around. Imagine spinning a rope with a weight tied to the end. The weight moves outward and away from the central point of your hand. The same thing happens with big objects, like planets and space clouds. Gravity is the rope holding everything to the star and centrifugal force makes the object, over time, flatten out as all its atoms try to move away from the axis of spin. In the case of a solar system, this happens before the planets form. The nebula destined to be a solar system flattens out into a disc. Then the space debris collides and collects to form the planets.
Under the current nebular model of solar system formation, we began with a big cloud of gas and dust, which began to collapse (probably after some sort of nudge from passing star or shock from a supernova or the like).
This cloud almost certainly had some slight rotation to it, as it’s nearly impossible to get an extended cloud without any. As the cloud collapses, it’s difficult for it to collapse towards the axis it’s rotating around because of the conservation of angular momentum–when you try, it just spins faster, like a figure skater pulling in their arms. This doesn’t apply to the parts of the cloud collapsing “down” parallel to the axis, so the cloud flattens out into a disk.
For a vaguely similar model, think about someone making a pizza–they start with a big ball of dough, but they spin it around and it ends up flattening into a disk, not just compressing into a smaller sphere.
Note that there are some parts of the cloud that didn’t collapse down like this, and are still out there in spherical form. We call this (hypothesized) part of the solar system the Oort cloud.
So the solar system started off as a big cloud of collapsing dust. By sheer statistical probability on average everything will average out to be slightly rotating around one axis.
During the collapse dust hits other dust, bigger blobs hit other blobs, and each time their angular momentum combines had brings it closer to the average.
Things orbiting at a tilt or orbit against the average in general have more chances to hit and cancel out as well.
So by the time we have the sun and planets mostly formed all the angular momentum left leaves everything in a plane orbiting around that original axis of average angular momentum.
Some smaller things, again just by chance ended up tilted a bit, not in circular orbits, etc and toward the end of the solar system formation there just wasn’t enough stuff spread out to hit anymore.
There are also some objects and anomalies like planet axis tilts that were a result of large collisions after things we’re mostly formed.
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